UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

The fluorides of vanadium Cavell, Ronald George 1962

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
831-UBC_1962_A1 C3 F4.pdf [ 8.69MB ]
Metadata
JSON: 831-1.0062265.json
JSON-LD: 831-1.0062265-ld.json
RDF/XML (Pretty): 831-1.0062265-rdf.xml
RDF/JSON: 831-1.0062265-rdf.json
Turtle: 831-1.0062265-turtle.txt
N-Triples: 831-1.0062265-rdf-ntriples.txt
Original Record: 831-1.0062265-source.json
Full Text
831-1.0062265-fulltext.txt
Citation
831-1.0062265.ris

Full Text

THE FLUORIDES OF VANADIUM by RONALD GEORGE CAVELL B . S c , M c G i l l U n i v e r s i t y , 1958 M.Sc., U n i v e r s i t y of B r i t i s h Columbia, i960 A t h e s i s submitted i n p a r t i a l f u l f i l m e n t of the requirements f o r the degree of DOCTOR OP PHILOSOPHY i n the DEPARTMENT OF CHEMISTRY We accept t h i s t h e s i s as conforming t o the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA August 1962 In presenting t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the requirements f o r an advanced degree a t the U n i v e r s i t y of B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission f o r extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by hxs r e p r e s e n t a t i v e s . I t i s understood t h a t copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gaxn s h a l l not be allowed without my w r i t t e n permission. Department of dMeU\%>T&V The U n i v e r s i t y of B r i t i s h Columbia, Vancouver 8, Canada. Date *\ Se^fr V^>1 GRADUATE STUDIES F i e l d of Study: Inorganic Chemistry Advanced Inorganic Chemistry Surface Chemistry S t a t i s t i c a l Mechanics Physical Organic Chemistry Quantum Chemistry Inorganic Reaction Mechanisms Related Studies: D i f f e r e n t i a l Equations Geophysics N. B a r t l e t t J. Halpern R.F. Snider R. Stewart Pmcock A. Coope R.E J J. Halpern .A. Jennings J.A. Jacobs PUBLICATIONS 1 R.G. C a v e l l and H.C. Clark "Amine Derivatives of Vanadium Pentafluoride" J Inorg. Nuclear Chemistry (1960) YT_ 257 2 R.G. C a v e l l and H.C. Clark "The Preparation and Properties of Vanadium T e t r a f l u o r i d e " Jour Chem Soc (London) (1962), 2692. The U n i v e r s i t y of B r i t i s h Columbia FACULTY OF GRADUATE STUDIES PROGRAMME OF THE FINAL ORAL EXAMINATION FOR THE DEGREE OF DOCTOR OF PHILOSOPHY of RONALD GEORGE CAVELL B.Sc., McGill University, 1958 M.Sc, The U n i v e r s i t y of B r i t i s h Columbia, 1960 FRIDAY, SEPTEMBER 7, 1962, at 9:30 A.M.-IN ROOM 342, CHEMISTRY BUILDING COMMITTEE IN CHARGE Chairman: F. H. SOWARD N. BARTLETT J. GRINDLAY H.C. CLARK K.B. HARVEY F.W. DALBY L.D. HAYWARD C. REID External Examiner. R.S. NYHOLM Department of Chemistry Un i v e r s i t y College, London, England THE FLUORIDES OF VANADIUM ABSTRACT The density, surface tension and v i s c o s i t y of l i q u i d vanadium pentafluoride have been measured. The high value of the v i s c o s i t y indicates that the l i q u i d i s probably associated i n a somewhat s i m i l a r manner to antimony pentafluoride, thus supporting recent evidence which has suggested that mter-molecular ass o c i a t i o n i s also an important process i n these associated f l u o r i d e s The i n f r a r e d spectrum of vanadium pentafluoride vapor has been measured i n the region 250 to 3500 cm"^ and the r e s u l t s have been interpreted i n terms of a monomeric t r i g o n a l bipyramid molecular structure i n view of the normal vapor density Vanadium pentafluoride formed a 1:1 complex with selenium t e t r a f l u o r i d e The apparently s i m i l a r sulphur t e t r a f l u o r i d e complex was extremely unstable Vanadium t e t r a f l u o r i d e i s best prepared by f l u o r i n a t m g vanadium t e t r a c h l o r i d e with anhydrous hydrogen f l u o r i d e i n trichlorofluoromethane solution Vanadium t e t r a f l u o r i d e disproportionates r e a d i l y at 100* i n vacuum into the t r i f l u o r i d e and the penta-f l u o r i d e S o l i d vanadium t e t r a f l u o r i d e also sublimes slowly at 100-120° i n vacuum The av a i l a b l e s t r u c t u r a l information suggests that i n vanadium t e t r a f l u o r i d e the vanadium atom i s surrounded by s i x f l u o r i n e atoms to form an octahedra VF5 unit Four f l u o r i n e s are shared with adjacent vanadium atoms thus forming a polymeric f l u o r i n e -bridge-bonded structure The i n f r a r e d spectrum of s o l i d VF4. has been interpreted i n terms of t h i s model Bromine t r i f l u o r i d e and gaseous f l u o r i n e r e a d i l y f l u o r i n a t e d vanadium t e t r a f l u o r i d e to the penta-f l u o r i d e In the presence of iodine pentafluoride, n i t r y l f l u o r i d e oxidised vanadium t e t r a f l u o r i d e and formed the n i t r y l s a l t NC^VFg. Ammonia, pyridine and selenium t e t r a f l u o r i d e formed 1.1 complexes with VF^ Vanadium t e t r a f l u o r i d e did not react with sulphur t e t r a f l u o r i d e , sulphur t r i o x i d e or sulphur dioxide Potassium hexafluorovanadate (IV) was prepared frpm potassium f l u o r i d e and vanadium t e t r a f l u o r i d e i n selenium t e t r a f l u o r i d e solution. The t r i g o n a l form was obtained with l a t t i c e constants of a = 5 68, £ = 4 66 A. Alkaline earth f l u o r i d e s did not form hexafluorovanadate s a l t s with vanadium t e t r a f l u o r i d e i n iodine pentafluoride KVF^ could not be prepared from equimolar proportions of potassium f l u o r i d e and vanadium t e t r a f l u o r i d e i n iodine pentafluoride solution A l l of the tetravalent vanadium f l u o r i d e compounds which have been studied obeyed the Curie-Weiss law, with very high values of the Weiss Constant Separation of antiferromagnetism and spm-orbit i n t e r a c t i o n i s not possible as both e f f e c t s are l i k e l y to arise from the proposed f l u o r i n e bridging The heat of hydrolysis of vanadium t e t r a f l u o r i d e in water has been found to be -27 5 kc a l /mole, and this value was used i n a Hess law c a l c u l a t i o n to obtain -332 kcal /mole for the heat of formation of vanadium t e t r a f l u o r i d e Vanadium pentafluoride was hydrolysed under similar conditions i n a d i l u t e a l k a l i solution and the resultant heat of hydrolysis, -141 kcal /mole, was used to calculate the heat of formation of - 352 kcal /mole for vanadium pentafluoride. L a t t i c e energies were estimated from a simple formula given by Kapustmskii and used i n a Born-Haber cycle to calculate heats of formation Using the calculated heat of formation of vanadium t r i f l u o r i d e and the experimental values for vanadium t e t r a f l u o r i d e and vanadium pentafluoride, the spontaneity of the disproportionation of vanadium t e t r a f l u o r i d e was confirmed. The heat of hydrolysis of vanadium t e t r a c h l o r i d e i n water i s -68 8 kcal /mole With t h i s value, the heat of formation of the aqueous vanadyl ion was calculated as -113 kcal /mole ACKNOWLEDGEMENT The work r e p o r t e d i n t h i s t h e s i s was done between May i960 and J u l y 1962 under the s u p e r v i s i o n of Dr. Howard C. C l a r k , t o whom the author wishes to express h i s s i n c e r e a p p r e c i a t i o n f o r u n f a i l i n g advice and encourage-ment. The author would a l s o l i k e t o thank a l l the other members of the Inorganic group f o r much s t i m u l a t i n g d i s c u s s i o n . The author thanks Dr. R. H. Wright and Dr. J . E. B l o o r of the B r i t i s h Columbia Research C o u n c i l f o r the use of t h e i r i n f r a r e d spectrometer w i t h caesium bromide o p t i c s . F i n a l l y the author wishes to thank the N a t i o n a l Research C o u n c i l f o r f i n a n c i a l support i n the form of stu d e n t s h i p s throughout the p e r i o d from May i960 t o the pr e s e n t . ABSTRACT i i i The d e n s i t y , s u r f a c e t e n s i o n and v i s c o s i t y of l i q u i d vanadium p e n t a f l u o r i d e have been measured. The h i g h value of the v i s c o s i t y i n d i c a t e s t h a t the l i q u i d i s probably a s s o c i a t e d i n a somewhat s i m i l a r manner to antimony p e n t a f l u o r i d e , thus s u p p o r t i n g r e c e n t evidence t h a t i n t e r m o l e c u l a r a s s o c i a t i o n i s an important process i n ' a s s o c i a t e d 1 f l u o r i d e s . The i n f r a r e d spectrum of vanadium p e n t a f l u o r i d e vapor has been measured i n the r e g i o n 250 to 3500 cmT^ " and the r e s u l t s have been i n t e r p r e t e d i n terms of a monomeric t r i g o n a l bipyramid molecular s t r u c t u r e i n view of the normal vapor d e n s i t y . Vanadium p e n t a f l u o r i d e formed a 1:1 complex w i t h selenium t e t r a f l u o r i d e . The a p p a r e n t l y s i m i l a r sulphur t e t r a f l u o r i d e complex was extremely u n s t a b l e . Vanadium t e t r a f l u o r i d e i s best prepared by f l u o r i n a t i n g vanadium t e t r a c h l o r i d e w i t h anhydrous hydrogen f l u o r i d e i n t r i c h l o r o f l u o r o m e t h a n e s o l u t i o n . Vanadium t e t r a f l u o r i d e d i s -p r o p o r t i o n a t e s r e a d i l y at 100° i n vacuum i n t o the t r i f l u o r i d e and the p e n t a f l u o r i d e . S o l i d vanadium t e t r a f l u o r i d e a l s o sublimes s l o w l y at 100-120° i n vacuum. The a v a i l a b l e s t r u c t u r a l i n f o r m a t i o n suggests t h a t i n vanadium t e t r a f l u o r i d e the vanadium atom i s surrounded by s i x f l u o r i n e atoms to form an o c t a h e d r a l VPg u n i t . Pour f l u o r i n e s are shared w i t h adjacent vanadium atoms thus forming a p o l y -meric f l u o r i n e bridge-bonded s t r u c t u r e . The i n f r a r e d spectrum of s o l i d vanadium t e t r a f l u o r i d e has been i n t e r p r e t e d i n terms i v of t h i s model. Bromine t r i f l u o r i d e and gaseous f l u o r i n e r e a d i l y f l u o r -i n a t e d vanadium t e t r a f l u o r i d e t o the p e n t a f l u o r i d e . In the presence of i o d i n e p e n t a f l u o r i d e , n i t r y l f l u o r i d e o x i d i s e d vanadium t e t r a f l u o r i d e and formed the n i t r y l s a l t NO^VFg. Ammonia, p y r i d i n e and selenium t e t r a f l u o r i d e formed 1:1 complexes w i t h vanadium t e t r a f l u o r i d e . Vanadium t e t r a f l u o r i d e d i d not r e a c t w i t h sulphur t e t r a f l u o r i d e , sulphur t r i o x i d e , sulphur d i o x i d e or i o d i n e p e n t a f l u o r i d e . Potassium hexafluorovanadate (IV) was prepared from potassium f l u o r i d e and vanadium t e t r a f l u o r i d e i n selenium t e t r a f l u o r i d e s o l u t i o n . The t r i g o n a l form was obtained w i t h l a t t i c e constants of a = 5 . 6 8 , _c = 4 . 6 6 A. A l k a l i n e e a r t h f l u o r i d e s d i d not form hexafluorovanadate s a l t s w i t h vanadium t e t r a f l u o r i d e i n i o d i n e p e n t a f l u o r i d e , KVF^ co u l d not be prepared from equimolar p r o p o r t i o n s of potassium f l u o r i d e and vanadium t e t r a f l u o r i d e i n i o d i n e p e n t a f l u o r i d e s o l u t i o n . A l l of the t e t r a v a l e n t vanadium f l u o r i d e compounds which have been s t u d i e d obeyed the Curie-Weiss law, w i t h very h i g h v a l u e s of the Weiss Constant. S e p a r a t i o n of a n t i f e r r o -magnetism and s p i n - o r b i t i n t e r a c t i o n i s not p o s s i b l e as both e f f e c t s are l i k e l y t o a r i s e from the proposed f l u o r i n e b r i d g i n g . The heat of h y d r o l y s i s of vanadium t e t r a f l u o r i d e i n water has been found t o be - 2 7 . 5 kcal./mole, and t h i s value was used i n a Hess law c a l c u l a t i o n t o o b t a i n - 3 3 2 kcal./mole f o r the heat of for m a t i o n of vanadium t e t r a f l u o r i d e . Vanadium V p e n t a f l u o r i d e was h y d r o l y s e d under s i m i l a r c o n d i t i o n s i n a d i l u t e a l k a l i s o l u t i o n and the r e s u l t a n t heat of h y d r o l y s i s , - l 4 l kcal./mole, was used t o c a l c u l a t e the heat of formation of -352 kcal./mole f o r vanadium p e n t a f l u o r i d e . L a t t i c e energies were estimated from a simple formula given by K a p u s t i n s k i i and used i n a Born-Haber c y c l e t o c a l c u l a t e heats of f o r m a t i o n . Using the c a l c u l a t e d heat of formation of vanadium t r i f l u o r i d e and the experimental values f o r vanadium t e t r a f l u o r i d e and vanadium p e n t a f l u o r i d e , the s p o n t a n e i t y of the d i s p r o p o r t i o n a t i o n of vanadium t e t r a f l u o r i d e was confirmed. The heat of h y d r o l y s i s of vanadium t e t r a c h l o r i d e i n water i s -68.8 kcal./mole. With t h i s value, the heat of formation of the aqueous vanadyl i o n was c a l c u l a t e d as -113 kcal./mole. v i CONTENTS CHAPTER Page 1. I n t r o d u c t i o n 1 2. A s s o c i a t i o n and D i s s o c i a t i o n i n F l u o r i d e s . . . 11 3. The P h y s i c a l and Chemical P r o p e r t i e s of Vanadium P e n t a f l u o r i d e . . . . 32 4. The P h y s i c a l and Chemical P r o p e r t i e s of Vanadium T e t r a f l u o r i d e 39 5. The Magnetic P r o p e r t i e s of T e t r a v a l e n t Vanadium F l u o r i d e s 6l 6 . The I n f r a r e d Spectra of Vanadium F l u o r i d e s . . . 75 7. The Thermochemistry of Vanadium F l u o r i d e s . . . 83 8. Experimental General Techniques 107 P r e p a r a t i o n and P h y s i c a l P r o p e r t i e s of Vanadium P e n t a f l u o r i d e 112 Chemical P r o p e r t i e s of Vanadium P e n t a f l u o r i d e . 125 P r e p a r a t i o n and P h y s i c a l P r o p e r t i e s of Vanadium T e t r a f l u o r i d e 127 Chemical P r o p e r t i e s of Vanadium T e t r a f l u o r i d e . 135 P r e p a r a t i o n and P r o p e r t i e s of Hexafluoro-vanadate S a l t s . . . . . l4 l Thermochemistry of Vanadium F l u o r i d e s . . . . 145 APPENDIX: Thermochemistry of the Vanadyl Ion . . . . 160 REFERENCES 164 v i i TABLES -PAGE 1. P h y s i c a l P r o p e r t i e s of the V o l a t i l e F l u o r i d e s . . . . 16 2. S p e c i f i c C o n d u c t i v i t y and D i e l e c t r i c Constant of Halogen F l u o r i d e s 24 3. P h y s i c a l P r o p e r t i e s of L i q u i d A s s o c i a t e d F l u o r i d e s . 28 4. Magnetic P r o p e r t i e s of T e t r a v a l e n t Vanadium F l u o r i d e s 63 5. Comparison of P r e d i c t e d and Experimental Magnetic Moments 69 6. I n f r a r e d Spectrum of Vanadium P e n t a f l u o r i d e 77 7. I n f r a r e d Spectra of Vanadium T r i f l u o r i d e and T e t r a f l u o r i d e 80 8. Heat of Formation of Vanadium T e t r a f l u o r i d e 8 4 9. Heat of Formation of Vanadium Pentaf l u o r i d e 87 10. Data f o r Heat of Formation C a l c u l a t i o n s 93 11. L a t t i c e E n e r g i e s and Heats of Formation of F l u o r i d e s . 94 12. S t a b i l i t y of T e t r a f l u o r i d e s ' 101 13. Bond E n e r g i e s of Some Group IV and V F l u o r i d e s . . . 1 0 4 1 4 . Bond E n e r g i e s of Vanadium C h l o r i d e s 104 15. D e n s i t y of Vanadium Pentaf l u o r i d e 115 16. V i s c o s i t y of Vanadium P e n t a f l u o r i d e 118 17. Surface Tension of Vanadium P e n t a f l u o r i d e 122 18. Molar Surface Energy of Vanadium P e n t a f l u o r i d e . . . 123 19. Magnetic S u s c e p t i b i l i t y of Vanadium T e t r a f l u o r i d e . . 131 20. X-ray D i f f r a c t i o n Data f o r Vanadium T e t r a f l u o r i d e . . 134 21. Magnetic S u s c e p t i b i l i t y of the VF^.SeF^ Adduct . . . 136 22. Magnetic S u s c e p t i b i l i t y of K^VFg 142 (continued) v i i i TABLES (continued) PAGE 23. X-ray D i f f r a c t i o n Data f o r T r i g o n a l &2 VF6 24. Heat of H y d r o l y s i s of Vanadium T e t r a f l u o r i d e . . . . .150 25. Heat of H y d r o l y s i s of Vanadium P e n t a f l u o r i d e 155 26. Heat of H y d r o l y s i s of Vanadium T e t r a c h l o r i d e . . . . .158 27. Heat of Formation of the V0 + +(aq) Ion l6l FIGURES To f o l l o w page 1. P r o p e r t i e s of L i q u i d Vanadium P e n t a f l u o r i d e 34 2. I n f r a r e d Spectra of Vanadium F l u o r i d e s . 77 3. Sample Holders f o r Vanadium P e n t a f l u o r i d e and Vanadium T e t r a f l u o r i d e H y d r o l y s i s Experiments . . . 148 1. CHAPTER 1: INTRODUCTION The development of f l u o r i n e chemistry was begun n e a r l y e i g h t y years ago by Moissan and continued by R u f f u n t i l the 1930's. These p i o n e e r s t u d i e s were not extended by other workers u n t i l r e c e n t l y when r e l i a b l e s u p p l i e s of elemental f l u o r i n e became a v a i l a b l e and vacuum techniques were developed to handle the more r e a c t i v e f l u o r i d e s . The i n c r e a s e d a c t i v i t y i s shown by the p r e p a r a t i o n of many new compounds, the complete c h a r a c t e r i s a t i o n of many p r e v i o u s l y d o u b t f u l compounds and an e v e r - i n c r e a s i n g amount of q u a n t i t a t i v e i n f o r m a t i o n on the p r o p e r t i e s of f l u o r i n e compounds. A s t r i k i n g example of the consequences of t h i s re-awakened i n t -e r e s t i s the r e c e n t i n d i c a t i o n t h a t osmium o c t a f l u o r i d e , long regarded as the prime example of the a b i l i t y of f l u o r i n e to b r i n g out the h i g h e s t v a l e n c i e s and c o o r d i n a t i o n , i s a c t u a l l y osmium h e x a f l u o r i d e ( l ) . Many of the r e c e n t advances have i n v o l v e d the use of p r o p e r t i e s of only r e c e n t l y a v a i l a b l e compounds. The use of bromine t r i f l u o r i d e as an o x i d i s i n g and f l u o r i n a t i n g agent or as an i o n i s i n g s o l v e n t has r e s u l t e d i n the p r e p a r a t i o n of com-pounds such as a u r i c f l u o r i d e , AuF^(2) and many complex f l u o r o -s a l t s , such as the VF^, NbFg s a l t s ; compounds which cannot otherwise be prepared because of t h e i r i n s t a b i l i t y t o heat and water. Selenium t e t r a f l u o r i d e a c t s as a m i l d r e d u c i n g agent i n a d d i t i o n to i t s s o l v e n t p r o p e r t i e s , thus lower valence f l u o r i d e s such as PdFQ(3,4) can be prepared from the higher 2 f l u o r i d e s . Sulphur t e t r a f l u o r i d e , now a v a i l a b l e commercially, i s a good non-oxidative f l u o r i n a t i n g agent f o r m e t a l l i c oxides and s u l p h i d e s and, i n c o n t r a s t to reagents such as selenium t e t r a f l u o r i d e and bromine t r i f l u o r i d e , has l i t t l e tendency t o s o l v a t e the products (eg. 5). Most of the i n f o r m a t i o n obtained i n the e a r l i e r s t u d i e s was q u a l i t a t i v e i n nature, mainly because of the experimental d i f f i c u l t i e s a r i s i n g from the e t c h i n g p r o p e r t i e s of many v o l a t i l e r e a c t i v e f l u o r i d e s . The use of i n e r t f l u o r o c a r b o n p l a s t i c s , such as T e f l o n , or metals such as s t a i n l e s s s t e e l , n i c k e l or pl a t i n u m can a v o i d such d i f f i c u l t i e s , however i n most cases the convenience and economy of g l a s s apparatus i s d e s i r -a b l e . R e c e n t l y (6,7) i t has been found t h a t many r e a c t i v e f l u o r i d e s , which were p r e v i o u s l y r e p o r t e d t o e t c h g l a s s , can be r e a d i l y handled i n g l a s s apparatus p r o v i d e d t h a t a l l t r a c e s of moisture and hydrogen f l u o r i d e are excluded. Thus q u a n t i -t a t i v e s t u d i e s of the p r o p e r t i e s of v o l a t i l e r e a c t i v e f l u o r i d e s are p o s s i b l e without the use of e l a b o r a t e metal or p l a s t i c apparatus. A c o n s i d e r a b l e p o r t i o n of the re c e n t work has centered on the f l u o r i d e s of the t r a n s i t i o n metals, the r e s u l t of rec e n t successes i n the t h e o r e t i c a l i n t e r p r e t a t i o n of the p r o p e r t i e s of t r a n s i t i o n metal compounds by means of the l i g a n d f i e l d theory (10). Almost a l l of the t r a n s i t i o n metals possess the a b i l i t y t o form compounds i n which the outermost set of s t a b l e ' d' e l e c t r o n o r b i t a l s i s only p a r t i a l l y occupied. I t i s t h i s 3 p a r t i a l occupancy of the 'd 1 o r b i t a l s a r i s i n g from the r e l a t i v e l y unusual r e l a t i o n s between the s u c c e s s i v e i o n i s a t i o n p o t e n t i a l s of the 'd ! e l e c t r o n s which causes the o u t s t a n d i n g f e a t u r e of t r a n s i t i o n metal chemistry, the v a r i a b l e valence of the metal. The p a r t i a l l y occupied 'd 1 o r b i t a l s are a l s o respon-s i b l e f o r the magnetic and s p e c t r a l p r o p e r t i e s of t r a n s i t i o n metal compounds, p r o p e r t i e s which were not s u c c e s s f u l l y i n t e r -p r e t e d u n t i l the advent of the l i g a n d f i e l d t h e o r y . In the halogen group, f l u o r i n e tends to b r i n g out the h i g h e s t o x i d a t i o n s t a t e s p o s s i b l e , f o r example, vanadium forms a p e n t a f l u o r i d e , but only a t e t r a c h l o r i d e and molybdenum forms a h e x a f l u o r i d e but o n l y a p e n t a c h l o r i d e . The unique p r o p e r t i e s of f l u o r i n e are l a r g e l y r e s p o n s i b l e f o r t h i s behaviour. I t has the h i g h e s t e l e c t r o n e g a t i v i t y and s m a l l e s t s i z e of the elements i n the halogen s e r i e s and the bond energy i n the f l u o r i n e molecule i s e x c e p t i o n a l l y low ( l l ) . The s i m i l a r i t y of the p r o p e r t i e s of f l u o r i n e to those of oxygen o f t e n c o n f e r s a g r e a t e r resemblance of the p r o p e r t i e s of f l u o r i d e s t o oxides r a t h e r than to the other h a l i d e s . Many t r a n s i t i o n metals form, i n a d d i t i o n to the f l u o r i d e w i t h the maximum o x i d a t i o n s t a t e , a s e r i e s of f l u o r i d e s of lower o x i d a t i o n s t a t e s , the p r o p e r t i e s of which become i n c r e a s i n g l y s a l t - l i k e as the valence of the metal decreases (10). Vanadium, f o r example, forms a v o l a t i l e p e n t a f l u o r i d e , an i n t e r m e d i a t e t e t r a f l u o r i d e and an i n v o l a t i l e t r i f l u o r i d e . The p r e s e n t study i s concerned w i t h the proper-t i e s of these three f l u o r i d e s . 4 The p r e p a r a t i o n and some p r o p e r t i e s of the three simple f l u o r i d e s of vanadium, the t r i f l u o r i d e , t e t r a f l u o r i d e and p e n t a f l u o r i d e , were f i r s t r e p o r t e d by Ruff and L i c k f e t t i n 1911 (12). One i n t e r e s t i n g f e a t u r e was the r e p o r t t h a t vanadium t e t r a f l u o r i d e d i s p r o p o r t i o n a t e s a t 325° i n t o the t r i f l u o r i d e and p e n t a f l u o r i d e , and t h i s i s the method t h a t they used to prepare vanadium p e n t a f l u o r i d e . The l a c k of subsequent work on the vanadium f l u o r i d e s l e a d to doubts as t o the e x i s -tence of vanadium t e t r a f l u o r i d e . In 19^ 5 Simons and Powell (13), i n a study of vanadium t e t r a c h l o r i d e , s t a t e d t h a t the e x i s t e n c e of vanadium t e t r a c h l o r i d e was s u r p r i s i n g " i n view of the e x i s t e n c e of the t r i - and p e n t a f l u o r i d e s and the non-e x i s t e n c e of a t e t r a f l u o r i d e . " In s p i t e of r e c e n t i n t e r e s t i n vanadium t r i f l u o r i d e (l4,2l), vanadium p e n t a f l u o r i d e (7,8,931^) and complex f l u o r o s a l t s of t e t r a v a l e n t vanadium (15,16,17) no study of the t e t r a f l u o r i d e has been r e p o r t e d s i n c e 1911. T h i s i s s u r p r i s -i n g i n view of the f a c t t h a t vanadium p e n t a f l u o r i d e was f i r s t prepared by the d i s p r o p o r t i o n a t i o n of the t e t r a f l u o r i d e (12), however r e c e n t work has shown t h a t the p e n t a f l u o r i d e can be more c o n v e n i e n t l y prepared by the f l u o r i n a t i o n of vanadium metal a t 350°, thus a v o i d i n g the l a b o r i o u s procedure of p r e p a r i n g the t e t r a f l u o r i d e . Ruff and L i c k f e t t (12) r e p o r t e d t h a t vanadium p e n t a f l u o r i d e was a v o l a t i l e orange s o l i d w i t h a m e l t i n g p o i n t "above 200°" and a b o i l i n g p o i n t of 111.2°C. As niobium p e n t a f l u o r i d e and tantalum p e n t a f l u o r i d e melt a t 5 80° and 95° r e s p e c t i v e l y and b o i l at 235° and 229° r e s p e c t i v e l y (l8), these values f o r vanadium p e n t a f l u o r i d e were c l e a r l y u n s a t i s f a c t o r y . Recent s t u d i e s (7,9) have shown t h a t vanadium p e n t a f l u o r i d e i s a c t u a l l y a white s o l i d which melts at 19.5° t o a straw-coloured l i q u i d and b o i l s at 4 8 ° , and thus the melt-i n g and b o i l i n g p o i n t s of the group (V) t r a n s i t i o n metal p e n t a f l u o r i d e s i n c r e a s e w i t h atomic s i z e of the metal as i s g e n e r a l l y observed w i t h i n a p e r i o d i c group. The s u s c e p t i b i l i t y of vanadium p e n t a f l u o r i d e to h y d r o l y s i s may be r e s p o n s i b l e f o r the d i s c r e p a n c y as Ruff and L i c k f e t t ' s measurements were prob-a b l y done on the o x y f l u o r i d e , r a t h e r than the p e n t a f l u o r i d e . For the same reason the value of 2.1766 g/cc. given by Ruff and L i c k f e t t (12) f o r the d e n s i t y of the p e n t a f l u o r i d e at 19° Is'probably u n r e l i a b l e . As vanadium p e n t a f l u o r i d e can be r e a d i l y handled i n a l l - g l a s s apparatus when i t i s f r e e of moisture ( j ) , i t has been p o s s i b l e t o determine s e v e r a l of i t s p h y s i c a l p r o p e r t i e s . The vapour pressure of vanadium p e n t a f l u o r i d e has been measured (7>9) and the heat of v a p o u r i s a t i o n calculated from the vapour pressure equation. The r a t i o of the heat of v a p o u r i s a t i o n to the temperature of v a p o u r i s a t i o n (the Trouton constant) i s 33.1 cal/deg.mole, much high e r than the normal value of 21 or 22. The s p e c i f i c c o n d u c t i v i t y of vanadium pentaf l u o r i d e , 2.4 x 10"^ ohm"1 c r r u 1 (7), i s between t h a t of bromine t r i f l u o r i d e (8.0 x 10 ohm" cm" (19)) and i o d i n e — 6 — 1 — 1 p e n t a f l u o r i d e (5.4 x 10" ohm- cmT (19)). High val u e s of the Trouton constant are found i n many other f l u o r i d e s , p a r t i c u l a r l y the halogen f l u o r i d e s . A h i g h Trouton constant i s g e n e r a l l y accompanied by r e l a t i v e l y h i g h m e l t i n g and b o i l i n g p o i n t s , a p p r e c i a b l y l a r g e l i q u i d ranges a f a i r l y h i g h s p e c i f i c c o n d u c t i v i t i e s . These p r o p e r t i e s are g e n e r a l l y i n t e r p r e t e d as i n d i c a t i v e of intermolecular a s s o c i a -t i o n or d i s s o c i a t i o n . Considerable success has attended the i n t e r p r e t a t i o n of the p r o p e r t i e s of the halogen f l u o r i d e s i n terms of an acid-base s o l v e n t system (19) s i m i l a r t o the s o l v e n t systems formed by water and l i q u i d ammonia, both of which have proper t i e s s i m i l a r t o the halogen f l u o r i d e s . The e l e c t r i c a l c o n d u c t i v i t y of bromine t r i f l u o r i d e , f o r example, i s presumed to a r i s e from the d i s s o c i a t i o n : 2BrF 3 ^ =2; B r F 2 + + BrF^~ forming the a c i d i c ( B r F 2 + ) and b a s i c (BrF^~) i o n s of the s o l v e n t system. The l a r g e s p e c i f i c c o n d u c t i v i t y of vanadium p e n t a f l u o r i d e has been a t t r i b u t e d t o an analogous s e l f -d i s s o c i a t i o n of vanadium p e n t a f l u o r i d e t o form an acid-base s o l v e n t system (7,8). Recent s t u d i e s of f l u o r i n e exchange i n v o l a t i l e f l u o r i d e s , such as the halogen f l u o r i d e s , by n u c l e a r 18 magnetic resonance (20) and r a d i o a c t i v e F t r a c e r (21) techniques have suggested t h a t i n t e r m o l e c u l a r a s s o c i a t i o n , r a t h e r than i o n i c d i s s o c i a t i o n , i s r e s p o n s i b l e f o r the behaviour of these f l u o r i d e s w i t h h i g h Trouton c o n s t a n t s . 7 The r e l a t i v e importance of these two hypotheses i s d i s c u s s e d more f u l l y i n Chapter 2. In the presen t i n v e s t i g a t i o n , f u r t h e r p h y s i c a l p r o p e r t i e s of vanadium p e n t a f l u o r i d e have been measured. In p a r t i c u l a r the d e n s i t y of the l i q u i d has been measured because of rec e n t i n d i c a t i o n s (j,9) t h a t the r e s u l t s of Ruff and L i c k f e t t (12) on vanadium p e n t a f l u o r i d e are not p a r t i c u l a r l y r e l i a b l e . The sur f a c e t e n s i o n and v i s c o s i t y have been measured t o p r o v i d e experimental data which are i n some p a r t i n d i c a t i v e of molecular a s s o c i a t i o n w i t h i n the l i q u i d . The i n f r a r e d spectrum of the vapour and some chemical p r o p e r t i e s of vanadium p e n t a f l u o r i d e have a l s o been i n v e s t i g a t e d . Vanadium t r i f l u o r i d e has been w e l l c h a r a c t e r i s e d s i n c e the i n i t i a l work of Ruff and L i c k f e t t (12). The s t r u c t u r e has been determined (22) and has proven t o be a b a s i c s t r u c t u r a l type f o r t r i f l u o r i d e s (23). The s t r u c t u r e i s based on a b i m o l e c u l a r rhombohedral u n i t c e l l i n which each vanadium i s surrounded by an almost r e g u l a r octahedron of f l u o r i n e atoms. The f l u o r i n e s are shared by two vanadium atoms and hence a c t as b r i d g i n g groups, so t h a t the t r i f l u o r i d e can be regarded as h i g h l y a s s o c i a t e d . Vanadium t r i f l u o r i d e appears t o be the lowest s t a b l e f l u o r i d e of vanadium, as attempts to prepare vanadium d i f l u o r i d e from the d i c h l o r i d e and hydrogen f l u o r i d e r e s u l t e d i n the form a t i o n of vanadium t r i f l u o r i d e (l4). Of the three f l u o r i d e s of vanadium, the l e a s t known i s the t e t r a f l u o r i d e , only the work of Ruf f and L i c k f e t t (12) has been r e p o r t e d . Therefore much of the presen t study i s 8 concerned w i t h e s t a b l i s h i n g the e x i s t e n c e of vanadium t e t r a -f l u o r i d e and an i n v e s t i g a t i o n of i t s p r o p e r t i e s , i n view of i t s i n t e r m e d i a t e p o s i t i o n i n the s e r i e s of known vanadium f l u o r i d e s . L i t t l e i s known about the thermodynamics of f l u o r i d e s , p a r t i c u l a r l y those o f the t r a n s i t i o n metals. Most of the data a v a i l a b l e i n the l i t e r a t u r e , u n t i l r e c e n t l y , has been c a l c u l a t e d by s t a t i s t i c a l methods from the s p e c t r a of f l u o r i d e s . The most e x t e n s i v e s e r i e s of c a l c u l a t i o n s i s t h a t on the hexa-f l u o r i d e s of the group 6 elements, rhenium and uranium (24). Complete experimental thermochemical data i s a v a i l a b l e only f o r simple f l u o r i d e s such as the a l k a l i metal and a l k a l i n e e a r t h f l u o r i d e s . Experimental thermochemical data i s a v a i l a b l e f o r a few t r a n s i t i o n metal f l u o r i d e s but i n many cases the r e l i a b i l i t y i s not e x c e p t i o n a l l y good because of the e x p e r i -mental d i f f i c u l t i e s a s s o c i a t e d w i t h s t u d i e s on f l u o r i d e s . Indeed, r e l i a b l e values f o r the d i s s o c i a t i o n energy of f l u o r i n e i t s e l f were determined not l o n g ago ( l l ) . The most s t r a i g h t f o r w a r d method of measuring the heats of formation of metal f l u o r i d e s , i n v o l v i n g the measurement of the heat evolved on r e a c t i n g f l u o r i n e w i t h the metal i n a c a l o r i m e t e r , was developed by von Wartenberg about twenty years ago (25). The values obtained are not con s i d e r e d t o be very r e l i a b l e because of the experimental d i f f i c u l t i e s encountered (26). R e c e n t l y workers a t the Argonne N a t i o n a l L a b o r a t o r y (26) have developed f l u o r i n e bomb c a l o r i m e t r y to a r e l i a b l e method. T h i s method has r e c e n t l y been used t o measure the heats of formation of z i r c o n i u m t e t r a f l u o r i d e (26) 9 and molybdenum h e x a f l u o r i d e (27). Gross has r e f i n e d the technique of measuring the heat of r e a c t i o n of gaseous f l u o r i n e , near atmospheric p r e s s u r e , w i t h another element, without r e q u i r i n g e l a b o r a t e bomb combustion apparatus. Using t h i s method the heat of formation of t i t a n i u m t e t r a f l u o r i d e (28) has been measured. Most of the thermochemical data a v a i l a b l e u n t i l the advent of r e l i a b l e f l u o r i n e combustion techniques had been obtained by i n d i r e c t methods, measuring the heat of a r e a c t i o n i n v o l v i n g the f l u o r i d e , which can be combined w i t h a s e r i e s of known r e a c t i o n s , by means of Hess' law, to y i e l d the r e q u i r e d heat of f o r m a t i o n . An example i s the r e c e n t d e t e r m i n a t i o n of the heat of formation of thorium t e t r a f l u o r i d e from the e q u i l i b r i u m constant of the r e a c t i o n of the f l u o r i d e w i t h s i l i c a (29). The d i f f i c u l t y encountered w i t h t h i s method i s u s u a l l y the choice of a s u i t a b l e r e a c t i o n . The vigorous exothermic h y d r o l y s i s r e a c t i o n s e x h i b i t e d by most f l u o r i d e s have o f t e n been used to o b t a i n heats of formation of the f l u o r i d e . S u f f i c i e n t r e l i a b l e data must be a v a i l a b l e t o complete a Hess' law c a l c u l a t i o n , a requirement which may not e a s i l y be f u l f i l l e d . Woolf used the heat of h y d r o l y s i s t o determine the heat of formation of i o d i n e p e n t a f l u o r i d e (30), and Meyers and Brady (31) have determined the heats of formation of molybdenum and tungsten hexa-f l u o r i d e s and niobium p e n t a f l u o r i d e from the measured heats of h y d r o l y s i s of these compounds. The accuracy of t h i s procedure i s i l l u s t r a t e d by a comparison of the two r e c e n t l y r e p o r t e d values f o r the heat of formation of molybdenum h e x a f l u o r i d e . Meyers and Brady (31) obtained -388.6 kcal/mole f o r the heat of formation from the measured heat of h y d r o l y s i s and a Hess' law c a l c u l a t i o n , w h i le S e t t l e et_. al_. (27)obtained -372.3 kcal./mole from the f l u o r i n e bomb combustion method. The d i f f e r e n c e i s about hfo, not e x c e s s i v e when i t i s c o n s i d e r e d t h a t the use of a Hess' law c a l c u l a t i o n i n v o l v e s the heats of s e v e r a l r e a c t i o n s , a l l of v a r y i n g r e l i a b i l i t y . The f i n a l p o r t i o n of the present i n v e s t i g a t i o n has been concerned w i t h the thermochemical p r o p e r t i e s of the vanadium f l u o r i d e s . The v i g o r o u s h y d r o l y s i s r e a c t i o n s of vanadium p e n t a f l u o r i d e and vanadium t e t r a f l u o r i d e have been used to o b t a i n heats of f o r m a t i o n of these two compounds. T h e o r e t i c a l c a l c u l a t i o n s of the heat of formation of t r i -f l u o r i d e s and t e t r a f l u o r i d e s of the f i r s t p e r i o d t r a n s i t i o n metals, based on l a t t i c e e n e r g i e s c a l c u l a t e d from an a p p r o x i -mate formula (32), have been made and the r e s u l t s are compared w i t h a v a i l a b l e experimental d a t a . An experimental value f o r the heat of formation of vanadium t r i f l u o r i d e was not determined i n the same manner as f o r the t e t r a f l u o r i d e and p e n t a f l u o r i d e because of the i n s o l u b i l i t y of vanadium t r i f l u o r i d e i n water. Therefore the c a l c u l a t e d value i s used to e valuate the thermochemical p r o p e r t i e s of s e v e r a l r e a c t i o n s i n v o l v i n g the simple f l u o r i d e s of vanadium. 11 CHAPTER 2: ASSOCIATION AND DISSOCIATION IN FLUORIDES The g e n e r a l theory of the s t r u c t u r e of i o n i c s o l i d s i s based on the assumptions (105) t h a t the c o o r d i n a t i o n number of the ions i s as l a r g e as p o s s i b l e , s u b j e c t to the r e s t r i c t i o n s imposed by the packing of ions ( g e n e r a l l y assumed to be s p h e r i c a l ) about a c e n t r a l i o n , and t h a t the c o o r d i n a t e d groups are arranged i n a c o n f i g u r a t i o n which minimises the r e p u l s i o n s between them. In g e n e r a l the c o o r d i n a t i o n about the c e n t r a l io n of a molecule w i l l be higher than i n d i c a t e d by the s t o i c h i o m e t r y of the molecule and the symmetry about the c e n t r a l i o n w i l l be q u i t e high, eg. o c t a h e d r a l or c u b i c , even i n low valence compounds. As the arrangement of ions of one charge about those of another charge i s s t a b l e only i f the c e n t r a l i o n i s i n contact w i t h the surrounding ions and does not ' r a t t l e ' , the packing i n an i o n i c s o l i d w i l l be governed by the r a d i u s r a t i o r u l e s (105). Thus i n an i o n i c s o l i d the c o o r d i n a t i o n of a small metal atom depends on the r a t i o of i t s r a d i u s to t h a t of the anion. The p r e f e r r e d c o n f i g u r a t i o n of AB^ and ABg groupings i n s o l i d s w i l l be t e t r a h e d r a l and o c t a h e d r a l r e s p e c t i v e l y as these c o n f i g u r a t i o n s minimise the e l e c t r o s t a t i c r e p u l s i o n s between the B 1 0 n s , m a i n t a i n i n g at the same time equivalence between a l l A-B i n t e r a c t i o n s (105). I t i s i n t e r e s t i n g to c o n s i d e r the s t r u c t u r e of an AB R complex i n view of the g e n e r a l theory of i o n i c s o l i d s . 12 The only AB^ c o n f i g u r a t i o n which maintains equivalence of a l l the A-B d i s t a n c e s i s the e n e r g e t i c a l l y unfavourable p l a n a r s t r u c t u r e (105). Low e l e c t r o s t a t i c r e p u l s i o n e n e r g i e s are a l s o given by the t r i g o n a l bipyramid and square pyramidal s t r u c t u r e s and these are the most f a v o u r a b l e AB,-groupings. However, r a d i u s r a t i o requirements f o r both the square pyramidal and the t r i g o n a l bipyramid s t r u c t u r e s are i d e n t i c a l to the o c t a h e d r a l requirements hence, i f A i s l a r g e enough t o form an AB,_ group, o c t a h e d r a l ABg groups can a l s o be formed. As the a x i a l - e q u a t o r i a l B-B d i s t a n c e s i n the t r i g o n a l bipyramid or square pyramid s t r u c t u r e s are s i m i l a r to those i n the o c t a h e d r a l s t r u c t u r e , conversion of an AB^ to ABg group i n t r o d u c e s no unfavourable i n t e r a c t i o n s . The tendency t o achieve maximum c o o r d i n a t i o n of the c e n t r a l i o n w i l l be dominant and w i l l make the s i x coor d i n a t e octahedron more fa v o u r a b l e than e i t h e r of the f i v e c o o r d i n a t e s t r u c t u r e s , hence p e n t a c o o r d i n a t i o n i s not to be expected i n i o n i c s o l i d s (105). In ge n e r a l t h i s i s found t o be the case; t e t r a h e d r a l and o c t a h e d r a l c o o r d i n a t i o n s are found i n s o l i d s i n p r e f e r e n c e to p e n tacoordinate s t r u c t u r e s . R i g o r o u s l y these arguments apply o n l y t o monatomic i o n i c s o l i d s , however Orgel suggests (105) t h a t these arguments can be q u a l i t a t i v e l y a p p l i e d t o the d i s p o s i t i o n of d i p o l a r molecules and polyatomic anions about a c e n t r a l atom or i o n . The occurrence of h i g h l y symmetric c o o r d i n a t i o n s t r u c t u r e s i n no way i m p l i e s e i t h e r c o v a l e n t or i o n i c bonding i n the 13 compounds. The stere o c h e m i c a l arrangements are simply those which achieve the maximum p o s s i b l e c o o r d i n a t i o n about a c e n t r a l atom or i o n , which i s mostly governed by the r e l a t i v e s i z e s of the atoms or i o n s , w i t h the minimum r e p u l s i o n s between the c o o r d i n a t i n g groups. The s t e r e o c h e m i s t r y i s not dependent on whether the c o o r d i n a t i n g groups are c o v a l e n t l y bonded t o the c e n t r a l atom, as i s presumed i n o c t a h e d r a l l y c o o r d i n a t e d C o ^ E L ^ g ions or i o n i c a l l y bonded as i s presumed i n the case of sodium c h l o r i d e , where the sodium i s o c t a h e d r a l l y c o o r d i n a t e d by c h l o r i d e i o n s . The small s i z e of the f l u o r i d e i o n (I.36A compared to 1.81A f o r a c h l o r i d e i o n ( l l ) ) ensures that maximum c o o r d i n a t i o n w i l l u s u a l l y be achieved i n s o l i d f l u o r i d e s . In ge n e r a l the maximum c o o r d i n a t i o n w i l l be much g r e a t e r than t h a t i n d i c a t e d by the molecular s t o i c h i o m e t r y . A t y p i c a l example i s the s t r u c t u r e of s o l i d vanadium t r i f l u o r i d e (22) i n which the vanadium atom i s surrounded by s i x f l u o r i n e atoms, forming an o c t a h e d r a l VFg group. Each f l u o r i n e i s shared between two vanadium atoms to maintain the s t o i c h i o m e t r y w h i l e a c h i e v i n g s i x c o o r d i n a t i o n about the vanadium. Many other examples of t h i s type are known (23) where the f l u o r i n e a c t s as a br i d g e between the metal atoms, but the s t r u c t u r e n e i t h e r i m p l i e s nor excludes covalent b r i d g e bonding. Thus while the c o o r d i n a t i o n s t a t e of a c e n t r a l atom of a molecule i s q u i t e e a s i l y e s t a b l i s h e d i n s o l i d s , through complete s t r u c t u r a l a n a l y s i s w i t h such techniques as X-ray d i f f r a c t i o n , t h i s knowledge does not e s t a b l i s h the nature of the bond formed 14 between the c e n t r a l atom and the c o o r d i n a t i n g l i g a n d s . Other evidence which w i l l be d i s c u s s e d l a t e r suggests t h a t an a p p r e c i a b l e amount of covalent b r i d g e bonding occurs i n the s o l i d f l u o r i d e s , thus the h i g h c o o r d i n a t i o n about the metal atom may be i n t e r p r e t e d i n terms of a s t r u c t u r a l u n i t i n v o l v -i n g p a r t l y covalent bonds. In the vapour s t a t e , the assumption of u n i m o l e c u l a r s p e c i e s i s u s u a l l y v a l i d . In c o n t r a s t t o the s o l i d s t a t e , where o r d e r i n g and i n t e r m o l e c u l a r i n t e r a c t i o n i s at a maxi-mum, molecular i n t e r a c t i o n s are minimised under the i n f l u e n c e of i n c r e a s i n g randomness. The c o o r d i n a t i o n i s g e n e r a l l y i d e n t i c a l t o the s t o i c h i o m e t r i c c o o r d i n a t i o n and c o o r d i n a t i o n s t a t e s not found i n s o l i d s , such as pentacoordinate t r i g o n a l bipyramid s t r u c t u r e s , are f r e q u e n t l y observed. The determin-a t i o n of the c o o r d i n a t i o n i s u s u a l l y a more i n t u i t i v e process, i n v o l v i n g molecular s p e c t r a and e l e c t r o n d i f f r a c t i o n techniques which n e v e r t h e l e s s y i e l d r e l i a b l e r e s u l t s . The problem of i n t e r m o l e c u l a r i n t e r a c t i o n and c o o r d i n -a t i o n of molecules i n the l i q u i d s t a t e i s more d i f f i c u l t . The i n t e r m e d i a t e degree of i n t e r a c t i o n found i n l i q u i d s and the i n t e r m e d i a t e degree of randomisation of the s t r u c t u r e p r e c l u d e s the use of X-ray d i f f r a c t i o n techniques and yet i t i s not p o s s i b l e to assume as i n gases, t h a t there i s l i t t l e or no i n t e r m o l e c u l a r i n t e r a c t i o n . Some i n s i g h t i n t o the behaviour of l i q u i d s has been obtained from q u a l i t a t i v e comparison of the p r o p e r t i e s of 15 l i q u i d s , n u c l e a r resonance techniques and many other methods. The problem i s o f t e n somewhat s i m p l i f i e d by the q u i t e reason-able assumption t h a t the bonding i n compounds v a r i e s c o n t i n u o u s l y through the s o l i d - l i q u i d - g a s phase changes, r a t h e r than d i s c o n t i n u o u s l y at each phase boundary. The v a l i d i t y of t h i s assumption i s q u i t e dependent upon the nature of the compound and upon the p a r t i c u l a r phase change i n v o l v e d ; n e v e r t h e l e s s i t o f t e n y i e l d s a u s e f u l i n s i g h t i n t o areas where data are s e v e r e l y l a c k i n g , and i n p a r t i c u l a r i n t o the problem of a s s o c i a t i o n i n l i q u i d s as t h i s assumption suggests t h a t a compound which i s a s s o c i a t e d i n the s o l i d s t a t e w i l l be s i m i l a r l y a s s o c i a t e d i n the l i q u i d phase. The behaviour of v o l a t i l e f l u o r i d e s shows a marked dependence upon the g e n e r a l nature of the compound. The apparent reason f o r t h i s behaviour i s the p o s s i b i l i t y of i n t e r m o l e c u l a r a s s o c i a t i o n i n c e r t a i n systems, i n the s o l i d , l i q u i d and even gaseous phases. A comparison of simple p r o p e r t i e s of a l a r g e number of v o l a t i l e f l u o r i d e s such as m e l t i n g and b o i l i n g p o i n t s and Trouton const a n t s , shown i n t a b l e 1, l e a d s to the c o n c l u s i o n t h a t these f l u o r i d e s can be r o u g h l y c l a s s i f i e d i n t o two groups; a s s o c i a t e d and non-a s s o c i a t e d f l u o r i d e s . 16 TABLE 1 THE PHYSICAL PROPERTIES OP THE VOLATILE FLUORIDES (Data are taken from Ref. 19 u n l e s s otherwise noted) F l u o r i d e m.p. °K b.p. l i q u i d - r a n g e Trouton constant °K bp-mp. Non-associated f l u o r i d e s S i F ^ 183 I78(subl) - -GeP 4 158 136(subl) - -PP C 5 190 198 8 21 SP 6 223 209(subl) - -SeP 6 238 227(subl) - -TeFg 235 234(subl) - -MoFg 290 308 18 21 WP6 275 290 15 21 UP6 337 330(subl) - 20 i s o c i a t e d F l u o r i d e s BrF^ 282 399 117 25.7 CIP 3 196 284 88 23.1 S F 4 152 233 81 27 SeF^ 264 381 117 30 AsP,-5 193 220 27 24 SbPr-5 282 416 134 26 BrP,-5 213 314 101 23 I F 5 283 374 91 26 V F C 293 321 29 33 NbF c 5 353 508 155 25 TaP,-5 368 503 133 26 (Ref 33) 17 The non-associated f l u o r i d e group, c o n t a i n i n g such f l u o r i d e s as s i l i c o n t e t r a f l u o r i d e , sulphur h e x a f l u o r i d e and molybdenum h e x a f l u o r i d e show q u i t e s i m i l a r p r o p e r t i e s . The m e l t i n g and b o i l i n g p o i n t s are q u i t e low and e x h i b i t a f a i r l y r e g u l a r i n c r e a s e w i t h molecular weight. The l i q u i d range i s very s m a l l , i n many cases i t i s non- e x i s t a n t as the s o l i d read-i l y sublimes. The Trouton constant; the r a t i o of en t h a l p y of v a p o u r i s a t i o n t o the b o i l i n g p o i n t , i s g e n e r a l l y c l o s e to the 'normal' value of 21 or 22, when i t can be determined. T y p i c a l l y these p r o p e r t i e s r e f l e c t a very small degree of i n t e r m o l e c u l a r a s s o c i a t i o n i n the s o l i d as w e l l as the l i q u i d . S i l i c o n t e t r a f l u o r i d e i s known to form cubic c r y s t a l s of d i s c r e t e t e t r a h e d r a l molecular u n i t s i n which the s i l i c o n atom maintains i t s t e t r a h e d r a l c o o r d i n a t i o n (36). The h e x a f l u o r i d e s have been shown t o possess r e g u l a r o c t a h e d r a l s t r u c t u r e s (see 19) and probably c r y s t a l l i s e as molecular c r y s t a l s as w e l l . The 'non-associated' behaviour of the h e x a f l u o r i d e s i s e a s i l y understood, as the maximum valence and c o o r d i n a t i o n has been a t t a i n e d . S i l i c o n t e t r a f l u o r i d e and phosphorus penta-_2 f l u o r i d e , however, r e a d i l y form the hexacoordinate ions SiPg and i n d i c a t i n g t h a t the maximum c o o r d i n a t i o n of the c e n t r a l atom has not been a t t a i n e d , hence they should behave as a s s o c i a t e d f l u o r i d e s . The reason t h a t they do not may be th a t the energy of the vacant 'd' o r b i t a l s of the c e n t r a l atom, u t i l i z e d i n complex i o n formation, may be too h i g h i n the n e u t r a l molecule t o allow p a r t i c i p a t i o n i n the formation of n e u t r a l a s s o c i a t e d complexes. 18 The other group, encompassing a wide v a r i e t y of f l u o r i d e s of metals and non-metals a l i k e such as bromine t r i f l u o r i d e , selenium t e t r a f l u o r i d e , antimony p e n t a f l u o r i d e and vanadium p e n t a f l u o r i d e a l l have p r o p e r t i e s (19) which are t y p i c a l of i n t e r m o l e c u l a r a s s o c i a t i o n . The me l t i n g and b o i l i n g p o i n t s are g r e a t e r than those of non-associated f l u o r i d e s of compar-able molecular weight. A l a r g e l i q u i d range of approximately 100°, and Trouton constants ranging from 23 to 33 are a l s o observed. They a l l conduct e l e c t r i c i t y w i t h v a r y i n g a b i l i t y (19) but a t l e a s t as w e l l as l i q u i d ammonia or water (33) and i n many cases the f l u o r i d e s are b e t t e r conductors. Abnormal behaviour i s a l s o observed i n the vapour phase, f o r example c h l o r i n e t r i f l u o r i d e d e v i a t e s from the p e r f e c t gas laws i n a manner which suggests d i m e r i s a t i o n (see 19). There i s no evidence to i n d i c a t e the behaviour of the s o l i d a s s o c i a t e d f l u o r i d e s , but i t i s reasonable t o assume that i f a s s o c i a t i o n occurs, i t w i l l be a maximum i n the s o l i d . I f the non-bonding e l e c t r o n p a i r s are assumed t o f u l f i l the s t e r e o c h e m i c a l f u n c t i o n of a bonded atom (37) then most of the a s s o c i a t e d f l u o r i d e s have an e f f e c t i v e c o o r d i n a t i o n number of f i v e about the c e n t r a l atom. T h i s assumption appears t o be j u s t i f i e d by the observed 'planar-T' s t r u c t u r e of c h l o r i n e t r i f l u o r i d e (19) i n which two lone p a i r s occupy p l a n a r s i t e s i n a t r i g o n a l bipyramid s k e l e t o n and the three f l u o r i n e s occupy the two a x i a l and one of the p l a n a r s i t e s . S i m i l a r l y the 'C^ 1 s t r u c t u r e of sulphur t e t r a f l u o r i d e (38,39) and selenium t e t r a f l u o r i d e (40) i s a l s o based on a t r i g o n a l 19 bipyramid w i t h one p l a n a r s i t e occupied by a non-bonding e l e c t r o n p a i r . Assuming t h a t a l l the a s s o c i a t e d f l u o r i d e s , except i o d i n e and bromine p e n t a f l u o r i d e s , have a maximum c o o r d i n a t i o n of s i x , these compounds are c o o r d i n a t i v e l y u n s a t u r a t e d and hence can i n c r e a s e the c o o r d i n a t i o n of the c e n t r a l atom through i n t e r m o l e c u l a r a s s o c i a t i o n . Iodine and bromine p e n t a f l u o r i d e s can probably a s s o c i a t e to hepta-coordinate s t r u c t u r e s , because of t h e i r l a r g e r s i z e , a n d hence a l s o tend t o achieve maximum c o o r d i n a t i o n . The marked change from a s s o c i a t e d behaviour t o non-a s s o c i a t e d behaviour on c o n v e r t i n g sulphur and selenium t e t r a f l u o r i d e s t o the h e x a f l u o r i d e s supports the p r o p o s a l t h a t the behaviour of the ' a s s o c i a t e d ' f l u o r i d e s i s due to a tendency to i n c r e a s e the c o o r d i n a t i o n of the c e n t r a l atom as much as p o s s i b l e . I t i s i n t e r e s t i n g t o n o t i c e that ' a s s o c i a t e d 1 f l u o r i d e s are mainly formed by the halogens and the t r a n s i t i o n metals, elements which l i k e l y have unoccupied 'd 1 o r b i t a l s of s u f f i c i e n t l y low energy t o be i n v o l v e d i n brid g e bond form-a t i o n . Low l y i n g 1 d' o r b i t a l s are probably a v a i l a b l e i n antimony, because of i t s l a r g e s i z e , and t e t r a v a l e n t sulphur and selenium because t h e i r maximum valence has not been s a t i s f i e d . Before c o n s i d e r i n g the nature of the a s s o c i a t e d s p e c i e s i t i s necessary t o co n s i d e r an a l t e r n a t i v e e x p l a n a t i o n f o r the p r o p e r t i e s of the ' a s s o c i a t e d ' f l u o r i d e s . I t was proposed that the h i g h s p e c i f i c c o n d u c t i v i t i e s e x h i b i t e d by most of these 20 f l u o r i d e s was due to a s e l f - d i s s o c i a t i o n of the type (see Ref 19); 2MP MP + n + MF " n n-1 n+1 to form an acid-base s o l v e n t system. A c i d s i n the system are compounds which c o n t a i n the MP^ "1"^  i o n and bases are those compounds which c o n t a i n the MF ^ i o n . To e s t a b l i s h the e x i s t e n c e of such a s o l v e n t system i t i s necessary to i s o l a t e both a c i d and base compounds, show a p a r t i a l i o n i c c h a r a c t e r i n these compounds and to perform n e u t r a l i s a t i o n r e a c t i o n s between the a c i d and base to y i e l d a s a l t and s o l v e n t . These c o n d i t i o n s have been completely f u l f i l l e d f o r only the bromine t r i f l u o r i d e s o l v e n t system (19) although most of the other ' a s s o c i a t e d 1 f l u o r i d e s s a t i s f y a t l e a s t one and f r e q u e n t l y more of the above c o n d i t i o n s (19). The p r o p o s a l t h a t the behaviour of ' a s s o c i a t e d ' f l u o r i d e s i s due to s e l f - d i s s o c i a t i o n does not i n v a l i d a t e the suggestion t h a t u n s a t u r a t e d c o o r d i n a t i o n of the molecular f l u o r i d e i s the b a s i c f a c t o r r e s p o n s i b l e f o r the behaviour of the ' a s s o c i a t e d ' f l u o r i d e s although i t suggests t h a t the behaviour of these f l u o r i d e s may be b e t t e r d e s c r i b e d i n terms of u n s t a b l e c o o r d i n a t i o n of the molecular f l u o r i d e . Consider, f o r example, the d i s s o c i a t i o n of bromine t r i f l u o r i d e i n t o a c i d i c and b a s i c i o n s : 2 B r F 3 ^=2s B r F 2 + + BrP^" Assuming as before that the two non-bonding p a i r s on the bromine atom are s t e r e o c h e m i c a l l y a c t i v e , the r e s u l t of t h i s 21 d i s s o c i a t i o n i s the conversion of two pentacoordinate molecules i n t o one t e t r a h e d r a l ( B r F 2 + ) i o n and one o c t a h e d r a l (BrF^~) i o n . The c o o r d i n a t i o n s t a t e s formed i n the i o n s , t e t r a h e d r a l and o c t a h e d r a l , are f r e q u e n t l y observed and may be regarded as being s t a b l e . The p e n t a c o o r d i n a t i o n of the n e u t r a l mole-c u l e , on the other hand, i s r a r e l y observed i n condensed systems and may be regarded as b e i n g u n s t a b l e . Thus the e f f e c t of the d i s s o c i a t i o n i s t o convert two s p e c i e s of un s t a b l e c o o r d i n a t i o n i n t o two s p e c i e s w i t h s t a b l e c o o r d i n -a t i o n . However, there i s no i n h e r e n t reason f o r a penta-c o o r d i n a t e molecule t o be u n s t a b l e r e l a t i v e t o t e t r a - and hexacoordinate molecules. A more l i k e l y e x p l a n a t i o n i s t h a t a pentacoordinate molecule i s unsaturated, t h a t i s the c e n t r a l atom can accommodate another l i g a n d without overcrowding the molecule. Where o r b i t a l s of s u f f i c i e n t l y low energy are a v a i l -a ble to p a r t i c i p a t e i n bonding another l i g a n d , a s s o c i a t i o n to crea t e a h i g h e r c o o r d i n a t i o n s t a t e occurs, whereas i f the r e q u i s i t e o r b i t a l s have e x c e s s i v e l y h i g h e n e r g i e s no a s s o c i a -t i o n o c c u r s . Other p r o p e r t i e s of the a s s o c i a t e d f l u o r i d e s are not f u l l y e x p l a i n e d by i o n i c d i s s o c i a t i o n . I f the a s s o c i a t e d f l u o r i d e s are p a r t i a l l y d i s s o c i a t e d a c c o r d i n g t o the p r e v i o u s l y o u t l i n e d i o n i c d i s s o c i a t i o n mechanism, exchange of f l u o r i n e 19 atoms between c e n t r a l atoms w i l l occur. The P n u c l e a r magnetic resonances s p e c t r a of l i q u i d a s s o c i a t e d f l u o r i d e s do indeed show t h i s behaviour (20,4l). In g e n e r a l broad 22 n u c l e a r resonance a b s o r p t i o n peaks are observed at normal temperatures, but on c o o l i n g the sample, the m u l t i p l e t s t r u c t u r e a r i s i n g from non-equivalent f l u o r i n e environments on the c e n t r a l nucleus can be r e s o l v e d . T h i s behaviour i s c o n s i s t e n t w i t h r a p i d f l u o r i n e exchange between non-equivalent f l u o r i n e atoms. The r a t e of exchange, measured as the l i f e t i m e of a f l u o r i n e atom i n a given environment, can be estimated from the s p l i t t i n g of the broad resonance upon r e s o l u t i o n . M u e t t e r t i e s and P h i l l i p s (20) concluded from t h e i r exchange measurements that the r a t e of f l u o r i n e exchange i n the halogen f l u o r i d e s decreases i n the order B r F 0 > C1F0 > IF.-> BrF,_. However the 3 3 5 5 s p e c i f i c c o n d u c t i v i t y decreases i n the order B r F ^ IF^> BrFp-> CIF^. Much b e t t e r agreement would be expected i f i o n i c d i s s o c i a t i o n were r e s p o n s i b l e f o r f l u o r i n e exchange. A s i m i l a r f l u o r i n e exchange behaviour has a l s o been observed i n sulphur t e t r a f l u o r i d e (39,^1) and selenium t e t r a -f l u o r i d e (4l). M u e t t e r t i e s and P h i l l i p s have shown (4l) t h a t the exchange i n sulphur t e t r a f l u o r i d e was independent of the w a l l area of the sample c o n t a i n e r and t h a t the exchange r e a c t i o n was a t l e a s t second o r d e r . A l s o a f r e e r a d i c a l d i s s o c i a t i o n mechanism of the type: SF^ SF 3- + F-was r i g o r o u s l y excluded because the s o l v e n t s used to d i l u t e the sulphur t e t r a f l u o r i d e were not f l u o r i n a t e d (4l). In the e a r l i e r paper (20) M u e t t e r t i e s and P h i l l i p s suggested t h a t as the r a t e of f l u o r i n e exchange d i d not f o l l o w the s p e c i f i c c o n d u c t i v i t y the exchange was due to an 23 a s s o c i a t i v e mechanism i n v o l v i n g f l u o r i n e b r i d g e dimers. T h i s i s c o n s i s t e n t w i t h the second order r a t e law (4l) suggested f o r exchange i n sulphur t e t r a f l u o r i d e and w i t h the o b s e r v a t i o n t h a t a r a d i c a l d i s s o c i a t i o n mechanism i s not p e r m i s s i b l e . 18 Rogers and Katz (21) observed r a p i d F exchange between tagged hydrogen f l u o r i d e or c h l o r i n e t r i f l u o r i d e and s e v e r a l halogen f l u o r i d e s and antimony p e n t a f l u o r i d e both i n the l i q u i d and gaseous phases. While l i q u i d exchange r e a c t i o n s are compatible w i t h the i o n i c d i s s o c i a t i o n , i t i s u n l i k e l y that the vapour phase exchange, which was shown to be homogeneous, occurs through an i o n i c d i s s o c i a t i o n mechanism. Rogers and Katz (21) concluded from these s t u d i e s t h a t i o n i c d i s s o c i a t i o n i s not r e s p o n s i b l e f o r the exchange, but r a t h e r t h a t exchange a r i s e s from the formation of a f l u o r i n e - b r i d g e bonded a s s o c i a t e d d i m e r i c s p e c i e s such as was proposed to e x p l a i n the a s s o c i a t i o n of c h l o r i n e t r i f l u o r i d e i n the gas^ phase (see 19) > f o r example, an exchange of the type: F ^ ' F ^ F F ^ B A s s o c i a t i o n by f l u o r i n e b r i d g i n g and i o n i c d i s s o c i a t i o n are not mutually e x c l u s i v e processes because a s s o c i a t i o n to dimers i s p robably a p r e l i m i n a r y step i n the i o n i s a t i o n p r o c e s s . Homogeneous d i s s o c i a t i o n of the dimer i n t o two molecular s p e c i e s leads to exchange i f the b r i d g e bonds become e q u i v a l e n t i n the dimer and t h e r e f o r e u n d i s t i n g u i s h a b l e when broken d u r i n g d i s s o c i a t i o n . Hetereogeneous d i s s o c i a t i o n of the dimer so t h a t 24 the two bri d g e bonds accompany one c e n t r a l atom w i l l l e a d t o the formation of the ions MP + , and MP ~ . The extent of n-1 n+1 d i s s o c i a t i o n i n t o ions w i l l t h e r e f o r e depend on the a b i l i t y of the l i q u i d f l u o r i d e t o s t a b i l i s e i t s i o n s , thus the magni-tude of the i o n i c d i s s o c i a t i o n constant ( r e p r e s e n t e d by the s p e c i f i c c o n d u c t i v i t y ) w i l l p a r a l l e l the d i e l e c t r i c constant of the s o l v e n t . A complete comparison i s not p o s s i b l e because of inadequate data, however the a v a i l a b l e data given i n Table 2 do show a correspondence between s p e c i f i c c o n d u c t i -v i t y and d i e l e c t r i c c onstant. TABLE 2 SPECIFIC CONDUCTIVITY & DIELECTRIC CONSTANT OP HALOGEN FLUORIDES I F C BrFr- 01F o Ref. 5 5 3 S p e c i f i c c o n d u c t i v i t y 5.4 0.09 0.004 -v (I06k s p) | 19 D i e l e c t r i c constant 36 8 4 of l i q u i d . The s i m i l a r i t y i n the p r o p e r t i e s of the a s s o c i a t e d f l u o r i d e s suggests t h a t a s i m i l a r type of i n t e r m o l e c u l a r bonding w i l l be found i n halogen f l u o r i d e s and a s s o c i a t e d metal f l u o r i d e s . In the halogen f l u o r i d e s , d i m e r i s a t i o n c o u l d i n v o l v e the non-bonding e l e c t r o n p a i r s i n a c l a s s i c a l donor-acceptor bond between the c e n t r a l atoms, however i t i s then d i f f i c u l t t o e x p l a i n a s s o c i a t i o n i n the metal f l u o r i d e s which have no e l e c t r o n s a v a i l a b l e f o r the form a t i o n of a metal to metal bond. The n o n - c l a s s i c a l f l u o r i n e b r i d g e bond proposed by M u e t t e r t i e s and P h i l l i p s (20) p r o v i d e s a bonding mechanism which can be a p p l i e d t o both the halogen f l u o r i d e s and the a s s o c i a t e d metal f l u o r i d e s . A c c o r d i n g t o t h i s p r o p o s a l , a s s o c i a t i o n i n v o l v e s the donation of e l e c t r o n s from the f l u o r i n e atom i n t o vacant o r b i t a l s of the c e n t r a l atom i n another molecule. Back donation from the second molecule t o the f i r s t by a s i m i l a r mechanism prevents the accumulation of e x c e s s i v e e l e c t r o n d e n s i t y on one molecule of the a s s o c i a t e d complex. Since f l u o r i n e i s not regarded as a good e l e c t r o n donor, the bridge bond may i n v o l v e a formal charge t r a n s f e r from f l u o r i n e atoms of one molecule to the f l u o r i n e atoms of a second molecule, r a t h e r l i k e the charge t r a n s f e r i n t e r -a c t i o n s proposed by Burbank and Bensey (42) to account f o r the d i f f e r e n t bond lengths i n c h l o r i n e t r i f l u o r i d e . As b efore, a second charge t r a n s f e r bond i s formed by a s i m i l a r mechanism t o prevent accumulation of e l e c t r o n i c charge on any one molecule of the a s s o c i a t e d complex. The two a l t e r n a t i v e s , which are e s s e n t i a l l y e q u i v a l e n t , are i l l u s t r a t e d below f o r the c h l o r i n e t r i f l u o r i d e dimer: s+ S -F .. F .. F F .. F, .. F \ 1 / \ I / \ < / - J / CI CI OR CI CI / l \ / | \ /\ ' / l \ F . . F . . F F . . F . . F In e i t h e r case the c o n f i g u r a t i o n about each c h l o r i n e atom can be regarded as o c t a h e d r a l w i t h the non-bonding e l e c t r o n p a i r s occupying the two a x i a l p o s i t i o n s of each o c t a h e d r a l 26 u n i t . A s i m i l a r argument a p p l i e s t o the formation of a s s o c i a t e d polymers, which i n some cases may be p r e f e r a b l e to dimers. M u e t t e r t i e s and P h i l l i p s (20), i n proposing f l u o r i n e b r i d g i n g i n halogen f l u o r i d e s by donation of e l e c t r o n s from the f l u o r i n e atoms i n t o the vacant 'd 1 o r b i t a l s of the c e n t r a l halogen atom, suggested t h a t the s t a b i l i t y of these f l u o r i n e b r i d g e bonds i n v o l v i n g 'd 1 o r b i t a l s would i n c r e a s e w i t h the atomic number and s i z e of the c e n t r a l halogen atom. They a l s o suggested t h a t f l u o r i n e b r i d g e bonds between XF^ mole-c u l e s would be more s t a b l e than those formed between XF,-5 molecules, presumably because of the i n c r e a s e d s t a b i l i t y of the l d f o r b i t a l b r i d g e bond accompanying the g r e a t e r s i z e of a t r i v a l e n t X i o n r e l a t i v e t o a p e n t a v a l e n t X i o n . Thus the s t a b i l i t y of dimers should decrease i n the order (20) B r F 0 > 01F o> I F r > B r F [ r .that i s the same as the order of 3 3 5 5' 19 d e c r e a s i n g f l u o r i n e exchange which was observed w i t h F n u c l e a r magnetic resonance measurements (20). While M u e t t e r t i e s and P h i l l i p s have not made e n t i r e l y c l e a r the reasons f o r t h i s order of s t a b i l i t y , e s p e c i a l l y the c o n c l u s i o n t h a t dimer s t a b i l i t y i s g r e a t e r i n t r i f l u o r i d e s than penta-f l u o r i d e s , t h e i r p r o p o s a l of a 'd' o r b i t a l bond can be extended s u c c e s s f u l l y to other systems. The i n c r e a s e i n the s t a b i l i t y of a'd 1 o r b i t a l bond would be expected to i n c r e a s e w i t h the i n c r e a s i n g s i z e and atomic number i n the s e r i e s phosphorus, a r s e n i c and antimony, l e a d i n g t o b r i d g e bond s t a b i l i t y which decreases i n the order: S b F ^ AsF,-^ PF . 27 The behaviour of antimony p e n t a f l u o r i d e suggests t h a t i t i s h i g h l y a s s o c i a t e d , while a r s e n i c p e n t a f l u o r i d e appears t o be a b o r d e r l i n e case of a s s o c i a t i o n and phosphorus p e n t a f l u o r i d e i s best c l a s s e d as non-associated, i n agreement w i t h pre-d i c t i o n s based on M u e t t e r t i e s and P h i l l i p s p r o p o s a l . F u r t h e r evidence f o r f l u o r i n e b r i d g e bonding i n a s s o c i a t e d f l u o r i d e s has been obtained from the n u c l e a r magnetic resonance spectrum of antimony p e n t a f l u o r i d e (43) which suggests;;that a polymer of o c t a h e d r a l SbFg u n i t s i s formed by s h a r i n g f l u o r i n e s w i t h adjacent octahedra a t r i g h t angles to each other. T h i s s t r u c t u r e i s c o n s i s t e n t w i t h the extremely h i g h v i s c o s i t y of antimony p e n t a f l u o r i d e (44). The molecular complexity of (SbF^)^ a t 150° and ( S b F ^ ) 2 a t 250° i n the vapour s t a t e (45) suggests t h a t the polymeric s t r u c t u r e i s p a r t i a l l y p r e s e r v e d upon v a p o u r i s a t i o n . The low c o n d u c t i -v i t y of antimony p e n t a f l u o r i d e (19) i s a l s o not s u r p r i s i n g i n view of the polymeric s t r u c t u r e of the l i q u i d . The extent of i n t e r m o l e c u l a r a s s o c i a t i o n i n a l i q u i d w i l l l i k e l y be r e f l e c t e d by p r o p e r t i e s such as s u r f a c e t e n s i o n and v i s c o s i t y which are l a r g e l y dependent on the s i z e and shape of the molecules i n the l i q u i d . V i s u a l o b s e r v a t i o n of the m o b i l i t y of l i q u i d f l u o r i d e s i s o f t e n s u f f i c i e n t to i n d i c a t e q u a l i t a t i v e l y t h a t a s s o c i a t e d f l u o r i d e s have a h i g h e r v i s c o s i t y than non-associated f l u o r i d e s , however q u a n t i t a t i v e data i s necessary t o e v a l u a t e the r e l a t i v e degree of a s s o c i a t i o n . The a v a i l a b l e s u r f a c e t e n s i o n and v i s c o s i t y data f o r a s s o c i a t e d f l u o r i d e s are shown w i t h t h e i r s p e c i f i c 28 c o n d u c t i v i t i e s i n Table 3. TABLE 3 PHYSICAL PROPERTIES OP LIQUID ASSOCIATED FLUORIDES (References are given i n br a c k e t s ) Compound Surface Tension V i s c o s i t y S p e c i f i c a t 250 at 25° C o n d u c t i v i t y 6 — 1 — 1 (dynes/cm ) ( c e n t i p o i s e ) (10 k , ohm" cm" ) — . . sp C I F 3 22.7 (19) 0 . 4 0 (19) 0.004 B r F 3 35.7 ( 4 6 ) 2.22 ( 4 6 ) 8000 SF 4 25.7*(47) SeF 4 35.3 ( 4 8 ) BrFr- 29.7 ( 4 6 ) 0.62 I F 5 22.6 ( 4 6 ) 2.19 -I SbF 5 43.3 (45) 460 ( 4 4 ) * o at -70 C. }(19) The s i m i l a r i t y i n the behaviour of the s u r f a c e t e n s i o n and v i s c o s i t y of the a s s o c i a t e d f l u o r i d e s i s q u i t e e v i d e n t . The val u e s of a l l these p r o p e r t i e s decrease i n the order: SbF^V B r F 3 > IF^> BrF^> CIF^. Agreement between the val u e s of the s u r f a c e t e n s i o n and the v i s c o s i t y i s t o be expected, as these two p r o p e r t i e s probably a r i s e from i n t e r m o l e c u l a r a s s o c i a t i o n . The order of d e c r e a s i n g molecular a s s o c i a t i o n i n d i c a t e d by the v i s c o s i t y and s u r f a c e t e n s i o n f o r the i n t e r h a l o g e n s i s not the order of d e c r e a s i n g Trouton c o n s t a n t s . 29 Likewise the n u c l e a r magnetic resonance study (20) i n d i c a t e d t h a t molecular a s s o c i a t i o n decreased i n the order: BrF^ y C I F ^ BrF,-, an e n t i r e l y d i f f e r e n t order to t h a t i n d i c a t e d by the v i s c o s i t y and su r f a c e t e n s i o n of the halogen f l u o r i d e s . The disagreement between the r e l a t i v e extent of a s s o c i a t i o n , as i n d i c a t e d by v i s c o s i t y or su r f a c e t e n s i o n , and th a t i n d i c a t e d by the Trouton constant i s not s e r i o u s as the Trouton constant i s only approximate i n any case. The reason f o r the disagreement between the r e l a t i v e " e x t e n t of a s s o c i a t i o n i n d i c a t e d by n u c l e a r magnetic resonance measurements and by the s u r f a c e t e n s i o n or v i s c o s i t y i s not r e a d i l y apparent but i t p r obably a r i s e s because measurements on m i c r o s c o p i c systems (eg. n u c l e a r magnetic resonance) are not d i r e c t l y comparable w i t h measurements such as s u r f a c e t e n s i o n or v i s c o s i t y on macroscopic systems because somewhat d i f f e r e n t p r ocesses are be i n g observed. The s p e c i f i c c o n d u c t i v i t y of the halogen f l u o r i d e s a l s o decreases i n the same order as the v i s c o s i t y or s u r f a c e t e n s i o n decrease which i s somewhat s u r p r i s i n g . A p r o p e r t y i n d i c a t i v e of molecular d i s s o c i a t i o n , such as s p e c i f i c con-d u c t i v i t y , would not be expected t o behave s i m i l a r l y to p r o p e r t i e s i n d i c a t i v e of molecular a s s o c i a t i o n . C o n s i d e r i n g a l s o t h a t i o n i c m o b i l i t y w i l l l i k e l y decrease w i t h i n c r e a s i n g v i s c o s i t y , a decrease i n s p e c i f i c c o n d u c t i v i t y w i t h i n c r e a s i n g v i s c o s i t y would be expected; j u s t the opposite i s observed. 30 The p a r a l l e l behaviour of a s s o c i a t i v e p r o p e r t i e s and s p e c i f i c c o n d u c t i v i t y suggests t h a t a s s o c i a t i o n t o dimers i s a necessary p r e l i m i n a r y step to i o n i c d i s s o c i a t i o n , hence the g r e a t e s t s p e c i f i c c o n d u c t i v i t y w i l l be found i n the most h i g h l y a s s o c i a t e d system. T h i s w i l l only be t r u e i f the molecules a s s o c i a t e i n t o e a s i l y i o n i z e d s p e c i e s , t h a t i s i n t o dimers and not polymers because polymeric s t r u c t u r e s w i l l not be as l i k e l y t o form h i g h c o n c e n t r a t i o n s of mobile i o n s . The low v i s c o s i t y of the halogen f l u o r i d e s r e l a t i v e t o , say, antimony p e n t a f l u o r i d e , suggests t h a t the halogen f l u o r i d e s tend to a s s o c i a t e i n t o dimers r a t h e r than polymers. Hence the s p e c i f i c c o n d u c t i v i t y w i l l f o l l o w the t r e n d of i n t e r m o l e c u l a r a s s o c i a t i o n i n d i c a t e d by v i s c o s i t y and s u r f a c e t e n s i o n . The p a r a l l e l behaviour of c o n d u c t i v i t y and a s s o c i a -t i o n , i n d i c a t e d by v i s c o s i t y does not extend to antimony p e n t a f l u o r i d e because of the tendency of antimony penta-f l u o r i d e t o form polymers r a t h e r than dimers. T h i s does not n e c e s s a r i l y imply t h a t the f l u o r i n e b r i d g e bonds are d i f f e r e n t i n the a s s o c i a t e d molecules, but only a d i f f e r e n c e i n the s t r u c t u r e of the r e s u l t a n t a s s o c i a t e d f l u o r i d e . The p r o p e r t i e s of ' a s s o c i a t e d 1 f l u o r i d e s are t h e r e f o r e probably due to the tendency of these f l u o r i d e s t o form d i m e r i c or polymeric a s s o c i a t e d s t r u c t u r e s , by means of f l u o r i n e - b r i d g e bonds, to achieve a more s t a b l e c o o r d i n a t i o n about the c e n t r a l atom. I o n i c d i s s o c i a t i o n , w h ile i t accounts f o r the s o l v e n t p r o p e r t i e s of these f l u o r i d e s , i s of secondary importance, e s p e c i a l l y i n h i g h l y a s s o c i a t e d , polymeric s t r u c t u r e s such as antimony p e n t a f l u o r i d e . The f l u o r i n e b r i d g e a s s o c i a t i o n proposed f o r these l i q u i d f l u o r i d e s appears t o be s i m i l a r to the b r i d g i n g o f t e n found i n s o l i d f l u o r i d e s , however i n a l i q u i d such a s s o c i a t i o n i m p l i e s the formation of a bond. The p r o p o s a l t h a t the a s s o c i a t i o n i n v o l v e s bond formation by donation of e l e c t r o n s from the f l u o r i n e atom i n t o the vacant 1 d 1 o r b i t a l s of the c e n t r a l atom i s supported by the agree-ment between p r e d i c t e d ' d' o r b i t a l bond s t a b i l i t y and the observed s t a b i l i t y of a s s o c i a t i o n i n a s e r i e s of r e l a t e d molecules. CHAPTER 3: PHYSICAL AND CHEMICAL PROPERTIES OF VANADIUM PENTAFLUORIDE 32 (1) P h y s i c a l P r o p e r t i e s of Vanadium P e n t a f l u o r i d e The h i g h s p e c i f i c c o n d u c t i v i t y and Trouton constant of vanadium p e n t a f l u o r i d e (7) suggested t h a t vanadium penta-f l u o r i d e i o n i z e s a c c o r d i n g t o the equation: 2VF 5 ^ V F ^ + + VFg" thus forming an acid-base s o l v e n t system. The e x i s t e n c e of s a l t s c o n t a i n i n g the VF^ - i o n (l4) and the occurrence of r e a c t i o n s which are best e x p l a i n e d as n e u t r a l i s a t i o n r e a c t i o n s (8) support t h i s p r o p o s a l . Vanadium p e n t a f l u o r i d e was t h e r e f o r e considered to resemble niobium and tantalum p e n t a f l u o r i d e s (49) and the halogen f l u o r i d e s (19), which have s i m i l a r s p e c i f i c c o n d u c t i -v i t i e s and Trouton c o n s t a n t s . Considerable success attended the i n t e r p r e t a t i o n of the p r o p e r t i e s of bromine t r i f l u o r i d e i n terms of acid-base s o l v e n t behaviour (19) however i t has not been p o s s i b l e to r i g o r o u s l y e s t a b l i s h acid-base s o l v e n t behaviour f o r the other halogen f l u o r i d e s or the group f i v e f l u o r i d e s (19). Recent s t u d i e s of f l u o r i n e exchange i n the halogen f l u o r i d e s by n u c l e a r magnetic resonance (20) or the use of a r a d i o a c t i v e f l u o r i n e t r a c e r (21), as d i s c u s s e d i n the p r e v i o u s chapter, i n d i c a t e d t h a t the important process was i n t e r -molecular a s s o c i a t i o n r a t h e r than i o n i c d i s s o c i a t i o n (19). F u r t h e r support of t h i s p r o p o s a l i s given by the n u c l e a r 33 magnetic resonance spectrum of antimony p e n t a f l u o r i d e which i n d i c a t e s t h a t the molecules a s s o c i a t e through f l u o r i n e b r i d g e bond formation to form polymeric chains i n the l i q u i d . The extent of i n t e r m o l e c u l a r a s s o c i a t i o n i n a l i q u i d appears to be r e f l e c t e d by p r o p e r t i e s such as s u r f a c e t e n s i o n and v i s c o s i t y as mentioned e a r l i e r . Q u a l i t a t i v e l y vanadium p e n t a f l u o r i d e appears t o be a h i g h l y v i s c o u s l i q u i d , suggesting t h a t the l i q u i d i s h i g h l y a s s o c i a t e d . In an attempt t o q u a n t i t a t i v e l y e s t a b l i s h the extent of i n t e r m o l e c u l a r a s s o c i a t i o n i n l i q u i d vanadium p e n t a f l u o r i d e the d e n s i t y , v i s c o s i t y and s u r f a c e t e n s i o n have been determined i n the p r e s e n t i n v e s t i g a t i o n . The d e n s i t y ) of vanadium p e n t a f l u o r i d e i s a l i n e a r f u n c t i o n of temperature over most of the l i q u i d range (19.5° to 4 8 ° ) of the compound. Between 20° and 40° the d e n s i t y i s given by the e x p r e s s i o n : <* = 2.483 - 0.00349(t-25°) g./cc. The d e n s i t y of s o l i d vanadium p e n t a f l u o r i d e i s probably c o n s i d e r a b l y g r e a t e r than the d e n s i t y of the l i q u i d , s i n c e on f r e e z i n g the p e n t a f l u o r i d e i n the d i l a t o m e t e r a very marked c o n t r a c t i o n o c c u r r e d . The value of 2.1766 g./cc. f o r the d e n s i t y of s o l i d vanadium p e n t a f l u o r i d e at 19°C i s prob-a b l y wrong, j u s t as the values of R u f f and L i c k f e t t (12) f o r the m e l t i n g and b o i l i n g p o i n t s of vanadium p e n t a f l u o r i d e were wrong (7). These erroneous r e s u l t s are probably due to e x t e n s i v e h y d r o l y s i s of Ruff and L i c k f e t t 1 s samples of the 34 p e n t a f l u o r i d e to the o x y t r i f l u o r i d e (7). The value of 2.483 g./cc. f o r the d e n s i t y of vanadium p e n t a f l u o r i d e a t 25° i s w i t h i n the expected range as i t i s c o n s i s t e n t w i t h the d e n s i t i e s ( l y ) of antimony p e n t a f l u o r i d e (2.99 g . / c c . ) , bromine t r i f l u o r i d e (2.80 g . / c c ) , bromine p e n t a f l u o r i d e (2.46 g./cc.) and i o d i n e p e n t a f l u o r i d e (3.19 g . / c c . ) . The average c o e f f i c i e n t of c u b i c a l expansion of vanadium p e n t a f l u o r i d e , determined i n the same experiments as the den-s i t y i s 1.26 x 10 cc/deg. The v i s c o s i t y of vanadium p e n t a f l u o r i d e was measured i n an Ostwald type viscometer m o d i f i e d , as d e s c r i b e d i n the experimental s e c t i o n , t o prevent h y d r o l y s i s of the f l u o r i d e by atmospheric moisture. The v i s c o s i t y v a r i e s l i n e a r l y w i t h temperature although a few p o i n t s d e v i a t e d c o n s i d e r a b l y from the s t r a i g h t l i n e as shown i n F i g u r e l a . The v i s c o s i t y (n. ) of vanadium p e n t a f l u o r i d e , between 25° and 35° i s given by the equation: rj_ = 125 - 7.2(t-25°) + 20$ c e n t i p o i s e . U n f o r t u n a t e l y the-value i s not e x c e p t i o n a l l y accurate because of r a t h e r l a r g e e r r o r s i n the measurement a r i s i n g from decomposition of the vanadium p e n t a f l u o r i d e and the n e c e s s a r i l y s m a l l c a p a c i t y of the v i s c o m e t e r . I t i s apparent, however, t h a t the v i s c o s i t y i s c o n s i d e r a b l y higher than t h a t of most l i q u i d f l u o r i d e s , which i s about 1-2 c e n t i p o i s e s . (Table 3). Only antimony p e n t a f l u o r i d e , w i t h a v i s c o s i t y of .•460 c e n t i p o i s e (44), has a higher v i s c o s i t y and i t has been Ul to o 0. z I-00 o o to F I G * I t o follow p. 34 PROPERTIES OF LIQUID VANADIUM PENTAFLUORIDE (a) VISCOSITY 130 h n6h &0h • = average of several determinations 70b 25 3& T E M P E R A T U R E ( D E G R E E S 35 C. ) •2. O \ CO UJ z > o 18 0 (b) SURFACE TENSION z o to z UJ H UJ o if cr CO 17 K 16 25 30 35 T E M P E R A T U R E ( D E G R E E S C.) 35 shown (43) t h a t antimony p e n t a f l u o r i d e i s a s s o c i a t e d i n t o f l u o r i n e b r i d g e d polymers. The h i g h v i s c o s i t y of vanadium p e n t a f l u o r i d e i n d i c a t e s c o n s i d e r a b l e polymeric a s s o c i a t i o n i n the l i q u i d , probably of a s i m i l a r type to t h a t i n antimony p e n t a f l u o r i d e . The s u r f a c e t e n s i o n of vanadium p e n t a f l u o r i d e , measured by two c a p i l l a r y r i s e techniques, was an approximately l i n e a r f u n c t i o n of the temperature as shown i n F i g u r e l b , although some of the values do not c o i n c i d e w i t h the s t r a i g h t l i n e . The r a t h e r h i g h l i m i t s of e r r o r a r i s e from the tendency of the l i q u i d t o form bubbles i n the c a p i l l a r i e s . The s u r f a c e t e n s i o n ( ) of vanadium p e n t a f l u o r i d e from 25° to 35° Is given by the equation: = 18.2 - 0.l42(t-25° ) +0.2 dynes/cm" . The standard d e v i a t i o n of +0.2 (or about Vfo e r r o r ) f o r measure-ments i n two types of c a p i l l a r y r i s e apparatus suggests t h a t the r e s u l t s are probably reasonably r e l i a b l e . The value of 18.2 dynes/cm f o r the s u r f a c e t e n s i o n of vanadium p e n t a f l u o r i d e i s lower than the values f o r a l l of the a s s o c i a t e d f l u o r i d e s (Table 3). T h i s low s u r f a c e t e n s i o n f o r such a v i s c o u s l i q u i d i s s u r p r i s i n g because i n the a s s o c i a t e d f l u o r i d e s the v i s c o s i t y and s u r f a c e t e n s i o n decrease i n the same order; SbF c> BrF 0 > IF._> BrF.-> C1F 0. While the v i s c o s i t y 5 J 5 5 J of vanadium p e n t a f l u o r i d e f a l l s between those of antimony p e n t a f l u o r i d e and bromine t r i f l u o r i d e the s u r f a c e t e n s i o n i s l e s s than t h a t of c h l o r i n e t r i f l u o r i d e . The h i g h v i s c o s i t y of the l i q u i d may have l e d to a f a l s e r e s u l t f o r the s u r f a c e t e n s i o n , s i n c e i t was found t h a t r e l i a b l e values f o r the sur-face t e n s i o n of antimony p e n t a f l u o r i d e c o u l d not be obtained from c a p i l l a r y r i s e methods, and i t was necessary t o use the maximum bubble pressure method (45). I f t h i s i s the case f o r vanadium p e n t a f l u o r i d e i t i s d i f f i c u l t to understand why two determinations of the s u r f a c e t e n s i o n i n two types of apparatus, although both are measuring c a p i l l a r y r i s e , should agree so w e l l w i t h each other. The change of the molar s u r f a c e energy (V ), given by the equation: V = tf(Mv)2/3 where Y i s the su r f a c e t e n s i o n , M i s the molecular weight and v i s the s p e c i f i c volume, w i t h temperature i s supposed t o be i n d i c a t i v e of molecular a s s o c i a t i o n (20). For non-associated l i q u i d s the slope of the molar s u r f a c e energy versus temperature curve (dV / d t ) i s a u n i v e r s a l constant (the Eotvos c o e f f i c i e n t ) w i t h a value of -2.1 (50). Using the data given i n Table 18, the slope of the molar s u r f a c e energy versus temperature graph f o r vanadium p e n t a f l u o r i d e i s -1.9. The degree of molecular a s s o c i a t i o n estimated from the r a t i o of Eotvos c o e f f i c i e n t s (50) i s not much g r e a t e r than u n i t y . However, the s u r f a c e t e n s i o n and the molar s u r f a c e energy, which i s d e r i v e d from the s u r f a c e t e n s i o n , are not r e l i a b l e i n d i c a t o r s of molecular a s s o c i a t i o n . Many a s s o c i a t e d as w e l l as n o n - a s s o c i a t e d l i q u i d s obey the Eotvos law (50) and attempts 37 t o c a l c u l a t e the degree of molecular a s s o c i a t i o n f o r l i q u i d s w i t h slopes l e s s than the Eotvos c o e f f i c i e n t have not been very s u c c e s s f u l (50). The v i s c o s i t y i s a more r e l i a b l e i n d i c a t i o n of a s s o c i a t i o n , and the h i g h v i s c o s i t y of vanadium p e n t a f l u o r i d e means t h a t the l i q u i d i s p robably h i g h l y a s s o c i a t e d i n s p i t e of the low s u r f a c e t e n s i o n and normal Etttvos c o e f f i c i e n t . The s i m i l a r i t y of the v i s c o s i t y of vanadium p e n t a f l u o r i d e to t h a t of antimony p e n t a f l u o r i d e , both of which are much g r e a t e r than t h a t of the halogen f l u o r i d e s , suggests t h a t both penta-f l u o r i d e s are a s s o c i a t e d i n t o chain polymers i n c o n t r a s t to the d i m e r i c a s s o c i a t e d s p e c i e s which pr o b a b l y form i n the halogen f l u o r i d e s . ( i i ) Chemical P r o p e r t i e s of Vanadium P e n t a f l u o r i d e Chemical i n t e r a c t i o n s between two f l u o r i d e s which e x h i b i t acid-base s o l v e n t p r o p e r t i e s sometimes r e s u l t i n the formation of s o l i d complexes c o n t a i n i n g one i o n from each s o l v e n t system. Bromine t r i f l u o r i d e (l4) or antimony penta-f l u o r i d e (7) are r e a d i l y m i s c i b l e w i t h vanadium p e n t a f l u o r i d e and form s o l u t i o n s from which the component f l u o r i d e s can not be separated by d i s t i l l a t i o n . S a l t formation i s not observed. Selenium t e t r a f l u o r i d e however forms a r e a c t i v e s o l i d 1:1 complex w i t h vanadium p e n t a f l u o r i d e . The most l i k e l y f o r m u l a t i o n of t h i s VP^.SeP^ complex i s as the s a l t S e P ^ V F g - , which combines the most s t a b l e i o n of the selenium t e t r a f l u o r i d e s o l v e n t system (4) w i t h the most s t a b l e i o n of the vanadium p e n t a f l u o r i d e s o l v e n t system (7,8,14), Sulphur t e t r a f l u o r i d e appears t o form a s o l i d complex w i t h vanadium p e n t a f l u o r i d e which i s s t a b l e only at low temper-a t u r e s . The a n a l y s i s i n d i c a t e s an approximate composition VF,_,0.5 SF^. T h i s may be due t o p a r t i a l decomposition of a 1:1 complex or the true formula of the adduct. On the b a s i s of present i n f o r m a t i o n i t i s not p o s s i b l e t o decide between the two a l t e r n a t i v e s . I t i s i n t e r e s t i n g t o note t h a t the r e a c t i o n of niobium pentoxide w i t h sulphur t e t r a f l u o r i d e (5) y i e l d e d a product of the composition NbF,-, 0.54 SF^ which again may be a p a r t i a l l y decomposed 1:1 SF^ complex w i t h niobium penta-f l u o r i d e or the tr u e formula of the adduct. The formation of a s t a b l e complex of vanadium penta-f l u o r i d e w i t h selenium t e t r a f l u o r i d e whereas bromine t r i -f l u o r i d e , antimony p e n t a f l u o r i d e and sulphur t e t r a f l u o r i d e form u n s t a b l e s o l i d s or no s o l i d complexes at a l l probably r e f l e c t s the g r e a t e r s t a b i l i t y of the S e F ^ + c a t i o n r e l a t i v e + + + to the SF^ and BrFg c a t i o n s . The i n s t a b i l i t y of the VF^ c a t i o n i s pro b a b l y r e s p o n s i b l e f o r the l a c k of a s o l i d complex between antimony p e n t a f l u o r i d e and vanadium penta-f l u o r i d e , as antimony p e n t a f l u o r i d e i s w e l l known to form complexes c o n t a i n i n g the SbFg i o n (19). 39 CHAPTER 4: THE PREPARATION AND PROPERTIES OF VANADIUM  TETRAFLUORIDE ( i ) P r e p a r a t i o n and P h y s i c a l P r o p e r t i e s of Vanadium T e t r a f l u o r i d e At the s t a r t of t h i s i n v e s t i g a t i o n , vanadium t e t r a -f l u o r i d e was prepared by f l u o r i n a t i n g vanadium t e t r a c h l o r i d e w i t h l i q u i d anhydrous hydrogen f l u o r i d e at -78°, e x a c t l y a c c o r d i n g t o the method of Ruff and L i c k f e t t (12). The vanadium t e t r a f l u o r i d e obtained was u s u a l l y contaminated w i t h a p p r e c i a b l e amounts of a brown, g r a n u l a r i m p u r i t y and the magnetic moment i n d i c a t e d t h a t a p p r e c i a b l e amounts of t r i v a l e n t vanadium were p r e s e n t . At -78° vanadium t e t r a c h l o r i d e i s s o l i d , and the i m p u r i t i e s probably r e s u l t e d from incomplete r e a c t i o n between s o l i d vanadium t e t r a c h l o r i d e and l i q u i d hydrogen f l u o r i d e due to the formation of a s o l i d f i l m of i n s o l u b l e t e t r a f l u o r i d e on the s u r f a c e of the t e t r a c h l o r i d e . L o c a l i s e d r e d u c t i o n t o form lower f l u o r i d e s may a l s o be due to the presence of s o l i d t e t r a c h l o r i d e . I n c r e a s i n g the r e a c t i o n temperature t o -25°, the m e l t i n g p o i n t of the t e t r a c h l o r i d e , p r o v i d e s a l i q u i d - l i q u i d r e a c t i o n system but s a c r i f i c e s the c o n t r o l over the r e a c t i o n r a t e which i s achieved a t -78°, and would probably l e a d t o g r e a t e r r e d u c t i o n t o the t r i f l u o r i d e . The a d d i t i o n of an i n e r t s o l v e n t such as t r i c h l o r o f l u o r o -methane (mp. -112°) p r o v i d e d a r e a c t i o n mixture which remained l i q u i d at -78° and could be c o n t i n u o u s l y s t i r r e d throughout the r e a c t i o n . With these improvements, a product w i t h a hi g h degree of p u r i t y was c o n s i s t e n t l y o b t a i n e d . 4o Vanadium t e t r a f l u o r i d e may a l s o be prepared from the metal and gaseous f l u o r i n e . At h i g h temperatures, eg. 350° t o 370°, the p e n t a f l u o r i d e i s obtained almost e x c l u s i v e l y , but i f the f l u o r i n e supply i s not s u f f i c i e n t f o r complete conversion t o the p e n t a f l u o r i d e , a brown l i q u i d i s formed which l e a v e s a small amount of a l i g h t brown r e s i d u e of vanadium t e t r a f l u o r i d e when the v o l a t i l e p e n t a f l u o r i d e i s removed. At lower temperatures, i n the neighbourhood of 200°, a l l three known f l u o r i d e s of vanadium, the t r i f l u o r i d e , t e t r a -f l u o r i d e and p e n t a f l u o r i d e are obtained although the p r i n c i p a l product i s s t i l l the p e n t a f l u o r i d e . The d i f f e r e n t v o l a t i l i t i e s of the three f l u o r i d e s p r o v i d e complete s e p a r a t i o n of the f l u o r i d e s d u r i n g the p r e p a r a t i o n , and reasonably pure vanadium t e t r a f l u o r i d e can be i s o l a t e d . For convenience, g r e a t e s t y i e l d and h i g h e s t p u r i t y , however, the f l u o r i n a t i o n of vanadium t e t r a c h l o r i d e w i t h hydrogen f l u o r i d e i n a s o l v e n t i s the p r e f e r r e d p r e p a r a t i v e method f o r vanadium t e t r a f l u o r i d e . Pure vanadium t e t r a f l u o r i d e , prepared from vanadium t e t r a c h l o r i d e and hydrogen f l u o r i d e i s a b r i l l i a n t , lime-green powder which on ,exposure t o moisture h y d r o l y s e s on the s u r f a c e t o a brown powder and e v e n t u a l l y t o a blue p a s t e . The product obtained on f l u o r i n a t i o n of vanadium metal i s a s l i g h t l y darker green, and h y d r o l y s e s i n much the same manner as the powder form but w i t h somewhat reduced r a t e presumably because of i t s g r e a t e r compactness. The brown c o l o u r of Ruff and L i c k f e t t ' s (12) p r e p a r a t i o n was probably due to s l i g h t hydro-l y s i s d u r i n g the p r e p a r a t i o n . Vanadium t e t r a f l u o r i d e d i s s o l v e d v i g o r o u s l y i n water forming a b r i g h t blue s o l u t i o n which i s c h a r a c t e r i s t i c of the +2 VO i o n . Vanadium t e t r a f l u o r i d e was completely i n s o l u b l e i n a v a r i e t y of organic and i n o r g a n i c s o l v e n t s and Lewis base s o l v e n t s r e a c t e d w i t h vanadium t e t r a f l u o r i d e to form dark, gummy r e s i d u e s . Vanadium t e t r a f l u o r i d e i s t h e r m a l l y u n s t a b l e . At 100-120° i n vacuum, the d i s p r o p o r t i o n a t i o n i n t o vanadium t r i f l u o r i d e and p e n t a f l u o r i d e , f i r s t r e p o r t e d by Ruff and L i c k f e t t (12), proceeded q u i t e r a p i d l y . The r a t e of d i s -p r o p o r t i o n a t i o n i n c r e a s e s w i t h temperature, however a h i g h temperature (eg. R u f f and L i c k f e t t ' s r e p o r t (12) of 325° as the d i s p r o p o r t i o n a t i o n temperature) i s not necessary to induce d i s p r o p o r t i o n a t i o n . Samples of vanadium t e t r a f l u o r i d e s t o r e d f o u r to s i x days at room temperature, s e a l e d i n dry g l a s s tubes, at atmospheric p r e s s u r e , contained about 5$ vanadium t r i f l u o r i d e p l u s a gas, which i s presumably s i l i c o n t e t r a -f l u o r i d e a r i s i n g from the r e a c t i o n of vanadium p e n t a f l u o r i d e w i t h the g l a s s . At -jQ°, vanadium t e t r a f l u o r i d e could be s t o r e d at l e a s t two weeks without e x c e s s i v e decomposition to the t r i - and p e n t a f l u o r i d e s . The d i s p r o p o r t i o n a t i o n of vanadium t e t r a f l u o r i d e i n t o the t r i - and p e n t a f l u o r i d e s i s a p p a r e n t l y i r r e v e r s i b l e and i s analogous to the d i s p r o p o r t i o n a t i o n of vanadium t r i c h l o r i d e i n t o the d i - and t e t r a c h l o r i d e s . Vanadium t e t r a f l u o r i d e i s one of the few simple f l u o r i d e s which spontaneously d i s p r o p o r -42 t i o n a t e s i n t o h i g h e r and lower f l u o r i d e s , although many higher f l u o r i d e s are known to l o s e f l u o r i n e under the I n f l u e n c e of heat or l i g h t to achieve a lower o x i d a t i o n s t a t e (23). The thermochemistry of t h i s d i s p r o p o r t i o n a t i o n w i l l be d i s c u s s e d i n Chapter J. The spontaneous d i s p r o p o r t i o n a t i o n of the t e t r a f l u o r i d e means t h a t a l l work must be done w i t h f r e s h l y prepared t e t r a f l u o r i d e as there i s no method a v a i l a b l e f o r s e p a r a t i n g the t r i - and t e t r a f l u o r i d e s . C o i n c i d e n t w i t h the d i s p r o p o r t i o n a t i o n , vanadium t e t r a f l u o r i d e was found t o sublime at 100-120° i n vacuum, d e p o s i t i n g , i n the c o l d p o r t i o n of the p y r o l y s i s tube, a dark green s o l i d resembling the product obtained on f l u o r i n -a t i o n of vanadium metal at 200°. The v o l a t i l i t y of vanadium t e t r a f l u o r i d e i s then between t h a t of vanadium t r i f l u o r i d e , which does not sublime up to r e d heat (51) and vanadium p e n t a f l u o r i d e , which sublimes q u i t e r e a d i l y at room tempera-t u r e s and b o i l s at 48° (7). Vanadium t e t r a f l u o r i d e resembles t i t a n i u m and chromium t e t r a f l u o r i d e s which sublime i n vacuum between 100° and 250° (51,52) but d i f f e r s from the t e t r a -f l u o r i d e s of z i r c o n i u m and hafnium, which sublime only a t r e d heat (51). Two d e t e r m i n a t i o n s of the d e n s i t y of vanadium t e t r a -f l u o r i d e under carbon t e t r a c h l o r i d e gave an average value of 3.15+0.15 g./cc. which i s s l i g h t l y h igher than Ruff and L i c k f e t t ' s v alue of 2.975 g./cc.(12), however t h e i r value was determined under toluene, which appears to r e a c t w i t h vanadium 43 t e t r a f l u o r i d e . A d e t e r m i n a t i o n of the d e n s i t y of the present sample of vanadium t e t r a f l u o r i d e under toluene y i e l d e d a value of 2.2 g./cc. and the s o l i d changed from a b r i g h t green to a dark brown c o l o u r when i n contact w i t h the t o l u e n e . Vanadium t e t r a f l u o r i d e appears to have a c r y s t a l s t r u c t u r e u n l i k e t h a t of any of the known t e t r a f l u o r i d e s t r u c t u r e s . The best agreement w i t h the powder photograph was obtained w i t h a hexagonal u n i t c e l l of dimensions a_=5.37> _c=5»l6 A., c o n t a i n i n g two formula u n i t s . The X-ray d e n s i t y of 3.28 g./cc. agrees s a t i s f a c t o r i l y w i t h the average pyknometric d e n s i t y of 3.15 g./cc. U n f o r t u n a t e l y a complete s t r u c t u r a l assignment i s not p o s s i b l e because of the u n i f o r m l y poor q u a l i t y of the d i f f r a c t i o n diagrams. Powder photographs of vanadium t e t r a f l u o r i d e from many sources, f o r example pre-p a r a t i o n s from (a) vanadium t e t r a c h l o r i d e and hydrogen f l u o r i d e , (b) vanadium metal and f l u o r i n e , both the s o l i d i s o l a t e d from low temperature f l u o r i n a t i o n and r e s i d u e s remaining on removal of vanadium p e n t a f l u o r i d e from the v o l a t i l e products of the h i g h temperature f l u o r i n a t i o n , and (c) sublimed vanadium t e t r a f l u o r i d e , s h o w e d e x a c t l y the same p a t t e r n . The powder p a t t e r n showed a number of d i f f u s e l i n e s amongst the sharper l i n e s suggesting t h a t these l i n e s should be s p l i t . Poor r e s o l u t i o n of the l i n e s above Bragg angles of 45° was c h a r a c t e r i s t i c of a l l photographs and may be due to poor c r y s t a l l i n i t y . I t i s however unusual t h a t poor c r y s t a l -U n i t y should p e r s i s t throughout a l l the methods of sample 44 p r e p a r a t i o n i n v o l v i n g w i d e l y d i f f e r e n t temperatures. The symmetry of the u n i t c e l l i n d i c a t e s t h a t vanadium t e t r a f l u o r i d e i s not i s o s t r u c t u r a l w i t h any known t e t r a -f l u o r i d e . A l a r g e group of t e t r a f l u o r i d e s , p a r t i c u l a r l y z i r c o n i u m and hafnium, and the a c t i n i d e and l a n t h a n i d e t e t r a -f l u o r i d e s c r y s t a l l i s e w i t h a m o n o c l i n i c u n i t c e l l , w i t h twelve formula u n i t s per u n i t c e l l (53). The c o o r d i n a t i o n of the metal atom i s not known hut i t has been suggested t h a t i t i s g r e a t e r than s i x (54). The molecular t e t r a f l u o r i d e s , such as s i l i c o n and germanium t e t r a f l u o r i d e s c r y s t a l l i s e w i t h a cubic u n i t c e l l i n which the t e t r a h e d r a l molecular u n i t s are pre-served. Vanadium t e t r a f l u o r i d e i s not i s o s t r u c t u r a l w i t h p l a t i n u m t e t r a f l u o r i d e , which has a d i s t o r t e d uranium t e t r a -c h l o r i d e s t r u c t u r e (55). I f one uses the v o l a t i l i t y of the t e t r a f l u o r i d e s as an i n d i c a t i o n of the degree of molecular a s s o c i a t i o n , i t i s evident t h a t there are three p r i n c i p a l c l a s s e s of v o l a t i l i t y , each of which can be c o r r e l a t e d w i t h a c e r t a i n extent of mole-c u l a r a s s o c i a t i o n . (a) H i g h l y v o l a t i l e - The molecular t e t r a f l u o r i d e s such as s i l i c o n t e t r a f l u o r i d e and germanium t e t r a f l u o r i d e , i n which the t e t r a h e d r a l molecular u n i t i s preserved, are h i g h l y v o l a t i l e i n d i c a t i n g t h a t the i n t e r m o l e c u l a r a s s o c i a t i o n i s very weak. (b) I n v o l a t i l e - The m o n o c l i n i c t e t r a f l u o r i d e s of the z i r -conium t e t r a f l u o r i d e type are h i g h l y i n v o l a t i l e , s u b l i m i n g 45 o n l y at red heat, suggesting a h i g h degree of molecular a s s o c i a t i o n (51). A s i m i l a r s o r t of i n v o l a t i l i t y i s found i n vanadium t r i f l u o r i d e which has been shown to have a h i g h l y a s s o c i a t e d s t r u c t u r e (22). (c) Moderate v o l a t i l i t y - To t h i s i n t e r m e d i a t e group belong t i t a n i u m t e t r a f l u o r i d e (5l)> chromium t e t r a f l u o r i d e (52), and vanadium t e t r a f l u o r i d e ; a l l of which sublime i n the range 100-250° and p r o b a b l y possess an i n t e r m e d i a t e degree of i n t e r m o l e c u l a r a s s o c i a t i o n . Thus the c o o r d i n a t i o n number of the c e n t r a l metal atom i s probably between f o u r (as found f o r S i F ^ e t c . ) and e i g h t or twelve which i s suggested f o r the z i r c o n i u m t e t r a f l u o r i d e type (5^ -). T h i s suggests a p o s s i b l e c o o r d i n a t i o n number of s i x i n the moderately v o l a t i l e t e t r a -f l u o r i d e s . A r e c e n t study of the heat c a p a c i t y of t i t a n i u m t e t r a -f l u o r i d e (54) shows t h a t t i t a n i u m t e t r a f l u o r i d e has a h i g h e r heat c a p a c i t y than zirconium, hafnium or uranium t e t r a f l u o r i d e s (54). W i t h i n an i s o s t r u c t u r a l s e r i e s , the heat c a p a c i t y i s expected to i n c r e a s e w i t h the molecular weight, and t h i s expected t r e n d i s observed f o r the i s o s t r u c t u r a l s e r i e s formed by zirconium, hafnium and uranium t e t r a f l u o r i d e s , suggesting t h a t t i t a n i u m t e t r a f l u o r i d e does not belong i n t h i s s e r i e s (54). The anomalous behaviour of t i t a n i u m t e t r a f l u o r i d e was a t t r i b u t e d to the formation of a s t r u c t u r a l type i n t e r m e d i a t e between t h a t of the n o n - a s s o c i a t e d s i l i c o n t e t r a f l u o r i d e and 46 the h i g h l y a s s o c i a t e d z i r c o n i u m t e t r a f l u o r i d e , and the s t r u c t u r e proposed by Huckel (51) was favoured (54). Huckel (51) proposed t h a t t i t a n i u m t e t r a f l u o r i d e c r y s t a l l i s e s w i t h an i n f i n i t e chain type s t r u c t u r e of octa-h e d r a l T i F g u n i t s , j o i n e d at f o u r corners to other o c t a h e d r a l u n i t s by the s h a r i n g of a f l u o r i n e atom. Thus f o u r of every s i x f l u o r i n e s i n the T i F g octahedra are shared between two t i t a n i u m atoms, and two would be h e l d by only one t i t a n i u m atom, l e a d i n g to a d i s t o r t e d o c t a h e d r a l u n i t about the t i t a n i u m atom. The s i m i l a r v o l a t i l i t i e s of t i t a n i u m t e t r a f l u o r i d e and vanadium t e t r a f l u o r i d e , and the i n d i c a t i o n t h a t both are not members of the z i r c o n i u m t e t r a f l u o r i d e s t r u c t u r a l type, suggests t h a t the s t r u c t u r e o u t l i n e d above f o r t i t a n i u m t e t r a f l u o r i d e i s e q u a l l y a p p l i c a b l e to vanadium t e t r a f l u o r i d e . Thus vanadium t e t r a f l u o r i d e would be c o n s t r u c t e d from polymeric chains of VFg octahedra so t h a t each vanadium would achieve i t s maximum c o o r d i n a t i o n of s i x . Vanadium t r i f l u o r i d e has been shown to c o n t a i n condensed VFg o c t a h e d r a l u n i t s (22) w i t h each f l u o r i n e atom shared by two vanadium atoms. I t has a l s o been suggested t h a t vanadium p e n t a f l u o r i d e condenses as a polymer or dimer c o n t a i n i n g hexacoordinate VFg u n i t s , each s h a r i n g two f l u o r i n e s w i t h adjacent octahedra (19,56). Thus the r e g u l a r i n c r e a s e In v o l a t i l i t y of the vanadium f l u o r i d e s from the t r i f l u o r i d e t o the p e n t a f l u o r i d e appears to f o l l o w the probable order of d e c r e a s i n g molecular condensation. The volume of the u n i t c e l l of a heavy metal f l u o r i d e can be approximated t o the volume of the f l u o r i d e i o n s , s i n c e the metal atoms are r e l a t i v e l y s m a l l and occupy the i n t e r s t i t i a l h oles between the f l u o r i d e i ons ( 5 7 ) . In the uranium and lanthanum f l u o r i d e s the volume occupied by each f l u o r i d e i o n v a r i e s only s l i g h t l y about a mean value of l8A Jj eg. from 17A3 i n ^  -UF 5 and UgF^ to 19A3 i n UF^ and UFg ( 5 7 ) . However t h i s volume i s not constant i n a l l f l u o r i d e s as i t depends on the s t r u c t u r a l type and the extent to which the approximation of a n e g l i g i b l e volume f o r the metal ion a p p l i e s . Since the i o n i c s i z e s of vanadium ions are c o n s i d e r a b l y l e s s than the s i z e s of uranium and lan t h a n i d e ions of the same valence ( 5 8 ) , the approximation of the volume of the u n i t c e l l to the volume of the f l u o r i d e ions i s more j u s t i f i a b l e . In vanadium t r i f l u o r i d e , the b i m o l e c u l a r rhombohedral u n i t c e l l of dimensions ._a=5.73 A, o<= 5 7 . 5 2 ° (22) has a volume of 3 3 7 3 . 5 A , or a volume of 12.2A^ per f l u o r i d e i o n . T h i s i s not much l a r g e r than the volume of 11A^ which i s occupied by a sphere w i t h a r a d i u s of I.36A, the accepted r a d i u s of the f l u o r i d e i o n ( l l ) , i n d i c a t i n g t h a t the packing i n vanadium t r i f l u o r i d e i s very e f f i c i e n t . In vanadium t e t r a f l u o r i d e , the b i m o l e c u l a r hexagonal c e l l , suggested by powder measurements, 3 3 has a volume of 129A , or a volume of l6A J per f l u o r i d e i o n . T h i s i n c r e a s e i n the volume occupied per f l u o r i d e i o n i s not unexpected because the decrease i n the extent of i n t e r m o l e c u l a r a s s o c i a t i o n , which would be necessary t o preserve s i x 48 c o o r d i n a t i o n about the vanadium, would l i k e l y l e a d t o a decrease i n the compactness of the s t r u c t u r e . ( i i ) Chemical P r o p e r t i e s of Vanadium T e t r a f l u o r i d e Vanadium t e t r a f l u o r i d e i s r e a d i l y o x i d i s e d t o vanadium p e n t a f l u o r i d e by f l u o r i n e gas a t 100°, and by l i q u i d bromine t r i f l u o r i d e . In the l a t t e r case the r e s u l t i n g vanadium penta-f l u o r i d e could not be separated from the bromine t r i f l u o r i d e , and so the s o l u t i o n was r e a c t e d w i t h potassium f l u o r i d e t o form the potassium hexaf luorovanadate * (*V) s a l t . The vigo r o u s e v o l u t i o n of bromine d u r i n g the d i s s o l u t i o n of vanadium t e t r a -f l u o r i d e i n bromine t r i f l u o r i d e was f u r t h e r evidence f o r oxid-a t i o n of vanadium which probably proceeds a c c o r d i n g t o the equation: 2VP^ + B r F 3 — 3 V F 5 + \ B r 2 Iodine p e n t a f l u o r i d e , however, i s not a s u f f i c i e n t l y s t r o n g f l u o r i n a t i n g agent t o o x i d i s e vanadium t e t r a f l u o r i d e to vanadium p e n t a f l u o r i d e . Vanadium t e t r a f l u o r i d e d i d d i s s o l v e s l i g h t l y i n i o d i n e p e n t a f l u o r i d e forming a red-brown s o l u t i o n , but when the more v o l a t i l e i o d i n e p e n t a f l u o r i d e was removed, unchanged vanadium t e t r a f l u o r i d e remained. L i q u i d n i t r y l f l u o r i d e d i d not o x i d i s e vanadium t e t r a -f l u o r i d e even when they were kept i n contact f o r two months at - 7 8 ° . No evidence f o r formation of a n i t r o n i u m s a l t , such as (N0 2) 2VFg, was observed, which may have been due to the i n s o l u b i l i t y of vanadium t e t r a f l u o r i d e i n n i t r y l f l u o r i d e . A c c o r d i n g l y an attempt was made to prepare the n i t r o n i u m ! s a l t , (NOg) 2^ "P5^  u s i n g i o d i n e pentaf l u o r i d e as a s o l v e n t , however when n i t r y l f l u o r i d e was mixed w i t h an i o d i n e p e n t a f l u o r i d e s o l u t i o n of vanadium t e t r a f l u o r i d e a gas, presumably NO^, was evolved and the n i t r o n i u m s a l t of vanadium p e n t a f l u o r i d e (59) > NCv-jVFg, was obtained. There i s no evidence t o suggest t h a t i o d i n e p e n t a f l u o r i d e was the o x i d i s i n g agent or that the t e t r a f l u o r i d e was merely d i s p r o p o r t i o n a t i n g t o the t r i - and p e n t a f l u o r i d e s , t h e r e f o r e n i t r y l f l u o r i d e must be regarded as the o x i d i s i n g agent. Thus, i n the presence of i o d i n e penta-f l u o r i d e , n i t r y l f l u o r i d e o x i d i s e d vanadium t e t r a f l u o r i d e a c c o r d i n g t o the equation: NOgF + VF^ — v VF^ + N0 2 which was then f o l l o w e d by s a l t formation between excess n i t r y l f l u o r i d e and vanadium p e n t a f l u o r i d e : NOgF + VF^ (N0 2)VF 6. Since the r e a c t i o n occurs only i n the presence of the s o l v e n t ( i o d i n e p e n t a f l u o r i d e ) the mechanism may i n v o l v e the formation of i o n i c s p e c i e s . A comparison of the r e l a t i v e f l u o r i n a t i n g powers of vanadium p e n t a f l u o r i d e and vanadium t e t r a f l u o r i d e r e s u l t s from a c o n s i d e r a t i o n of the behaviour of these f l u o r i d e s w i t h sulphur d i o x i d e and sulphur t r i o x i d e . Vanadium p e n t a f l u o r i d e r e a d i l y f l u o r i n a t e s both sulphur d i o x i d e and sulphur t r i o x i d e (8) forming t h i o n y l f l u o r i d e from the former and p y r o s u l p h u r y l f l u o r i d e from the l a t t e r , a c c o r d i n g to the equations: 50 VP^ + S0 2 — — V 0F 3 + S0F 2 V F C + 2 SO, — V 0 F o + S o 0 c F o 5 J O 2 5 Vanadium t e t r a f l u o r i d e does not r e a c t w i t h e i t h e r S 0 2 or SO^ i l l u s t r a t i n g t h a t i t i s not an e f f e c t i v e f l u o r i n a t m g agent f o r sulphur oxides. I t has been suggested t h a t the f l u o r i n a t i o n of sulphur d i o x i d e and t r i o x i d e w i t h vanadium p e n t a f l u o r i d e i n v o l v e s the proposed a u t o i o n i s a t i o n of VF^ ( 8 ) , i n t h a t the r e a c t i o n proceeds through the formation of i n t e r -mediate i o n i c complexes, such as S 0 ( V F g ) 2 and (VF^gSO^ i n the case of sulphur d i o x i d e , and the f l u o r o s u l p h o n a t e , VF^ (SO^F), i n the case of sulphur t r i o x i d e . Vanadium t e t r a f l u o r i d e does not r e a c t because i t does not d i s s o l v e m sulphur d i o x i d e or sulphur t r i o x i d e to form the r e q u i s i t e complex intermediates. Of course, h e a t i n g vanadium t e t r a f l u o r i d e i n contact w i t h sulphur d i o x i d e or sulphur t r i o x i d e would r e s u l t i n the e v o l u t i o n of vanadium p e n t a f l u o r i d e which would then r e a c t a c c o r d i n g to the above r e a c t i o n s . Vanadium t e t r a f l u o r i d e c o u l d thus f u n c t i o n as a f l u o r m a t i n g agent i n t h a t i t can be induced to supply the p e n t a f l u o r i d e , the r a t e c o n t r o l l i n g f a c t o r being the r a t e of e v o l u t i o n of vanadium p e n t a f l u o r i d e . Vanadium t e t r a f l u o r i d e r e a d i l y forms 1:1 complexes w i t h ammonia and p y r i d i n e . Amminotetrafluorovanadium (IV) and p y r i d i n e t e t r a f l u o r o v a n a d i u m (TV) were p r e v i o u s l y obtained (6o) from the r e a c t i o n of vanadium p e n t a f l u o r i d e w i t h ammonia and p y r i d i n e r e s p e c t i v e l y . These complexes, which are p r o b a b l y e l e c t r o n donor-acceptor complexes, resemble the 1:1 complexes of metal t e t r a f l u o r i d . e s , such as t i t a n i u m , zirconium, s i l i c o n and molybdenum t e t r a f l u o r i d e s , w i t h p y r i d i n e or t r i m e t h y l amine, r e p o r t e d by M u e t t e r t i e s (6l). The 1:1 complexes obtained by M u e t t e r t i e s were s o l i d s , without sharp m e l t i n g p o i n t s . They were i n s o l u b l e i n p o l a r non-protonic s o l v e n t s and were r e a d i l y decomposed by p r o t o n i c s o l v e n t s such as water. These p r o p e r t i e s suggested t h a t the 1:1 t e t r a f l u o r i d e - b a s e complexes were polymeric r a t h e r than monomeric pent a c o o r d i n a t e s p e c i e s and i t was suggested (6l) that p o l y m e r i s a t i o n occurred through the formation of f l u o r i n e bridge bonds. The 1:1 ammonia or p y r i d i n e complexes w i t h vanadium t e t r a f l u o r i d e are a l s o non-melting, i n s o l u b l e s o l i d s which are decomposed by water (60), suggesting t h a t these complexes are f l u o r i n e b r i d g e d polymers, s i m i l a r t o those obtained by M u e t t e r t i e s (6l). In these f l u o r i n e b r i d g e d polymers, vanadium i s at l e a s t s i x c o o r d i n a t e . I t has been p r e v i o u s l y suggested (56) t h a t r e d u c t i o n of vanadium p e n t a f l u o r i d e by ammonia and p y r i d i n e to form 1:1 t e t r a f l u o r i d e - b a s e adducts occurs because of the i n s t a b i -l i t y of c o o r d i n a t i o n numbers g r e a t e r than s i x f o r vanadium, but t h i s does not e x p l a i n why the r e a c t i o n between vanadium t e t r a f l u o r i d e and the n i t r o g e n bases forms only the 1:1 t e t r a -f l u o r i d e - b a s e adducts when many of the t e t r a f l u o r i d e s s t u d i e d by M u e t t e r t i e s (6l) formed 1:2 t e t r a f l u o r i d e - b a s e complexes as w e l l as the 1:1 complexes which are s i m i l a r to those formed by vanadium t e t r a f l u o r i d e . Comparisons of the behaviour of the vanadium f l u o r i d e s w i t h the behaviour of the t e t r a f l u o r i d e s s t u d i e d by M u e t t e r t i e s (6l) are not p a r t i c u l a r l y v a l i d because M u e t t e r t i e s g e n e r a l l y prepared the adducts from s o l u t i o n s of the t e t r a f l u o r i d e , whereas vanadium p e n t a f l u o r i d e and t e t r a -f l u o r i d e were r e a c t e d d i r e c t l y w i t h the base, i n which both the f l u o r i d e s and the r e a c t i o n products were i n s o l u b l e . The r e a c t i o n of vanadium p e n t a f l u o r i d e w i t h a Lewis base probably i n v o l v e s the i n i t i a l f ormation of a VP^.Base adduct which i s u n s t a b l e and l o s e s f l u o r i d e t o form the VF^.Base product. The i n s t a b i l i t y of t h i s p e n t a v a l e n t i n t e r m e d i a t e suggests t h a t vanadium p e n t a f l u o r i d e i s the only s t a b l e h a l i d e compound of p e n t a v a l e n t vanadium. The decomposition of the i n t e r m e d i a t e adduct i s probably a i d e d by the gain i n l a t t i c e energy accompanying the t r a n s f o r m a t i o n of a penta-v a l e n t , hexacoordinate complex of the type VP^.Base, i n which the maximum c o o r d i n a t i o n of vanadium has been s a t i s f i e d and the complex can only form non-associated, molecular c r y s t a l s of low l a t t i c e energy, to a VP^.Base complex i n which a s s o c i a t i o n occurs to s a t i s f y the c o o r d i n a t i o n requirements of the vanadium atom and forms a " g i a n t molecule" c r y s t a l w i t h a h i g h l a t t i c e energy. The r e a c t i o n between vanadium t e t r a f l u o r i d e and the base probably r e s u l t s i n the formation of a 1:1 base adduct as the i n i t i a l s t e p . Presuming t h a t vanadium t e t r a f l u o r i d e I t s e l f Is a f l u o r i n e b r i d g e polymer i n which vanadium i s hexa-c o o r d i n a t e , t h i s adduct now c o n t a i n s heptacoordinate vanadium. 53 To maintain h e x a c o o r d i n a t i o n r e q u i r e s only t h a t one b r i d g e bond r e v e r t to a normal m e t a l - f l u o r i n e bond, l e a v i n g the s o l i d as an a s s o c i a t e d " g i a n t molecule" c r y s t a l w i t h a h i g h l a t t i c e energy. F u r t h e r r e a c t i o n w i t h the base to form a VF^.2Base complex probably does not occur because to maintain hexa-co o r d i n a t e vanadium t h i s complex must be non-associated. The r e s u l t i n g VF^.2Base compound can only form molecular c r y s t a l s which probably have a low l a t t i c e energy, thus the r e a c t i o n : VF^.Base + Base — V F ^ . 2 B a s e i n v o l v e s a c o n s i d e r a b l e l o s s i n l a t t i c e energy of the s o l i d complex. I f the t e t r a f l u o r i d e and i t s complexes were s o l u b l e i n the base or a s o l v e n t , the mode of r e a c t i o n would probably be q u i t e d i f f e r e n t because the l o s s of l a t t i c e energy on break-in g down the c r y s t a l i s compensated by a gain i n the s o l v a t i o n energy upon d i s s o l u t i o n . The l a t t i c e energy l o s e s i t s impor-tance i n the r e a c t i o n , and i t becomes p o s s i b l e to form adducts w i t h two or more base molecules, the product of the r e a c t i o n depending only on the maximum c o o r d i n a t i o n of the c e n t r a l atom. Since M u e t t e r t i e s formed the t e t r a f l u o r i d e - b a s e complexes i n s o l u t i o n , 1:2 complexes as w e l l as 1:1 complexes were formed ( 6 l ) . Selenium t e t r a f l u o r i d e , but not sulphur t e t r a f l u o r i d e , forms a 1:1 adduct w i t h vanadium t e t r a f l u o r i d e , SeF^.VF^. T h i s adduct may be e i t h e r a 1:1 donor-acceptor adduct analogous to the n i t r o g e n base adducts above; an i o n i c s a l t such as S e F ^ V F ^ - or some e q u i v a l e n t s t r u c t u r e ; or f i n a l l y an a s s o c i a t e d complex i n which selenium and vanadium are bonded v i a f l u o r i n e 54 b r i d g e s . Since selenium and sulphur t e t r a f l u o r i d e s do not show any tendency t o form donor-acceptor complexes (4), the 1:1 complex of selenium t e t r a f l u o r i d e w i t h vanadium t e t r a -f l u o r i d e i s probably not a donor-acceptor complex s i m i l a r t o the VF^.Base complexes w i t h the n i t r o g e n bases. Recent work on the sulphur and selenium t e t r a f l u o r i d e complexes w i t h boron t r i f l u o r i d e , antimony p e n t a f l u o r i d e and a r s e n i c p e n t a f l u o r i d e (4) suggests t h a t these complexes c o n t a i n the SF^ 4" or SeF^"*" i o n s . Thus i t seems reasonable t o formulate the VF^.SeF^ complex as SeF-^VF^" . I f t h i s i s so i t Is d i f f i -c u l t t o understand why the VF^.2SeF^ complex, which can be + — 2 formulated as (SeF^ )2^ff ' ^s n o ^ £°rmed, p a r t i c u l a r l y when complex s a l t s such as KgVFg (15,16,17) are w e l l known whereas VFj- - s a l t s have not been r e p o r t e d . T r i v a l e n t vanadium r e a d i l y forms complex s a l t s such as _2 KgVF,-, which a p p a r e n t l y c o n t a i n the VF^ i o n . Therefore i t was p o s s i b l e t h a t s i m i l a r complex s a l t s of t e t r a v a l e n t vanadium (eg. KVFj-) could be prepared which would c o n f i r m the e x i s t e n c e of the VFp~ i o n proposed i n the S e F ^ V F j - - complex. Combination of s t o i c h i o m e t r i c amounts of potassium f l u o r i d e and vanadium t e t r a f l u o r i d e i n i o d i n e p e n t a f l u o r i d e however, r e s u l t e d only i n the formation of K^VF^ l e a v i n g c o n s i d e r a b l e unreacted vanadium t e t r a f l u o r i d e . T h i s i n d i c a t e s t h a t the YF~ i o n i s 5 not too r e a d i l y formed, but i t s formation w i t h selenium t e t r a f l u o r i d e may a r i s e from the s t a b i l i z i n g e f f e c t of the l a r g e SeF^"1" c a t i o n . The i n a b i l i t y of sulphur t e t r a f l u o r i d e to complex wi t h vanadium t e t r a f l u o r i d e can he a t t r i b u t e d t o the l e s s e r s t a b i l i t y of the SF^"1" c a t i o n (4). I t has been suggested (23) t h a t the pentafluorovanadate i o n s of t r i v a l e n t vanadium (VF^~ ) are d i m e r i c , a c h i e v i n g -4 hexacoordinate vanadium through the formation of vV^io f l u o r i n e - b r i d g e d s p e c i e s . S i m i l a r l y the i o n i c s t r u c t u r e of the VF^.SeF^ complex may be based on d i m e r i c anions of the type: ^2^10 ' a s s o c i a t e d through the formation of f l u o r i n e b r i d g e bonds. The SeF^"1" ions are not a s s o c i a t e d and a c t as simple c a t i o n s i n the s t r u c t u r e . C e r t a i n l y a more s a t i s f y i n g p i c t u r e of the s t r u c t u r e of the SeF^.VF^ complex r e s u l t s i f f l u o r i n e bridge bonding i s r e s t r i c t e d to the complex vanadium f l u o r i d e anion r a t h e r than attempting t o propose polymeric s t r u c t u r e s i n v o l v i n g f l u o r i n e b r i d g e s between selenium and vanadium, which maintain h e x a c o o r d i n a t i o n about both the vanadium and selenium atoms. The a v a i l a b l e evidence, however, does not permit the assignment of a d e f i n i t e s t r u c t u r e to the complex. ( i i i ) The P r e p a r a t i o n and P r o p e r t i e s of Hexafluorovanadate (IV)  S a l t s The apparent s o l u b i l i t y of vanadium t e t r a f l u o r i d e i n i o d i n e p e n t a f l u o r i d e without complex formation or r e a c t i o n suggested t h a t complex s a l t s of the a l k a l i and a l k a l i n e e a r t h _2 metals, c o n t a i n i n g the VFg i o n , could be prepared i n t h i s s o l v e n t from vanadium t e t r a f l u o r i d e and the r e q u i s i t e a l k a l i or a l k a l i n e e a r t h f l u o r i d e . Although there i s no o x i d a t i o n of 56'' vanadium t e t r a f l u o r i d e by i o d i n e p e n t a f l u o r i d e i t s e l f , u n f o r t u n a t e l y i n the presence of a l k a l i metal f l u o r i d e s e x t e n s i v e o x i d a t i o n of the vanadium occurred, as i n d i c a t e d by f the presence of c o n s i d e r a b l e amounts of i o d i n e i n the p r o d u c t s . The s a l t s were mixtures of M 2VFg, MVFg and unreacted a l k a l i metal f l u o r i d e . Calcium and barium f l u o r i d e s d i d not form complex f l u o r i d e s , nor d i d they cause o x i d a t i o n of the vanadium t e t r a f l u o r i d e , presumably because of t h e i r i n s o l u -b i l i t y i n i o d i n e p e n t a f l u o r i d e . Attempts to prepare t e t r a v a l e n t hexafluorovanadate s a l t s of potassium and caesium from vanadium t e t r a f l u o r i d e i n selenium t e t r a f l u o r i d e were s u c c e s s f u l only i n the case of potassium where the product was pure potassium h e x a f l u o r o -vanadate ( I V ) . The t r i g o n a l form of K^VFg w i t h the l a t t i c e c onstants a = 5.68A, c_ = 4.66A, was obtained and corresponds to the low temperature form obtained by L i e b e , Weise and Klemm (17) from the f l u o r i n a t i o n of t r i v a l e n t K^VF,-. The pyknometric d e n s i t y of 2.56 g./cc. does not agree too w e l l w i t h the c a l -c u l a t e d d e n s i t y of 3.09 g./cc. nor w i t h the value of 2.99 g./cc. obtained by Liebe eJb.^aJL. (l7)> however only one d e n s i t y measurement was made and e r r o r s due to s l i g h t h y d r o l y s i s of the sample may have been c o n s i d e r a b l e . The magnetic p r o p e r t i e s of potassium hexafluorovanadate (IV) are d i s c u s s e d i n Chapter 5* Caesium f l u o r i d e r e a c t e d w i t h vanadium t e t r a f l u o r i d e i n selenium t e t r a f l u o r i d e to form an inhomogeneous product c o n t a i n i n g some hexagonal CSgVFg ( p r e v i o u s l y obtained by L i e b e 57 e t . a l . (17)) a l o n g w i t h i m p u r i t i e s , p r i n c i p a l l y t r i v a l e n t vanadium complexes as i n d i c a t e d by the magnetic moment of 2.5 Bohr magnetons at 295°K. I t i s not known why the a d d i t i o n of an a l k a l i metal f l u o r i d e t o a s o l u t i o n of vanadium t e t r a f l u o r i d e i n i o d i n e p e n t a f l u o r i d e or selenium t e t r a f l u o r i d e r e s u l t s i n v a r y i n g degrees of o x i d a t i o n or r e d u c t i o n , p a r t i c u l a r l y i n view of the many s u c c e s s f u l p r e p a r a t i o n s of s a l t s of lower f l u o r i d e s i n these s o l v e n t s (23,62). However a c o n s i d e r a t i o n of the r e a c t i o n products i n d i c a t e s t h a t s e v e r a l f a c t o r s are respon-s i b l e f o r t h i s behaviour. The a l k a l i metal f l u o r i d e s are bases i n the i o d i n e p e n t a f l u o r i d e and selenium t e t r a f l u o r i d e s o l v e n t systems, t h a t i s d i s s o l u t i o n of an a l k a l i metal f l u o r i d e i n c r e a s e s the c o n c e n t r a t i o n of the b a s i c anions IFg and SeF,- r e s p e c t i v e l y . The s o l u b i l i t y of a l k a l i metal f l u o r i d e s i n f l u o r i d e s o l v e n t s w i l l be p a r t i a l l y dependent on the f l u o r i d e acceptor p r o p e r t i e s of the n e u t r a l s o l v e n t molecule; the stronger f l u o r i d e a c c e p t o r s y i e l d i n g the more concentrated a l k a l i metal f l u o r i d e s o l u t i o n . Iodine p e n t a f l u o r i d e shows a st r o n g e r tendency t o form compounds c o n t a i n i n g i t s b a s i c i on ( l F g ~ ) than selenium t e t r a f l u o r i d e , which appears t o p r e f e r to form i t s a c i d i c i o n SeF^"1" (4) thus i o d i n e p e n t a f l u o r i d e can be regarded as the stronger f l u o r i d e a c c e p t o r . A l k a l i metal f l u o r i d e s w i l l thus tend t o be more s o l u b l e m i o d i n e p e n t a f l u o r i d e than i n selenium t e t r a f l u o r i d e . 58 S o l u b i l i t y i s a l s o governed by the l a t t i c e e n e r g i e s of the s o l i d s and s o l v a t i o n e n e r g i e s of the i o n s . For d i s s o l u t i o n t o be a f e a s i b l e process the l a t t i c e energy r e q u i r e d to d i s s o c i a t e the s o l i d i n t o the c o n s t i t u e n t ions must be balanced by the gain i n energy on s o l v a t i o n of the i o n s . In g e n e r a l l a t t i c e e n e r g i e s and s o l v a t i o n e n e r g i e s i n c r e a s e w i t h d e c r e a s i n g i o n i c s i z e and i n c r e a s i n g i o n i c charge, although l a t t i c e e n e r g i e s g e n e r a l l y i n c r e a s e more r a p i d l y than s o l v a t i o n e n e r g i e s (33). For example the Increased s o l v a t i o n e n e r g i e s of d i v a l e n t a l k a l i n e e a r t h i o n s , r e l a t i v e to the a l k a l i metal i o n s , i s not s u f f i c i e n t t o compensate the three f o l d i n c r e a s e i n l a t t i c e e n e r g i e s of a l k a l i n e e a r t h f l u o r i d e s r e l a t i v e t o a l k a l i metal f l u o r i d e s (32), and so the a l k a l i n e e a r t h f l u o r i d e s are i n s o l u b l e i n the f l u o r i d e s o l v e n t s . Thus complex f l u o r i d e s a l t s of a l k a l i n e e a rths can not be prepared i n f l u o r i d e s o l v e n t s . The v a r y i n g extent of o x i d a t i o n and r e d u c t i o n observed i n the p r e p a r a t i o n of a l k a l i metal he xafluorovanadate s a l t s i n f l u o r i d e s o l v e n t s Is probably connected w i t h the d i f f e r e n c e i n s o l u b i l i t y of the r e a c t a n t s and p r o d u c t s . The d e c r e a s i n g l a t t i c e e n e r g i e s from potassium to caesium (32) and the slower decrease i n s o l v a t i o n energy from potassium to caesium (33), both of which are due to the i n c r e a s i n g i o n i c s i z e of the metal, r e s u l t s i n the s o l u b i l i t y of an a l k a l i metal f l u o r i d e i n c r e a s -i n g from potassium to caesium f l u o r i d e . A s i m i l a r i n c r e a s e i n s o l u b i l i t y from potassium to caesium w i l l probably occur f o r the M^VFg s a l t s as w e l l , thus the c o n c e n t r a t i o n of hexa-fluorovanadate (IV) i o n i n the f l u o r i d e s o l u t i o n w i l l probably i n c r e a s e from potassium to caesium. A l s o as selenium t e t r a -f l u o r i d e i s a poorer s o l v e n t f o r a given metal f l u o r i d e than i o d i n e p e n t a f l u o r i d e , the s o l u b i l i t y of a given metal f l u o r i d e or fluorovanadate s a l t w i l l l i k e l y i n c r e a s e on going from the selenium t e t r a f l u o r i d e to t h e ' i o d i n e p e n t a f l u o r i d e s o l v e n t system. During the p r e p a r a t i o n of the s a l t s i t was observed t h a t c o n s i d e r a b l e s o l u t i o n of the reagents oc c u r r e d i n a l l systems except w i t h potassium f l u o r i d e and vanadium t e t r a -f l u o r i d e i n selenium t e t r a f l u o r i d e . The above trends p r e d i c t t h a t a minimum s o l u b i l i t y w i l l be found w i t h potassium s a l t s i n selenium t e t r a f l u o r i d e , as observed. The appearance of o x i d a t i o n - r e d u c t i o n r e a c t i o n s appears to be a f u n c t i o n of the c o n c e n t r a t i o n of hexafluorovanadate i o n i n s o l u t i o n . T h i s i s a l s o i n agreement w i t h the a p p a r e n t l y g r e a t e r degree of o x i -d a t i o n i n caesium s a l t s r e l a t i v e t o the potassium s a l t s i n any one s o l v e n t . The extent of the o x i d a t i o n - r e d u c t i o n r e a c t i o n probably depends, i n a d d i t i o n to the c o n c e n t r a t i o n e f f e c t s o u t l i n e d above, upon the r e l a t i v e ease of o x i d a t i o n or r e d u c t i o n of the s o l v e n t s . The appearance of o x i d i s e d s a l t s and i o d i n e i n the i o d i n e p e n t a f l u o r i d e r e a c t i o n s and reduced s a l t s and probably selenium h e x a f l u o r i d e m the selenium t e t r a f l u o r i d e r e a c t i o n s suggests t h a t i o d i n e p e n t a f l u o r i d e i s p r e f e r a b l y reduced to i o d i n e whereas selenium t e t r a f l u o r i d e i s p r e f e r a b l y o x i d i s e d to selenium h e x a f l u o r i d e by t e t r a v a l e n t vanadium. That i s i o d i n e p e n t a f l u o r i d e tends to a c t as an o x i d i s i n g s o l v e n t 6o whereas selenium t e t r a f l u o r i d e tends to a c t as a r e d u c i n g s o l v e n t . T h i s e f f e c t w i l l i n f l u e n c e the r e l a t i v e o x i d a t i o n p o t e n t i a l s of the v a r i o u s o x i d a t i o n s t a t e s of vanadium i n the f l u o r i d e s o l v e n t s . The s t a b i l i t i e s of the d i f f e r e n t v"Fg _ n ions w i l l not have a great e f f e c t on the r e a c t i o n s because the r e l a t i v e s t a b i l i t i e s of these ions are probably somewhat s i m i l a r to each o t h e r . I t must be noted t h a t these f l u o r i d e s o l v e n t s are not r e a d i l y o x i d i s e d or reduced by vigorous f l u o r i n a t i n g agents ( f o r example selenium t e t r a f l u o r i d e i s not o x i d i s e d by bromine t r i f l u o r i d e (62)) however the a c t i o n of a t r a n s i t i o n metal such as vanadium may be markedly d i f f e r e n t from t h a t of o x i d i s i n g agents such as bromine t r i f l u o r i d e because of the p o s s i b i l i t y of c a t a l y t i c e f f e c t s w i t h the t r a n s i t i o n metals. The s u c c e s s f u l p r e p a r a t i o n of K^VFg (IV) i n selenium t e t r a f l u o r i d e was t h e r e f o r e due l a r g e l y t o the i n s o l u b i l i t y of the r e a c t a n t s and products m the f l u o r i d e s o l v e n t which prevented a t t a c k on the s o l v e n t by the hexafluorovanadate (IV) product. However because of the s c a r c i t y of q u a n t i t a t i v e i n f o r m a t i o n on the behaviour of the f l u o r i d e s o l v e n t s i n the presence of a l k a l i metal and t r a n s i t i o n metal f l u o r i d e s , i t i s almost i m p o s s i b l e to p r e d i c t whether a s p e c i f i c s a l t can be s u c c e s s f u l l y prepared i n a c e r t a i n s o l v e n t . 6i CHAPTER 5: THE MAGNETIC PROPERTIES OF TETRAVALENT VANADIUM FLUORIDES The magnetic s u s c e p t i b i l i t i e s of the t e t r a v a l e n t vanadium f l u o r i d e s were found to obey the Curie-Weiss law ( 6 3 ) : y 1 _ c A M ~ T+§ * (5--1) denotes the molar s u s c e p t i b i l i t y , c o r r e c t e d f o r diamagnetic c o n t r i b u t i o n s w i t h the values given i n Selwood ( 6 4 ) . These values were chosen because they p r o v i d e a set of s e l f - c o n s i s t e n t c o r r e c t i o n s f o r the diamagnetism of a l l the ions encountered i n t h i s study. Qis the Weiss constant and T the temperature In °K. In a l l cases a very h i g h Weiss constant was observed. Magnetic moments were c a l c u l a t e d from the Curie-'Weiss law; j j i e f f = 2 .84 J % ^ T + B ) (5-2) and the Curie law; / e f f = 2'ml tw T (5"3) Both c a l c u l a t i o n s were made because of the d i s p u t e concerning > the method of c a l c u l a t i n g the e f f e c t i v e magnetic moment. Selwood ( 6 4 ) c o n s i d e r s 0 to be a s i g n i f i c a n t quan-t i t y and t h a t magnetic moments of substances obeying the Curie-Weiss law should be c a l c u l a t e d from equation (5-2), thus y i e l d i n g moments Independent of temperature. 62 Other workers (63) have questioned the v a l i d i t y of the c o r r e c t i o n f o r 0 on the grounds t h a t c o r r e c t i o n of the moment f o r d e v i a t i o n s of the s u s c e p t i b i l i t y from the Curie law i s q u i t e meaningless u n l e s s the source of the d e v i a t i o n i s known. As t h i s i s r a r e l y so, e s p e c i a l l y i n cases where t9 i s l a r g e , t h i s c o r r e c t i o n can not u s u a l l y be j u s t i f i e d . The most f r e q u e n t l y observed v a l u e s of 0 (e.g. 0° to 40°) are so small t h a t they may be i n c l u d e d or n e g l e c t e d i n the c a l c u l a t i o n of the moment w i t h l i t t l e e f f e c t . I t i s only when 6 becomes g r e a t e r than 100° t h a t the problem becomes important. C a l c u l a t i o n of the magnetic moment from the Curie law (eqn. 5-3) when the substance obeys the Curie-Weiss law, i n t r o d u c e s a temperature dependence i n t o the magnetic moment, consequently i f t h i s p r a c t i c e i s to be f o l l o w e d , jXQ^^ i s s i g n i f i c a n t only a t the temperature quoted. Since the e f f e c t i v e magnetic moment i n paramagnetic compounds can be regarded as the temperature independent c o n t r i b u t i o n t o the s u s c e p t i b i l i t y i t i s probably more j u s t i f i a b l e to c a l c u l a t e the moment from the same law as i s obeyed by the measured s u s c e p t i b i l i t y , thus f o l l o w i n g Selwood. However i f moments are c a l c u l a t e d i n t h i s way the e r r o r s c o n t r i b u t e d by 0 , which are f a i r l y l a r g e because 0 i s determined by e x t r a p o l a t i o n , must be r e c o g n i s e d and the r e s u l t i n g magnetic moments must always be q u a l i f i e d by the value of 0 used i n t h e i r c a l c u l a t i o n . Magnetic moments c a l c u l a t e d from both the Curie-Weiss and Curie laws, as w e l l as the parameters of the s u s c e p t i b i l i t y equation (eqn 5-1) are given i n Table 4 . The r e s u l t s of L i e b e , Weise and Klemm (17) are i n c l u d e d f o r comparison and i n a d d i t i o n the room temperature moments of NH^.VF^ and Py.VF^ ( c a l c u l a t e d from the Curie law (eqn. 5-3) are i n c l u d e d . The experimental data are given i n Tables 19, 21 and 22 i n the experimental s e c t i o n , and the experimental data f o r the MgVFg s a l t s s t u d i e d by L i e b e , Weise and Klemm w i l l be found i n r e f e r e n c e (17). TABLE 4 MAGNETIC PROPERTIES OF TETRAVALENT VANADIUM FLUORIDES Compound VF, VF^.SeF^ K 2 V F 6 NrL^.VF^ Py.VF,. c (cgs u n i t s ) e (degrees) a t 295°K(B.m.) Ref. 0.5835 198 O.665 133 0.515 118 f - e f f C u r i e Curie-Weiss 1.68 1.86 1.71 1.82 1.79 2.17 2.32': 2.05 Present work. K 2 V F 6 R b 2 V F 6 C s 2 V F 6 78 1.63,1.73 1.92,2.01 101 1.60,1.63 1.96,1.99 103 1.62,1.64 2.03 1 (17) The agreement of the e f f e c t i v e moments of KgVFg determined i n the prese n t i n v e s t i g a t i o n w i t h those of L i e b e , Weise and Klemm (17) i s very good. The d i f f e r e n c e i n the va l u e s of ^ > 118° i n the present work and 78° i n Liebe e t . a l . 64 i s l a r g e l y due to the d i f f e r e n c e i n the s t r a i g h t l i n e chosen to r e p r e s e n t the Curie-Weiss law. In the present work, the magnetic s u s c e p t i b i l i t y of KgVFg was measured a t e i g h t tempera-t u r e s between 8 l ° and 295°K and a l l but the 81° value c o i n -c i d e d w i t h the s t r a i g h t l i n e r e p r e s e n t i n g the Curie-Weiss law. The s u s c e p t i b i l i t y at 8l°K was s l i g h t l y higher than t h a t i n d i c a t e d by t h i s Curie-Weiss law. Liebe e t . a l . measured the s u s c e p t i b i l i t y at only three p o i n t s i n t h i s temperature range and these v a l u e s agree q u i t e w e l l w i t h the s u s c e p t i b i l i t i e s obtained i n the present work. The lower value of 0 obtained by Liebe e_t.aJL. i s due to t h e i r i n c l u s i o n of the s u s c e p t i b i l i t y at 8l°K i n t h e i r Curie-Weiss law, however the present work has shown t h a t the s u s c e p t i b i l i t y at t h i s temperature i s s l i g h t l y h igher than t h a t i n d i c a t e d by the Curie-Weiss law which f i t s a l l the other p o i n t s . The l o n g e x t r a p o l a t i o n (about 200°) i n v o l v e d i n e s t a b l i s h i n g © magnifies small d i f f e r e n c e s i n the s u s c e p t i b i l i t y measurements c o n s i d e r a b l y r e s u l t i n g i n t h i s l a r g e d i f f e r e n c e i n 0 . The small i n c r e a s e i n the s u s c e p t i b i l i t y of K^VF^ a t 8l°K over t h a t p r e d i c t e d by the Curie-Weiss law obtained i n the present work i s a p p a r e n t l y r e a l , because of the s i m i l a r i t y of the measured s u s c e p t i b i l i t i e s i n both the presen t work and th a t of L i e b e , Weise and Klemm (17). T h i s i n c r e a s e i n s u s c e p t i b i l i t y a t 8l°K may i n d i c a t e the presence of anomalous magnetic behaviour such as antiferromagnetism, however exten-s i o n of the s u s c e p t i b i l i t y measurements t o temperatures somewhat -65 below 8l°K would be vnecessary t o f u l l y e s t a b l i s h any d e v i a t i o n from normal paramagnetic behaviour which may be p r e s e n t . The e f f e c t i v e magnetic moments c a l c u l a t e d assuming Curie law behaviour (eqn. 5-3) are a l l f a i r l y c l o s e t o the s p i n -only value of 1.73 Bohr magnetons f o r one e l e c t r o n , t e n d i n g _2 i n general t o be low f o r VF^ and the VFg s a l t s , and h i g h m the VF^.Base and VF^.SeF^ complexes. C o r r e c t i o n of the e f f e c t i v e moments f o r the Weiss constant, (eqn. 5-2) y i e l d s magnetic moments c o n s i s t e n t l y higher (0.2 to 0.5 Bohr magnetons) than the s p i n - o n l y v a l u e . The Weiss constants of t e t r a v a l e n t vanadium f l u o r i d e compounds have c o n s i s t e n t l y h i g h p o s i t i v e v a l u e s , the lowest (which i s somewhat too low a c c o r d i n g t o the present work) being 78°, observed by Liebe e t . a l . (17) f o r K 2VFg, and the h i g h e s t 198° f o r vanadium t e t r a f l u o r i d e . I t i s i n t e r e s t i n g t o note that the anomalous 9 v a l u e s are not a f u n c t i o n of the d 1 e l e c t r o n i c c o n f i g u r a t i o n of vanadium ( I V ) . Vanadium t e t r a c h l o r i d e has a n e g l i g i b l e Weiss constant, and a magnetic moment of I.67 Bohr magnetons, s l i g h t l y below that expected f o r one unp a i r e d s p i n (65). The v a r i o u s vanadyl complexes which have been s t u d i e d e x h i b i t normal moments f o r one unpaired s p i n and have low Weiss con-s t a n t s of the order of 20° to 50° (66). In t r i v a l e n t vanadium compounds, which have two unpaired e l e c t r o n s , a s i m i l a r behaviour i s observed. Vanadium t r i c h l o r i d e has a magnetic moment of 2.74 Bohr magnetons, compared t o the s p i n - o n l y value of 2.83 66 Bohr magnetons, and a Weiss constant of 35° (67), whereas vanadium t r i f l u o r i d e has a Weiss constant of 146° and an e f f e c t i v e moment of 2.71 Bohr magnetons c a l c u l a t e d from the Curie law (eqn. 5-3) and 3.32 Bohr magnetons c a l c u l a t e d from the Curie-Weiss law (eqn. 5-2). While a l l the vanadium com-pounds have magnetic moments, c a l c u l a t e d from the Curie law, s l i g h t l y lower than the s p i n - o n l y value, only the f l u o r i d e s e x h i b i t anomalously h i g h & values w i t h a concomitantly l a r g e d i f f e r e n c e between the magnetic moments c a l c u l a t e d from the Curie law and Curie-Weiss law. The source of t h i s h i g h 9 value i s not known but i t i s reasonable t o a t t r i b u t e t h i s behaviour to i n t e r m o l e c u l a r a s s o c i a t i o n , because i n s o l i d h a l i d e s , the g r e a t e s t amount of i n t e r m o l e c u l a r a s s o c i a t i o n Is u s u a l l y found i n f l u o r i d e s . Van V l e c k (69) suggested t h a t the behaviour of 9 i n d i c a t e d t h a t i t c o u l d a r i s e from two sources. Since 9 i s u s u a l l y s m a l l e s t f o r S s t a t e s having no angular moment, © can a r i s e from s p i n - o r b i t c o u p l i n g which c o n t r i b u t e s angular momentum to the system. A l t e r n a t i v e l y as 9 i s u s u a l l y s m a l l e s t i n m a g n e t i c a l l y d i l u t e compounds, the source of 9 c o u l d be some form of exchange i n t e r a c t i o n ("antiferromagnetism") which v i o l a t e s a b a s i c assumption of the theory of paramag-netism; t h a t the magnetic c e n t r e s are n o n - i n t e r a c t i n g . I t must be noted t h a t compounds which obey the Curie-Weiss law, which i s a paramagnetic s u s c e p t i b i l i t y law, are not t r u e a n t i f e r r o m a g n e t i c compounds but paramagnetic compounds w i t h 67 p o s s i b l e small i n t e r a c t i o n s of an a n t i f e r r o m a g n e t i c type. ture behaviour of the s u s c e p t i b i l i t y e s p e c i a l l y i f the a n t i -f erromagnetic i n t e r a c t i o n s are small ( f o r example when the paramagnetism i s i n f l u e n c e d but not destroyed) or i f the s p i n -o r b i t c o u p l i n g s occur between n e a r l y degenerate l e v e l s (eg. the s p l i t t i n g i s l e s s than the magnitude of the thermal energy k T ) . The magnetic s u s c e p t i b i l i t y ( % ) of a system i n v o l v i n g a n t i f e r r o m a g n e t i c exchange i n t e r a c t i o n s o c c u r r i n g as pure s p i n - s p i n i n t e r a c t i o n s between neighbouring paramagnetic i o n s , to the f i r s t order i n J (the i n t e r a c t i o n c o n s t a n t ) , i s given by (70) the e x p r e s s i o n : where N i s the Avogadro number, k the Boltzmann constant, T the a b s o l u t e temperature and ^ the Bohr magneton. T h i s i s e x a c t l y the form of the Curie-Weiss law (eqn. 5-1) hence 8 can be a t t r i b u t e d t o a n t i f e r r o m a g n e t i c i n t e r a c t i o n s . However the magnetic moment of a s p i n - o r b i t coupled d 1 system i s given by K o t a n i (71) as: G r i f f i t h has shown (70) t h a t there i s l i t t l e b a s i s f o r d i f f e r e n t i a t i n g between e i t h e r source f o r 0 from the tempera-(5-4) k(T-J/4k) 2 8 + (3x-8) e " 3 x / 2 (5-5) where x = £ /kT and £ is the spin-orbit coupling constant. If j? is approximately as large as kT and the exponentials are replaced by series expansions to the f i rst power in £ only, then substitution of the resulting approximate expression p for JULeff into the Curie expression for the magnetic suscepti-bi l i ty d): % N E 2 / 4 f f (5-6) 3kT gives an approximate expression for the magnetic suscepti-bi l i ty of a paramagnetic system with spin-orbit interaction; <1 = §S|\1 . (5-7) *• 3k(T + 2£/ 5k) All the symbols are as given above. This also has the form of the Curie-Weiss law. Hence antiferromagnetic interactions and spin-orbit interactions lead to the same kind of departures from true paramagnetic behaviour and therefore i t is not possible to choose one or the other as being responsible for the high 0 values. The magnetic moments of the tetravalent vanadium fluorides, calculated from the Curie-Weiss law (eqn. 5-2) are, of course, independent of temperature, but the magnetic moments calculated from the Curie law (eqn. 5-3) do decrease with decreasing temperature. Kotani (71) has predicted the temperature variation of the magnetic moment that would be expected for a paramagnetic system with spin-orbit interaction. 69 The decrease i n moment w i t h temperature f o r a d^ e l e c t r o n i c c o n f i g u r a t i o n i s given by equation (5-5). Assuming the s p i n - o r b i t c o u p l i n g constants of vanadium t o be between 200 and 400 cm.\ the u s u a l range f o r the f i r s t p e r i o d of t r a n s i -t i o n elements, magnetic moments a t three r e p r e s e n t a t i v e temperatures have been c a l c u l a t e d and are shown i n Table 5. For comparison, the magnetic moments of vanadium t e t r a f l u o r i d e , potassium hexaf luorovanadate (IV") and the adduct, VF^.SeF^ have been c a l c u l a t e d f o r the same temperatures u s i n g the experimental s u s c e p t i b i l i t y equation and the Curie law (eqn. 5-2) and are a l s o shown i n Table 5. TABLE 5 COMPARISON OF PREDICTED (71) AND EXPERIMENTAL MAGNETIC MOMENTS (Bohr magnetons)) P r e d i c t e d moments by Kotani's theory (71) 200 300 400 cmT1 T°K 300 1.77 1.60 1 .40 200 1.60 1.35 1.15 100 1.15 O.85 0.65 Experimental moments, from Curie law. T o K V P 4 K 2 W 6 S e P 4 ' V F 4 300 1.68 1.71 1.92 200 1.54 l . 6 l 1.79 100 1.25 1.45 1.52 • In general the K o t a n i theory p r e d i c t s a more r a p i d decrease w i t h temperature than i s observed thus s p i n - o r b i t c o u p l i n g i s not s o l e l y r e s p o n s i b l e f o r the magnetic behaviour. T h i s does not exclude the presence of s p i n - o r b i t c o u p l i n g i n systems which a l s o have a n t i f e r r o m a g n e t i c i n t e r a c t i o n s s i n c e K o t a n i 1 s p r e d i c t i o n s are based on the assumption t h a t no a n t i f e r r o m a g n e t i c exchange i n t e r a c t i o n s occur (71). Because the magnetic behaviour observed i n t e t r a v a l e n t vanadium f l u o r i d e s i s not t h a t p r e d i c t e d by Kotani's theory alone i t seems reasonable to propose that both i n t e r - i o n ( a n t i f e r r o - ' magnetic) and i n t r a - i o n ( s p i n - o r b i t ) i n t e r a c t i o n s occur. The proposed f l u o r i n e b r i d g e p o l y m e r i s a t i o n i n vanadium t e t r a f l u o r i d e and the 1:1 VF^ complexes p r o v i d e s a r e a d i l y a v a i l a b l e means of t r a n s m i t t i n g a n t i f e r r o m a g n e t i c type i n t e r -a c t i o n s between.the vanadium atoms. The s i m i l a r i t y of the magnetic p r o p e r t i e s of vanadium t r i f l u o r i d e , which has been shown t o have a h i g h l y a s s o c i a t e d , b r i d g e bonded s t r u c t u r e (22), to those of vanadium t e t r a f l u o r i d e and i t s complexes, suggests that the h i g h Weiss constants may be due to exchange i n t e r a c t i o n s i n v o l v i n g f l u o r i n e b r i d g e bonds. The magnetic behaviour of the adduct SeP^.VP^ i s more + — 2 c o n s i s t e n t w i t h i t s f o r m u l a t i o n as (SeF^ ) 2 ("^ 2^ 10 ^ i n which the VF^~ ions have formed a f l u o r i n e b r i d g e d complex ion w i t h hexacoordinate vanadium, than with.the f o r m u l a t i o n as a f l u o r i n e b r i d g e d VP^.SeP^ polymer. Exchange i n t e r a c t i o n between the vanadium atoms i n the complex anion, where the 71 vanadium atoms are l i n k e d by f l u o r i n e b r i d g e s , i s more e a s i l y v i s u a l i s e d than i n the b r i d g e d polymer, where the i n t e r a c t i o n s between neighbouring vanadium atoms must be t r a n s m i t t e d through an F-Se-F b r i d g e . These h i g h Weiss constants do not n e c e s s a r i l y I n d i c a t e t h a t formation of the f l u o r i n e b r i d g e bonds causes a n t i f e r r o -magnetic exchange, because the Weiss constant can a l s o a r i s e from s p i n - o r b i t i n t e r a c t i o n s . The s h a r i n g of two or f o u r f l u o r i n e atoms of each VFg or VF^.Base u n i t c r e a t e s two d i f f e r e n t bond types and l e a d s to a d i s t o r t i o n of the octa-h e d r a l c o o r d i n a t i o n about the vanadium atom. T h i s In t u r n l e a d s t o a s p l i t t i n g of the low l y i n g (d€) o r b i t a l s , and t h i s s p l i t t i n g i s l i k e l y t o be q u i t e small (e.g. l e s s than kT) thus c r e a t i n g a s i t u a t i o n where s p i n - o r b i t c o u p l i n g can occur to p r o v i d e Curie-Weiss magnetic s u s c e p t i b i l i t y behaviour. Thus f l u o r i n e bridge bonding can give r i s e t o h i g h Weiss constants through e i t h e r exchange I n t e r a c t i o n or through s p i n - o r b i t type i n t e r a c t i o n s a r i s i n g from d i s t o r t i o n s of the symmetry accompanying the formation of the b r i d g e bonds. These polymeric compounds and complexes can not be regarded as " m a g n e t i c a l l y d i l u t e " , hence the simple t h e o r i e s of para-magnetism are l i k e l y inadequate. I t i s more d i f f i c u l t to e x p l a i n the behaviour of the hexafluorovanadate s a l t s because the s t r u c t u r e s are most _2 l i k e l y composed of d i s c r e t e VFg u n i t s , i n which the vanadium atom i s hexacoordinate and supposedly m a g n e t i c a l l y d i l u t e . 72 _2 The Weiss constants are c e r t a i n l y lower f o r the VPg s a l t s , i n agreement w i t h the expected t r e n d on magnetic d i l u t i o n , but they are s t i l l much l a r g e r than the u s u a l l y observed Weiss c o n s t a n t s . A s i m i l a r case has been r e p o r t e d by Hargreaves and Peacock (J2) f o r the AMFg s a l t s of molybdenum, rhenium and tungsten. Large p o s i t i v e values of the Weiss constant (as hig h as 218°) were observed f o r the molybdenum and rhenium s a l t s . The tungsten s a l t s were a c t u a l l y a n t i T ferromagnetic w i t h Neel p o i n t s i n the range 110-l40°K. The magnetic moment d i d not vary w i t h temperature a c c o r d i n g t o Kotan i ' s p r e d i c t i o n s and thus the h i g h © val u e s were a t t r i -buted t o a n t i f e r r o m a g n e t i c i n t e r a c t i o n s . These i n t e r a c t i o n s were proposed f o r AMoFg and ARePg and a c t u a l l y observed i n AWPg s a l t s (72). Hargreaves and Peacock (72) suggested t h a t the a n t i -f e rromagnetic i n t e r a c t i o n s o c c u r r i n g between the MFg~ ions were s i m i l a r t o the a n t i f e r r o m a g n e t i c i n t e r a c t i o n s observed i n I r C l g - s a l t s by G r i f f i t h s and co-workers (73). These a n t i f e r r o m a g n e t i c i n t e r a c t i o n s were supposed to occur through superexchange i n t e r a c t i o n s i n v o l v i n g the formation of _2 M-X-X-M type b r i d g e s between the I r C l 5 i o n s , u t i l i z i n g p o r b i t a l s on the b r i d g i n g atoms as proposed by Anderson (74). The e f f e c t s observed by G r i f f i t h and co-workers (73) are much sma l l e r than observed e i t h e r here or by Hargreaves and Peacock (72), as the Weiss constant f o r K Q I r C l / - i s only 33°. Since the a n t i f e r r o m a g n e t i c e f f e c t s observed i n oxides, s u l p h i d e s and s e l e n i d e s decrease i n t h i s order (7 )^ t h a t i s , f o l l o w i n g the decrease i n e l e c t r o n e g a t i v i t y and the i n c r e a s e i n the s i z e of the anion, superexchange appears to i n c r e a s e w i t h i n c r e a s i n g i o n i c c h a r a c t e r of the bond, and w i t h d e c r e a s i n g s e p a r a t i o n between exchanging c e n t r e s as would be expected (7^). A n t i f e r r o m a g n e t i c i n t e r a c t i o n s i n h a l i d e s , would t h e r e f o r e probably i n c r e a s e from i o d i d e s to f l u o r i d e s f o l l o w i n g the i n c r e a s i n g i o n i c c h a r a c t e r of the bond and the d e c r e a s i n g s i z e of the b r i d g i n g i o n . Exchange e f f e c t s are t h e r e f o r e l i k e l y t o be more important i n f l u o r i d e s than i n other h a l i d e s and t h i s appears to be the case. Anderson's theory of a n t i f e r r o m a g n e t i s m (7 )^ i s based on a cubic c r y s t a l s t r u c t u r e and i s not d i r e c t l y a p p l i c a b l e to the present complexes or to those of Hargreaves and Peacock (72) which have lower symmetry. Hargreaves and Peacock (72) proposed t h a t small t e t r a g o n a l d i s t o r t i o n s have a d i s p r o p o r -t i o n a t e l y l a r g e e f f e c t on the a n t i f e r r o m a g n e t i c exchange. T e t r a g o n a l d i s t o r t i o n s are observed i n many AMF^ s a l t s and are q u i t e l i k e l y t o be found i n the molybdenum and rhenium s a l t s (72). To suggest t h a t s p e c i f i c a n t i f e r r o m a g n e t i c e f f e c t s a r i s e from these d i s t o r t i o n s i s not necessary, as c r y s t a l d i s t o r t i o n s which might i n f l u e n c e a n t i f e r r o m a g n e t i c i n t e r -a c t i o n s may a l s o cause s p i n - o r b i t c o u p l i n g s , w i t h the same e f f e c t on the temperature dependence of the magnetic s u s c e p t i b i l i t y . T e t r a g o n a l d i s t o r t i o n s i n these s a l t s w i l l 74 l e a d , as before, to a s p l i t t i n g of the lower t r i p l e t of d e l e c t r o n s (d£) and thus c r e a t e l o w - l y i n g ( l e s s than kT above the ground s t a t e ) non-degenerate o r b i t a l s which can be i n v o l v e d i n s p i n - o r b i t c o u p l i n g . T e t r a g o n a l d i s t o r t i o n s of t h i s type need not be l a r g e to p r o v i d e a s u f f i c i e n t degree of s p l i t t i n g of the de l e v e l f o r l a r g e c o u p l i n g e f f e c t s , i n f a c t i f the d i s t o r t i o n s are too l a r g e the r e s u l t a n t l e v e l s w i l l probably be too g r e a t l y separated to be e x t e n s i v e l y populated by thermal e x c i t a t i o n and c o u p l i n g w i l l not occur to any great e x t e n t . At t h i s time i t i s only p o s s i b l e to conclude t h a t the observed magnetic p r o p e r t i e s of the t e t r a v a l e n t vanadium f l u o r i d e s w h ile c o n s i d e r a b l y d i f f e r e n t from other d 1 vanadium compounds are c o n s i s t e n t w i t h the e f f e c t s expected from a s s o c i a t i o n s and d i s t o r t i o n s which probably occur i n the f l u o r i d e s . Since the e f f e c t s are l a r g e , even i n the "magneti-c a l l y d i l u t e " VPg s a l t s , i t i s l i k e l y t h a t both exchange i n t e r a c t i o n s and d i s t o r t i o n induced s p i n - o r b i t c o u p l i n g s are r e s p o n s i b l e , however i t i s not p o s s i b l e to separate the two e f f e c t s . 75 CHAPTER 6: INFRARED SPECTRA OF VANADIUM FLUORIDES The i n f r a r e d spectrum of vanadium p e n t a f l u o r i d e has -1 -1 been obtained f o r the r e g i o n 250 cm. to 4000 cm. . The r e s u l t s are shown i n Table 6 and F i g u r e 2. Estimated i n t e n -s i t i e s and such assignments as are p o s s i b l e at t h i s time are a l s o given i n Table 6. Although l i q u i d vanadium p e n t a f l u o r i d e i s thought to be a s s o c i a t e d by means of f l u o r i n e bridge bonds, the normal vapour d e n s i t y (7) suggests t h a t the p e ntacoordinate monomer i s present i n the gas phase. There are however two p o s s i b l e s t r u c t u r e s f o r a p e ntacoordinate molecule; the t r i g o n a l bipyramid ( D ^ ) o r the square pyramidal (C^ v) s t r u c t u r e . The only f l u o r i d e known to have the l a t t e r s t r u c t u r e i s bromine p e n t a f l u o r i d e (19), however i n t h i s case the non-bonded e l e c t r o n p a i r of the bromine atom i s c o n s i d e r e d to be occupying one c o o r d i n a t i o n s i t e making the c o o r d i n a t i o n about the bromine atom o c t a h e d r a l . Thus bromine p e n t a f l u o r i d e has a d i s t o r t e d octa-h e d r a l r a t h e r than a square pyramidal s t r u c t u r e . As there are no non-bonding e l e c t r o n s i n vanadium p e n t a f l u o r i d e , I t i s reasonable to suppose t h a t , even i f o c t a h e d r a l c o o r d i n a t i o n i s achieved i n the l i q u i d , the gaseous molecule assumes the t r i g o n a l bipyramid s t r u c t u r e and thus Is i s o s t r u c t u r a l w i t h compounds such as phosphorus p e n t a f l u o r i d e and phosphorus p e n t a c h l o r i d e . The assignment of the i n f r a r e d spectrum w i l l l a r g e l y be based on p r e v i o u s assignments of the s p e c t r a of phosphorus p e n t a c h l o r i d e (75), antimony p e n t a c h l o r i d e (76) and antimony p e n t a f l u o r i d e (77). Spectra of phosphorus p e n t a f l u o r i d e (78) and bromine p e n t a f l u o r i d e (79) have been i n t e r p r e t e d i n terms of a t r i g o n a l bipyramid (D^^) and a square pyramidal ( C^ v) s t r u c t u r e r e s p e c t i v e l y but complete a s s i g n -ments have not been made. Assuming t h a t vanadium p e n t a f l u o r i d e vapour has the t r i g o n a l bipyramid (D.^) s t r u c t u r e , there w i l l be e i g h t funda-mental v i b r a t i o n a l f r e q u e n c i e s ; two of symmetry s p e c i e s a^, n 1 11 two of s p e c i e s a 2 , three of s p e c i e s e and one of sp e c i e s e . t 11 11 The a^ and e f r e q u e n c i e s are only Raman a c t i v e , the a 2 only t i n f r a r e d a c t i v e and the e i s both Raman and i n f r a r e d a c t i v e , hence the i n f r a r e d spectrum w i l l have f i v e fundamental f r e -quencies; two of sp e c i e s a^ and three of s p e c i e s e . I f the V-F bond d i s t a n c e i n vanadium p e n t a f l u o r i d e i s assumed to be 2.00+0.1 A, the moments of i n e r t i a of the mole-c u l e , which i s a symmetric top, are I ^ = 38o(+4o) x 10~^® and I x = I y = 44o(+8o) x 10 _^°g.cm. 2 where 1^ , i s the moment of i n e r t i a a l o n g the Z a x i s (the a x i s of the molecule) and 1^  and I y are moments about the C 2 axes, l y i n g i n the XY p l a n e . F o l l o w i n g the c o r r e l a t i o n of band shape w i t h moments of i n e r t i a by Badger and Zumwalt (80), which has been a p p l i e d t o a t r i g o n a l bipyramid molecule by Wilmshurst (76), the p a r a l l e l a 2 bands should have a weak Q, branch w i t h s t r o n g PR branches. The PR branch s e p a r a t i o n , which i s governed by the 1^ . and I y moments of i n e r t i a , (8l) should be about 18+4 cm""1". The p e r p e n d i c u l a r e bands w i l l have a s t r o n g Q branch and weak 77 TABLE 6 INFRARED SPECTRUM OF VANADIUM PENTAFLUORIDE. VAPOR Wavenumber Approximate Assignment and Comments. I n t e n s i t y ~  ! 271 1.1 [e fundamental 274 1.3 1 VF^ In plane bend. 1 287 1.9 Je fundamental 293 3.1 \ VF 2 l i n e a r bend. 320 2.5 -j „ 326 sh ? I a ? fundamental 332 4.5 I (VFo out of plane bend.) 339 3.0 5 343 sh ? 351 sh ? ( t . 1 e M fundamental! [V-F and V-F Y y ± a2 f u n d a m l ^ a l ) \ ^ ^ ^ e t r i c 810 J 880 sh 887 3.8 890 273+290+332=895 930 0.6 995 0.6 1060 0.6 784+273=1057 1385 3.0 1417 0.6 1497 3.5 1520 sh. 1615 broad. 0.8 sh. = shoulder F 1 i n d i c a t e s a x i a l and F i n d i c a t e s e q u a t o r i a l f l u o r i n e atoms r e s p e c t i v e l y . 100 To f o l l o w p. 77 FIG. 2 INFRARED SPECTRA OF VANADIUM FLUORIDES (o) VF g 250-400 cm'" (VAPOR, 100 mm) 100 260 275 300 360 400 (b) Vljj 700-1700 cmH (VAPOR, " 100' mm) 1200 1400 1600 (C) VF. 4 400-1100 cm (SOLID, MULL) -I 100 400 600 600 1000 (d) VF. 400-1100 cm"1, (SOLID, MULL) 5 X 400 6 00 800 1000 WAVENUMBER (cm"1) PR branches. The most prominent bands i n the spectra of t r i g o n a l bipyramid molecules are the two antisymmetric s t r e t c h f r e q u e n c i e s , one each of a^ and e symmetry. These bands, which are the antisymmetric s t r e t c h f r e q u e n c i e s of the a x i a l atoms and e q u a t o r i a l atoms r e s p e c t i v e l y , are g e n e r a l l y -1 " observed i n the range 400-1000 cm. i n h a l i d e s . The a 2 bands u s u a l l y show a p a r a l l e l s t r u c t u r e and the e' band a perpendi-c u l a r s t r u c t u r e as expected. The s e p a r a t i o n of the two — 1 — 1 bands i s 75 cnu i n phosphorus p e n t a f l u o r i d e (78), 127 cmT i n phosphorus p e n t a c h l o r i d e (75) and 14 cm."'" i n antimony p e n t a c h l o r i d e (76). Only one strong a b s o r p t i o n band was observed f o r vanadium p e n t a f l u o r i d e i n the range 400 to 2000 cmT''", and i t i s q u i t e p o s s i b l e that the odd s t r u c t u r e of t h i s band (see F i g u r e 2b), which looks l i k e an unsymmetrical PQ.R band, i s due to co i n c i d e n c e of the two antisymmetric s t r e t c h f r e q u e n c i e s . The 775 and 791 cm""'" peaks, separated by 16 cm."'" it are probably the PR branches of the p a r a l l e l a^ frequency (with the weak Q branch having a frequency of 783 cmT"'") and the peak a t 784 cm."'' i s probably a combination of the str o n g Q, branch of the p e r p e n d i c u l a r e 1 band and the weak Q branch of the a^ band; thus the antisymmetric f r e q u e n c i e s are degenerate. The three remaining s t r o n g f r e q u e n c i e s at 271-4, 288-94, and at 332 cm."*" are probably the other three i n f r a r e d a c t i v e fundamentals. The PQR s t r u c t u r e of the 332 cm~.^ band, w i t h PR se p a r a t i o n of approximately 19 cmT"^ (339-320 cmT"'") suggests t h a t t h i s i s the second a" frequency (the VF, out of plane 79 bending). The remaining two str o n g f r e q u e n c i e s are probably the e ! , VP 2 l i n e a r bending and the e*, VF^ i n plane bending f r e q u e n c i e s . Few combination bands can be deduced from t h i s i n f r a r e d spectrum, and those t h a t have been t e n t a t i v e l y a ssigned are shown. Other assignments of the observed i n f r a r e d fundamentals do not l e a d to any b e t t e r agreement w i t h the observed spectrum, nor does an assignment based on the l e s s l i k e l y square pyramidal model. I t i s p o s s i b l e t h a t one or more of the e' fundamentals which have been a s s i g n e d t o the lowest frequency peaks may be below the lower l i m i t of the instrument and have not been observed. The three fundamentals which are only Raman a c t i v e are unknown and these w i l l a l s o be i n v o l v e d i n forming i n f r a r e d a c t i v e combination bands. One of these, the e" frequency w i l l l i k e l y be found i n the range 300-400 c m T 1 and the two a^ f r e q u e n c i e s (the symmetric s t r e t c h i n g f r e q u e n c i e s of a x i a l and e q u a t o r i a l f l u o r i n e s ) w i l l p r o b a b l y occur between 500 and 800 c r ru " L , by analogy w i t h s i m i l a r p e n t a h a l i d e s . U n t i l the Raman spectrum of vanadium p e n t a f l u o r i d e i s a v a i l a b l e , no f u r t h e r assignments can be made. As i t was not p o s s i b l e t o measure the i n f r a r e d spectrum of the l i q u i d due to the r e a c t i v i t y of the c e l l window m a t e r i a l s r e q u i r e d t o i n v e s t i g a t e the r e g i o n s of i n t e r e s t , no comparison between l i q u i d and vapour i s a v a i l a b l e . Such a comparison could perhaps l e a d to f u r t h e r i n f o r m a t i o n about the a s s o c i a t i o n of vanadium p e n t a f l u o r i d e i n the l i q u i d s t a t e . 86 The i n f r a r e d s p e c t r a of vanadium t r i f l u o r i d e and vanadium t e t r a f l u o r i d e were a l s o measured over the range 400 to 4000 cm"1 as mulls i n N u j o l , and the r e s u l t s are shown i n Table J and F i g u r e 2. Since these compounds are s o l i d s , broader peaks and l e s s d e t a i l e d s p e c t r a were o b t a i n e d . TABLE 7 INFRARED SPECTRA OF VANADIUM TRIFLUORIDE AND TETRAFLUORIDE V F 3 VF^ 1060 b r . w. 1025 b r . w 970,940 doublet, w. 837 s. 890 vw. 780 s. 540+ 20 s.br. 530+20 s.br. s = strong, w = weak, vw = very weak, br = broad. While both VF^ and VF^ have a s t r o n g broad band of somewhat s i m i l a r shape at approximately 530 cm.'1' the remainder of the spectrum i s markedly d i f f e r e n t . With VF^ only a few weak bands are observed from 600 to 4000 cm~.^~, whereas vanadium t e t r a f l u o r i d e shows a s t r o n g doublet band at 780-837 cmT"1'. The d i f f e r e n c e i n s p e c t r a may be due t o the d i f f e r e n c e i n the s t r u c t u r e of the s o l i d . Vanadium t r i f l u o r i d e has a s t r u c t u r e (22) i n which each vanadium atom i s c o o r d i n a t e d by s i x f l u o r i n e atoms form-i n g a n e a r l y r e g u l a r octahedron. A s p e c i e s w i t h o c t a h e d r a l (0 )^ symmetry has two i n f r a r e d a c t i v e fundamentals ( t r i p l y degenerate F, s p e c i e s ) which are a s s o c i a t e d w i t h the a n t i -81 symmetric v i b r a t i o n f r e q u e n c i e s . The l a r g e r value Is u s u a l l y denoted V^ and the s m a l l e r , v?^. The broad band at 5^ 0 CHIT 1 i s p robably the V ^ frequency, and thus s i m i l a r t o the value of 511 observed in^K^VFg (82). The lower f r e q u e n c y , ^ ^, i s probably below the observed r e g i o n as suggested by Peacock and Sharp (82). I f vanadium t e t r a f l u o r i d e e x h i b i t e d r e g u l a r o c t a h e d r a l symmetry about the vanadium, the i n f r a r e d spectrum of vanadium t e t r a f l u o r i d e would be expected to be very s i m i l a r t o t h a t of vanadium t r i f l u o r i d e , perhaps showing only a s l i g h t i n c r e a s e i n the value of ^ ^ p a r a l l e l i n g the change i n \> ^  from 511 cm. 1 i n K^VFg t o 583 c m T 1 i n K 2VFg (82). However the i n f r a r e d spectrum of vanadium t e t r a f l u o r i d e has, i n a d d i t i o n to a broad s t r o n g band s i m i l a r to the \? band i n vanadium t r i f l u o r i d e , a s t r o n g 3 double band a t 780 and 837 cnu s u g g e s t i n g t h a t the s o l i d s t r u c t u r e i s not the same i n the two f l u o r i d e s . I t was suggested e a r l i e r t h a t vanadium t e t r a f l u o r i d e has an a s s o c i a t e d s t r u c t u r e i n which f o u r f l u o r i n e s of the VFg o c t a h e d r a l u n i t are shared w i t h adjacent octahedra (eg. as f l u o r i n e b r i d g e bonds) and two are unshared, probably l e a d i n g to f o u r short vanadium-fluorine bonds and two l o n g bonds i n the octahedron or v i c e v e r s a . The symmetry about the vanadium i s t h e r e f o r e t e t r a g o n a l r a t h e r than o c t a h e d r a l . A molecule of t e t r a g o n a l symmetry (D||h) has three i n f r a r e d a c t i v e funda-mentals (doubly degenerate E s p e c i e s ) r a t h e r than the two t r i p l y degenerate i n f r a r e d a c t i v e fundamentals of the o c t a h e d r a l molecule. 82 Assuming t h a t small changes i n symmetry such as t h i s w i l l not cause l a r g e s h i f t s i n the v i b r a t i o n a l f r e q u e n c i e s (eg. the s h i f t s are probably l e s s than 100 c m T 1 ) , the sudden appearance of the 780-837 c m T 1 band i n vanadium t e t r a f l u o r i d e i s probably due to a c t i v a t i o n of a p r e v i o u s l y i n a c t i v e i n f r a r e d band because of the lower symmetry of the VFg u n i t i n vanadium t e t r a f l u o r i d e r e l a t i v e to t h a t i n vanadium t r i f l u o r i d e . The str o n g broad band a t 5^0 cm. i n vanadium t e t r a f l u o r i d e probably corresponds t o the 583 c n u 1 band observed i n K^VFg (82) and the t h i r d fundamental i s probably below 400 c m T 1 as b e f o r e . The reason f o r the doublet s t r u c t u r e (see F i g u r e 2c) of the 780-837 c m T 1 band i s not known. The two peaks are not i d e n t i c a l , the 780 c r r u 1 band being much sharper than the 837 c r r u 1 band. I t i s p o s s i b l e t h a t the degenerate v i b r a t i o n i s s p l i t by c r y s t a l f o r c e s and d i f f e r e n c e s i n bonding and t h a t the two bands are a s s o c i a t e d w i t h non-bridge and brid g e f l u o r i n e s . A complete a n a l y s i s of the v i b r a t i o n a l spectrum r e q u i r e s a Raman spectrum but because VF^ i s a s o l i d the measurement of the Raman spectrum would l i k e l y be d i f f i c u l t . CHAPTER 7: THE THERMOCHEMISTRY OP VANADIUM FLUORIDES ( i ) The Standard Heat of Formation of Vanadium T e t r a f l u o r i d e Vanadium t e t r a f l u o r i d e was d i s s o l v e d i n d i s t i l l e d water i n a c a l o r i m e t e r , probably a c c o r d i n g to the r e a c t i o n : (a) VP^(c) + H 20(1) > V 0 + 2 ( a q ) + 4F~(aq) + 2H +(aq) and the average heat evolved d u r i n g t h i s h y d r o l y s i s ( H a ) , c a l c u l a t e d from the experimental data given i n Table 24, was -27.5+0.5 kcal/mole. The thermochemical c y c l e f o r the heat of formation was completed by combining r e a c t i o n (a) w i t h the f o l l o w i n g r e a c t i o n s : (b) V 0 + 2 ( a q ) + 2H +(aq) + 4Cl"(aq) < V C l ^ ( l ) + H 2 0 ( l ) (c) VC1 4(1) < V(c) + 2 C l 2 ( g ) (d) 2 C l 2 ( g ) > 4Cl~(aq) (e) 4F"(aq) < 2P 2(g) Adding r e a c t i o n s (a) to (e) together r e s u l t s i n r e a c t i o n ( g ) , (g) V P 4 ( s ) < V(c) + 2 F 2 ( g ) which i s the re v e r s e of the formation r e a c t i o n of vanadium t e t r a f l u o r i d e . The heat of r e a c t i o n (g) i s the negative of the standard heat of formation of vanadium t e t r a f l u o r i d e , and i s given by: H = -^H„ 0(VFi,) = H+H,+H +H.+H g " f u v 4 ' a b o d e The necessary data f o r the c a l c u l a t i o n and the r e s u l t i n g heat of formation of vanadium t e t r a f l u o r i d e are given i n Table 8. 84 The experimental heats of h y d r o l y s i s of vanadium t e t r a f l u o r i d e ( r e a c t i o n (a)) and vanadium t e t r a c h l o r i d e ( r e a c t i o n (b)) were measured at 2 9 8°K. A l l the other values are a l s o those given ( 8 3 ) a t 2 9 8°K. TABLE 8 HEAT OP FORMATION OF VANADIUM TETRAFLUORIDE Reaction D e s c r i p t i o n Numerical Values Reference Heat kcal/mole kcal/mole  H heat of h y d r o l y s i s - 2 7 . 5 Present work of VF^ (Table 2 4 ) Hfe - ( h e a t of h y d r o l y s i s Present work of VC14) - ( - 6 8 . 6 ) 6 8 . 6 (Table 2 6 ) - A H f 0 ( V C l 4 ) - ( - 1 3 6 . 2 ) 136.<2 84 4 A H f 0 C l " ( a q ) 4 ( _ 4 o . 0 2 ) - 1 6 0 . 1 H e - 4 A H f 0 F " ( a q ) - 4 ( - 7 8 . 6 6 ) 3 1 4 . 6 n C d ] 8 3 T h e r e f o r e : H = - A H F Q ( V F U ) = 3 3 1 . 8 kcal/mole. g X H-The standard heats of formation at 2 9 8°K of the ions are the values f o r each i o n at i n f i n i t e d i l u t i o n . The e r r o r a r i s i n g from t h i s approximation i s no g r e a t e r than 0 . 1 kcal./mole and i s not s i g n i f i c a n t i n t h i s study. The only e r r o r s estimated are those a r i s i n g from the experimental heats of h y d r o l y s i s , which are approximately + 0 . 5 kcal./mole f o r the heat of h y d r o l y s i s of vanadium t e t r a f l u o r i d e and +1 kcal./mole f o r vanadium t e t r a -c h l o r i d e , g i v i n g an o v e r a l l e r r o r of +1.5 kcal./mole. There-f o r e the standard heat of formation of vanadium t e t r a f l u o r i d e i s A H f 0 ( V F ^ ) - - 3 3 2 + 1 . 5 kcal./mole, R e a c t i o n (b) was the most f e a s i b l e method of completing the thermochemical c y c l e , because i t s i m p l i f i e s the assumption of the form i n which the vanadium (IV) e x i s t s i n the s o l u t i o n . Using r e a c t i o n (b) i t i s only necessary t o assume t h a t vanadium t e t r a f l u o r i d e and vanadium t e t r a c h l o r i d e h y d r o l y s e to the same c a t i o n i c s p e c i e s under s i m i l a r c o n d i t i o n s , r a t h e r than s p e c i f i c a l l y assuming t h a t vanadium t e t r a f l u o r i d e hydrolyse +2 to VO . However, the heat of r e a c t i o n (b) was not a v a i l a b l e i n the l i t e r a t u r e ; the de t e r m i n a t i o n of the heat of formation of vanadium of vanadium t e t r a c h l o r i d e by Ruff and F r e i d r i c h ( 8 5 ) from the heat of h y d r o l y s i s i s not a p p l i c a b l e because they hydr o l y s e d V C l ^ i n an a l k a l i n e peroxide s o l u t i o n , e s s e n t i a l l y measuring the heat of the r e a c t i o n : V C l ^ ( l ) + 9 / 2 0H~(aq) + 1/ 2 0 2H"(aq) V0~(aq) + 5 / 2 H 20 + 4 c i " ( a q ) which i s a combination of the heat of h y d r o l y s i s of vanadium t e t r a c h l o r i d e , n e u t r a l i z a t i o n of the evolved a c i d and the heat of o x i d a t i o n of V ( l V ) to V(V) i n aqueous s o l u t i o n . A l t e r n a t i v e l y , the standard heat of formation of the +2 aqueous vanadyl ion (VO ) could have been estimated from the standard f r e e energy of formation which has been determined from e l e c t r o c h e m i c a l c e l l p o t e n t i a l measurements ( 9 5 ) . The u n c e r t a i n t i e s i n t r o d u c e d by t h i s approximation however c o u l d have been as l a r g e as 2 0 kcal/mole i n the f i n a l heat of formation of vanadium t e t r a f l u o r i d e , so t o a v o i d i n t r o d u c i n g 86 unnecessary approximations, the heat of h y d r o l y s i s of vanadium t e t r a c h l o r i d e ( r e a c t i o n (b)) was a l s o determined. The average heat of h y d r o l y s i s of vanadium t e t r a c h l o r i d e , under s i m i l a r c o n d i t i o n s to those used i n the h y d r o l y s i s of vanadium t e t r a -f l u o r i d e , was -68.8+1.0 kcal./mole (Table 26). The h y d r o l y s i s of vanadium t e t r a c h l o r i d e i s f u r t h e r d i s c u s s e d i n Appendix 1, +2 r \ where i t i s shown that the standard heat of formation of VO (aq) i s -113 kcal./mole. Thus the approximation of & F ^ o ( V 0 + + ( a q ) ) (-109 kcal./mole) to A H ^ o ( V 0 + + ( a q ) ) i n order to c a l c u l a t e the heat of formation of vanadium t e t r a f l u o r i d e would have i n t r o -duced an e r r o r of 4 kcal./mole, i n s t e a d of the 20 kcal./mole estimated. ( i i ) The Standard Heat of Formation of Vanadium P e n t a f l u o r i d e Vanadium p e n t a f l u o r i d e was h y d r o l y s e d i n d i l u t e a l k a l i , p robably a c c o r d i n g to the r e a c t i o n : (a) V F 5 ( l ) + 60H"(aq) > V0~(aq) + 3H 20(l) + 5F~(aq) The average heat of h y d r o l y s i s of vanadium p e n t a f l u o r i d e ( H a ) , c a l c u l a t e d from the experimental data i n Table 25 i s -l4l+3 kcal./mole. The thermochemical c y c l e f o r the heat of formation of vanadium p e n t a f l u o r i d e was completed by combining r e a c t i o n (a) w i t h r e a c t i o n s (b) to (e) i n c l u s i v e : (b) VO'(aq) « V(c) + 3/2 0 2(g) (c) 3H 2(g) + 30 2(g) 60H~(aq) (d) 3H 20(l) < 3H 2(g) + 3/2 0 2(g) (e) 5F"(aq) -« 5/2 F 2(g) 87 The a d d i t i o n of r e a c t i o n s (a) to (e) together, r e s u l t s i n r e a c t i o n ( g ) , which i s the r e v e r s e of the standard formation r e a c t i o n of vanadium p e n t a f l u o r i d e : (g) VP 5(1) •< V(c) + 5/2 P 2 ( g ) The heat of r e a c t i o n (g), which i s the negative of the standard heat of formation of vanadium p e n t a f l u o r i d e i s given by the sum of the heats of r e a c t i o n s (a) to (e) i n c l u s i v e , t h a t i s : H g = - * H f 0 ( V P 5 ( l ) ) = H ^ + H ^ + V The data necessary f o r the c a l c u l a t i o n of the standard heat of formation of vanadium p e n t a f l u o r i d e are given i n Table 9. TABLE 9 HEAT OP FORMATION OP VANADIUM PENTAFLUORIDE Reaction D e s c r i p t i o n Numerical Values References Heat - kcal./moTe kcal./mole - - H 0 heat of h y d r o l y s i s of VPj- -l4l -141 Present work. (Table 25) -&H f 0(VO~(aq)) _(_224.5) 224.5 H c 6 &H f 0OH~(aq)) 6(-54.96) -329.8 H d -3 ^ H f O ( H 2 0 ( l ) ) -3(.-68.32) 204.96 H e - 5 & H f 0 ( F ~ ( a q ) ) -5(-78.66) 393.3 T h e r e f o r e : H = - A EL 0 ( V P l - ( l ) ) = 351.9 kcal./mole. No e r r o r i s c a l c u l a t e d f o r the standard v a l u e s used i n the thermochemical c y c l e . A l l heat values used are those given f o r 298°K as before and the standard heat of formation of each i o n i s the given heat of formation of the i o n at i n f i n i t e a H b 83 88 d i l u t i o n (83). The e r r o r c o n t r i b u t e d by t h i s approximation i s n e g l i g i b l e amounting t o no more than 0.1 kcal./mole. The only e r r o r estimated i s th a t a r i s i n g from the experimental heat of h y d r o l y s i s which i s probably about +3 kcal./mole. Therefore the standard heat of formation of vanadium p e n t a f l u o r i d e a t 298°K i s : & H f 0 ( V F 5 ( l ) ) = -352 + 3 kcal./mole ( i i i ) T h e o r e t i c a l E s t i m a t i o n of Heats of Formation from  L a t t i c e E n e r g i e s The Born-Haber c y c l e r e l a t e s the thermodynamic p r o p e r t i e s of a c r y s t a l l i n e compound to the thermodynamic p r o p e r t i e s of i t s c o n s t i t u e n t elements. The component e n t h a l p i e s of a f l u o r i d e , MF n, are shown below i n the f a m i l i a r g r a p h i c a l r e p r e s e n t a t i o n (32). MP (c) > M"™(g) + nP"(g) t I & H f 0 ( M F n ) - £ 3 ^ + nE (-L -nD/2) I M(s) + n/2 P 2 ( g ) < M(g) + P(g) where: U i s the l a t t i c e energy of MP^. S t r i c t l y , at 298°K the l a t t i c e enthalpy (u + (n+l)RT) should be used, however f o r the high e r f l u o r i d e s at 298°K the d i f f e r e n c e i n l a t t i c e energy and enthalpy i s about 1 p a r t i n 1000 and much sm a l l e r than the e r r o r s In e s t i m a t i n g U i t s e l f . A l l the remaining values are those given f o r 2y8°K. 1^  r e p r e s e n t s the s u c c e s s i v e i o n i z a t i o n p o t e n t i a l s of the metal M. E i s the e l e c t r o n a f f i n i t y of f l u o r i n e . L i s the enthalpy of s u b l i m a t i o n of the metal. D i s the enthalpy of d i s s o c i a t i o n of f l u o r i n e . A H f 0(MP ) i s the heat of formation of MF^ from i t s elements. As the sum of a l l terms i n the c y c l e must be zero, the heat of formation i s given by: A H f 0 ( M F n ) = £ I. + L - n(E - D/2) - U. (7-1) Since a l l the q u a n t i t i e s on the r i g h t hand s i d e of equation (7-1) except the l a t t i c e energy (U) are r e a d i l y a v a i l a b l e i n thermodynamic t a b l e s , the problem of c a l c u l a t i n g the heat of formation becomes one of e v a l u a t i n g the l a t t i c e energy. Exact l a t t i c e energy c a l c u l a t i o n s are lengthy and r a t h e r i n v o l v e d , however s u f f i c i e n t accuracy f o r the present purposes may be obtained w i t h the simple formula of Born and Mayer (32) which c o n s i d e r s only the coulombic f o r c e s and the nearest neighbour r e p u l s i v e f o r c e s w i t h i n the c r y s t a l . The l a t t i c e energy (U) i s given (32) by: 90 where: M i s the Madelung constant, N i s the Avogadro number, Z^ and Z^ are the a n i o n i c and c a t i o n i c charges r e s p e c t i v e l y , r Q i s the i n t e r n u c l e a r s e p a r a t i o n , e i s the e l e c t r o n i c charge, and b i s the i n t e r i o n i c r e p u l s i o n constant. The Madelung constant i s a f u n c t i o n of the c r y s t a l s t r u c t u r e and i t s c a l c u l a t i o n r e q u i r e s a d e t a i l e d knowledge of the c r y s t a l s t r u c t u r e . Furthermore the d i f f i c u l t y of the c a l c u -l a t i o n i n c r e a s e s w i t h d e c r e a s i n g c r y s t a l symmetry. F u r t h e r s i m p l i f i c a t i o n i s achieved by r e p l a c i n g ( 3 2 ) the Madelung constant (M), which i s p r o p o r t i o n a l t o the number of ions i n a molecule, by a constant (o() which i s the "Madelung constant per i o n " and i s given by: « = f (7-3) where v i s the number of ions i n a formula u n i t . The l a t t i c e energy i s then given ( 3 2 ) by: NotvZ f lZ„e 2 U = - A 0 2 r o 1 -r o (7-4) where a l l symbols are the same as given above. Although the "Madelung constant per i o n " (<* ) i s not i d e n t i c a l f o r d i f f e r e n t l a t t i c e types, changes i n o( w i t h l a t t i c e type are p r o p o r t i o n a l t o the i n t e r n u c l e a r s e p a r a t i o n of the i o n s ( 3 2 ) . A c c o r d i n g t o K a p u s t i n s k i i , every c r y s t a l 91 can be r e p r e s e n t e d by a sodium c h l o r i d e l a t t i c e w i t h s u i t a b l e m o d i f i c a t i o n s t o the c o e f f i c i e n t s d. and r without change i n o to l a t t i c e energy (86). The l a t t i c e energy can t h e r e f o r e be c a l c u l a t e d by assuming o( =1.754, the value f o r the sodium c h l o r i d e l a t t i c e , and t h a t r = r n + r„, the sum of the i o n i c ' o A C r a d i i ( i n Angstrom u n i t s ) f o r s i x c o o r d i n a t i o n of the i o n s ; the c o o r d i n a t i o n found i n sodium c h l o r i d e . Making these s u b s t i t u t i o n s and t a k i n g b=0.345, and Ne =329.7 kcal./mo] the K a p u s t i n s k i i formula (32,86) i s obtained; 0.345 287.2 v Z A Z B U r A + r C 1 " r A + r C (7-5) The l a t t i c e energy of a c r y s t a l (u) can t h e r e f o r e be c a l c u l a t e d from only the i o n i c r a d i i of the i o n s ( r A and r ^ ) . As b e f o r e , v i s the number of ions i n a formula u n i t and Z^ and Z-g are the a n i o n i c and c a t i o n i c charges r e s p e c t i v e l y . L a t t i c e e n e r g i e s r e s u l t i n g from the K a p u s t i n s k i i formula are not as exact as those obtained by e l a b o r a t e c a l -c u l a t i o n s (32) however the formula r e a d i l y g i v e s u s e f u l estimates of l a t t i c e e n e r g i e s where exact c a l c u l a t i o n s are o f t e n d i f f i c u l t or i m p o s s i b l e . L a t t i c e e n e r g i e s were estimated from the K a p u s t i n s k i i formula (eqn. 7-5) f o r a s e r i e s of t r i - and t e t r a f l u o r i d e s of the f i r s t l o n g p e r i o d t r a n s i t i o n metals and the h i g h e s t f l u o r i d e s of two t r a n s i t i o n metals of the second l o n g p e r i o d , z i r c o n i u m and niobium. These l a t t i c e e n e r g i e s were used i n equation (7-3) to c a l c u l a t e the heats of formation of the f l u o r i d e s . 92 The i o n i c r a d i i of the metals chosen f o r the c a l c u l a t i o n s were P a u l i n g ' s c r y s t a l r a d i i (58) which form a s e l f - c o n s i s t e n t set of s i x coo r d i n a t e r a d i i f o r a l l p o s s i b l e v a l e n c i e s of a metal i o n and y i e l d c l o s e r agreement w i t h measured i n t e r a t o m i c d i s t a n c e s i n the f l u o r i d e s . Where such r a d i i were not a v a i l a b l e (eg. f o r i r o n and c o b a l t ) e m p i r i c a l i o n i c r a d i i ( P a u l i n g , (58)) or estimated i o n i c r a d i i were used. These r a d i i are i n ge n e r a l l a r g e r than c r y s t a l r a d i i and y i e l d lower values of the l a t t i c e energy and hence of the heat of formation ( A H f 0 ) . The a c t u a l values used are shown i n Table 10. The r a d i u s of f l u o r i d e was taken as I.36 A ( l l ) . The r e s u l t i n g l a t t i c e e n e r g i e s of the t r i - and t e t r a f l u o r i d e s of the f i r s t l o n g p e r i o d elements from t i t a n i u m to c o b a l t , of z i r c o n i u m t e t r a -f l u o r i d e and niobium p e n t a f l u o r i d e are shown i n Table 11. Using the c a l c u l a t e d l a t t i c e e n e r g i e s and the thermo-chemical data given i n Table 10, the standard heats of form-a t i o n were c a l c u l a t e d f o r the same f l u o r i d e s as above. The r e s u l t s are shown i n Table 11 along w i t h the a v a i l a b l e e x p e r i -mental heats of formation of these f l u o r i d e s . Where the i o n i c r a d i i were extremely u n c e r t a i n , t h a t +4 +4 +4 i s f o r Mn , Fe and Co , upper and lower l i m i t s were assig n e d to the r a d i i and the two values given f o r each l a t t i c e energy and heat of formation r e p r e s e n t the extreme values of these q u a n t i t i e s f o r the range of i o n i c r a d i u s . 93 TABLE 10 DATA FOR HEAT OF FORMATION CALCULATIONS Element La t e n t heat of subl.(87) (L) (kcal./mole) I o n i s a t i o n P o t e n t i a l s (88) ( I , n = valence of ion) (kcal./mole) I I Z2 X3 J 4 T i 112 147 314 649 997 V 122 155 327 685 1106 Cr 94 156 380 713 1143 Mn 68 171 360 777 1199 Fe 99 182 373 706 1153 Co 102 182 392 772 (1153) Zr 125 160 323 572 783 Nb 185 156 323 648 883b a (E - D/2) f o r f l u o r i n e =66.3 kcal./mole (89) a) ) 1^  f o r Nb = 1153 kcal./mole, estimated value b 94 TABLE 11 LATTICE ENERGIES AND HEATS OP FORMATION OF FLUORIDES Element +n r U(MF n) A H f(MF n) A H f(MF n) A kc/mole kc/mole ( c a l c ) kc/mole ( e x p t l ) Ref kc/mole TRIFLUORIDE S, v = 4, n = 3, n(E - D/2) = 199 kcal./mole T i 0.73 1376 1121 -345 -315 83 V 0.67 1409 1167 -319 -Cr 0.64 1426 1250 -281 -265 90 Mn 0.62 1438 1308 -261 -238 91 Fe 0.60 1449 1261 -288 -Co 0.63 1432 1346 -183 -187 83 TETRAFLUORIDES, v = 5, n = 4, n(E -• D/2) = 265 kcal./mole. T i 0.68 2339 2114 -385 -393 28 V 0.62 2413 2241 -315 -332 Prese: Cr 0.56 2454 2412 -233 -287 90 Mn to 0.54-j 0.60 J 2474-1 2415J 2507 -165-1 -105 J -(Fe) to 0.52-j o.6o J 2494T 24l4 J 2414 -246-1 -167J -(Co) to 0.50-] o .6o J 2516-, 2415-1 (2499)* -180-) - 79 J -Zr 0.80 2234 1825 -536 -457 26 PENTAFLUORIDES, v = 6, n = 5> n(E - D/2) = : 332 kcal./mole V 0.59 . not c a l c u l a t e d . . . . -352 Prese: Nb 0.70 3482 3163 -466 -432 31 i n c l u d e s an estimated v a l u e . 95 C o n s i d e r i n g the approximate nature of the l a t t i c e energy c a l c u l a t i o n s , the agreement between the experimental and c a l c u l a t e d heats of formation, when both values are a v a i l -a b l e , i s s u r p r i s i n g l y good. The agreement i n d i c a t e s t h a t c a l -c u l a t e d values of the heat of formation may be used when experimental values are not a v a i l a b l e , such as i n the case of vanadium t r i f l u o r i d e . In g e n e r a l , c a l c u l a t e d heats of formation of the t r i -f l u o r i d e s of the elements of the f i r s t long p e r i o d are g r e a t e r than the experimental v a l u e s , whereas the c a l c u l a t e d values f o r the t e t r a f l u o r i d e s of the same elements are g e n e r a l l y lower than the experimental ones. The only c a l c u l a t i o n s done f o r f l u o r i d e s of the second long p e r i o d elements are those on niobium penta-f l u o r i d e and z i r c o n i u m t e t r a f l u o r i d e , both of which are higher than experimental v a l u e s . The d i s c r e p a n c i e s between experimental and c a l c u l a t e d values are probably due to small e r r o r s i n the l a t t i c e energy e s t i m a t i o n . The K a p u s t i n s k i i formula appears t o overestimate the l a t t i c e energy of the t r i f l u o r i d e s of the elements of the f i r s t l o n g p e r i o d whereas i t underestimates the l a t t i c e energy of the t e t r a f l u o r i d e s of the same elements probably because of the i n c r e a s e In covalent c h a r a c t e r of the M-F bonds accompany-i n g the i n c r e a s e d v a l e n c e . The c a l c u l a t e d values of the heat of formation of z i r c o n i u m t e t r a f l u o r i d e and niobium penta-f l u o r i d e are l a r g e r than the experimental values, i n d i c a t i n g t h a t the K a p u s t i n s k i i formula probably overestimates the l a t t i c e e n e r g i e s of f l u o r i d e s of elements i n the second long p e r i o d . Again t h i s o v e r e s t i m a t i o n i s probably due t o the l a r g e r degree of i o n i c c h a r a c t e r i n the bonds formed by second l o n g p e r i o d elements r e l a t i v e t o f i r s t l o n g p e r i o d elements, because of the l a r g e r i o n i c s i z e i n the second long p e r i o d elements. E f f e c t i v e l y the K a p u s t i n s k i i formula a r b i t r a r i l y compensates f o r a constant amount of covalent c h a r a c t e r , whereas i n f a c t c o v a l e n t c h a r a c t e r changes w i t h valence and i o n i c s i z e . There are a few anomalies i n the c a l c u l a t e d heat of formation values i n Table 11 which cannot be a t t r i b u t e d to systematic v a r i a t i o n s of c o v a l e n t c h a r a c t e r . The value of the heat of formation c a l c u l a t e d f o r i r o n t r i f l u o r i d e does not obey the f a i r l y r e g u l a r decrease of c a l c u l a t e d heat of form-a t i o n values observed f o r the t r i f l u o r i d e s from t i t a n i u m to c o b a l t . T h i s may be due to the use of e m p i r i c a l r a d i i r a t h e r than c r y s t a l r a d i i and i t i s p o s s i b l e t h a t the r a d i u s of Fe J should be taken as somewhat g r e a t e r than 0.60A. Barber, L i n n e t and T a y l o r (89), u s i n g a much more approximate formula f o r the l a t t i c e energy, have a l s o c a l -c u l a t e d heat of formation values f o r the t r i f l u o r i d e s of the f i r s t l o n g p e r i o d elements. In g e n e r a l t h e i r r e s u l t s e x h i b i t the same t r e n d as those i n Table 11 but were about 100 kcal./mole g r e a t e r than the p r e s e n t c a l c u l a t e d v a l u e s , and thus showed a poorer agreement wi t h the experimental v a l u e s . In g e n e r a l the more approximate method of e v a l u a t i n g the l a t t i c e energy which they used to c a l c u l a t e the heats of formation of mono, d i - , and t r i h a l i d e s of the t r a n s i t i o n metals of the f i r s t l o n g p e r i o d , y i e l d e d heats of formation which showed good 9 7 agreement w i t h experimental v a l u e s only i n the case of c h l o r i d e s . The p r e s e n t e v a l u a t i o n of the l a t t i c e energy y i e l d s heats of formation of f l u o r i d e s which are i n f a i r l y good agreement w i t h experimental v a l u e s . T h i s suggests t h a t agreement w i t h e x p e r i -ment i s obtained through a r a t h e r f o r t u i t o u s c a n c e l l a t i o n of unevaluated f a c t o r s , such as the e f f e c t of v a r i o u s amounts of covalency i n the M-X bonds, r a t h e r than the achievement of a complete e v a l u a t i o n of the l a t t i c e energy through a v a l i d t h e o r e t i c a l e x p r e s s i o n . The heat of formation of vanadium p e n t a f l u o r i d e was not c a l c u l a t e d i n the above manner because vanadium penta-f l u o r i d e i s a l i q u i d at 298°K, hence e v a l u a t i o n of a l a t t i c e energy and a heat of formation from i t i s completely meaningless f o r comparison w i t h the experimental v a l u e . The experimental value of the heat of formation of vanadium p e n t a f l u o r i d e i s i n c l u d e d i n the t a b l e t o f a c i l i t a t e comparison w i t h the heats of formation of vanadium t e t r a f l u o r i d e and niobium p e n t a f l u o r i d e . ( i v ) Thermodynamic S t a b i l i t y of Vanadium F l u o r i d e s Consider f i r s t the d i s p r o p o r t i o n a t i o n of vanadium t e t r a f l u o r i d e i n t o the t r i - and p e n t a f l u o r i d e s ; 2VFij_ > VF^ + V F 3 . The heat of the d i s p r o p o r t i o n a t i o n r e a c t i o n (AH^) i s given by the d i f f e r e n c e between the t o t a l heat of formation of the products and r e a c t a n t s , t h a t i s : A = A H f 0 ( V F 5 ) + A H f 0 ( V F 3 ) - 2 A H f 0 ( V F 4 ) 98 Using the experimental values of -332 and -352 kcal./mole f o r the heats of formation of vanadium t e t r a f l u o r i d e and vanadium p e n t a f l u o r i d e r e s p e c t i v e l y , and the c a l c u l a t e d value of -319 kcal./mole f o r the heat of formation of vanadium t r i f l u o r i d e , the heat of the above d i s p r o p o r t i o n a t i o n r e a c t i o n i s : & H D = -352 - 319 - 2(-332) = -7 kcal./mole. C o n s i d e r i n g A t o be an approximate c r i t e r i o n , the d i s p r o p o r t i o n a t i o n r e a c t i o n Is spontaneous. The assumption t h a t A S.^  i s p o s i t i v e f o r the above r e a c t i o n , as i s the ge n e r a l case f o r r e a c t i o n s which produce gaseous products (89), means tha t AF^, which i s given by A F = AH - T A S , w i l l have a gr e a t e r magnitude than A H^, thus c o n f i r m i n g the s p o n t a n e i t y . C e r t a i n l y the observed r a p i d d i s p r o p o r t i o n a t i o n of vanadium t e t r a f l u o r i d e i n t o vanadium p e n t a f l u o r i d e and vanadium t r i -f l u o r i d e supports the thermodynamic i n d i c a t i o n of s p o n t a n e i t y . Approximating A H^ to AF-jy the e q u i l i b r i u m constant (K) of the above d i s p r o p o r t i o n a t i o n r e a c t i o n can be estimated from the r e l a t i o n A F D < u LE^ = RTlnK rr U s i n g -7 kcal./mole f o r AH^* K = 10 , c e r t a i n l y s t r o n g l y i n favour of d i s p r o p o r t i o n a t i o n . T h i s suggests t h a t vanadium p e n t a f l u o r i d e w i l l not r e a d i l y combine w i t h vanadium t r i f l u o r i d e to form vanadium t e t r a f l u o r i d e . I t i s p o s s i b l e to prepare and study vanadium t e t r a -f l u o r i d e i n s p i t e of the exothermic nature of the d i s p r o p o r -t i o n a t i o n i n t o the t r i - and p e n t a f l u o r i d e s because the a c t u a l 99 s t a b i l i t y of the compound a l s o depends on the k i n e t i c s of the d i s p r o p o r t i o n a t i o n r e a c t i o n . Vanadium t e t r a f l u o r i d e d i s -p r o p o r t i o n a t e s q u i t e slowly, even at 120°, r e q u i r i n g about 20-30 h r s . to decompose approximately f o u r grams, suggesting t h a t the a c t i v a t i o n energy of the r e a c t i o n i s q u i t e h i g h . T h i s may i n d i c a t e t h a t the r e a c t i o n mechanism i n v o l v e s the simultaneous b r e a k i n g of s e v e r a l bonds. A complete k i n e t i c study of t h i s r e a c t i o n would be of i n t e r e s t . Through s i m i l a r c a l c u l a t i o n s the d i s p r o p o r t i o n a t i o n can be shown t o be the favoured mode of decomposition of vanadium t e t r a f l u o r i d e . The heat of d i s s o c i a t i o n of vanadium t e t r a f l u o r i d e i n t o the t r i f l u o r i d e and f l u o r i n e a c c o r d i n g to the equation VF^ — > VF^ + 1/2 F 2 i s given by the r e l a t i o n : A H d i s s < V V = A H f o ( V F 3 } - A H f G ( V F 4 ) Using the experimental value of -332 kcal./mole f o r AH^Q(VF^) and the c a l c u l a t e d value of -319 kcal./mole f o r A H ^ 0 ( V F 3 ) , the heat of d i s s o c i a t i o n i s A H d . s s ( V F 4 ) = -319 - (-332) = +13 kcal./mole That i s , the d i s s o c i a t i o n of vanadium t e t r a f l u o r i d e i n t o the t r i f l u o r i d e and f l u o r i n e i s endothermic and much l e s s favour-able than the d i s p r o p o r ^ t i o n a t i o n i n t o vanadium t r i f l u o r i d e and vanadium p e n t a f l u o r i d e . C e r t a i n l y , there i s no e x p e r i -mental evidence to suggest t h a t vanadium t e t r a f l u o r i d e d i s s o c i a t e s i n t o vanadium t r i f l u o r i d e and f l u o r i n e . 100 The p o s s i b i l i t y of forming vanadium t e t r a f l u o r i d e by d i s s o c i a t i o n of vanadium p e n t a f l u o r i d e can be estimated from the heat of the r e a c t i o n : V F 5 - _ ^ V F 4 + 1/2 F 2 which i s given by: A H d i s s ( V V = A H f 0 ( V F 4 ) - A H f Q ( V F 5 ) . S u b s t i t u t i n g the experimental values f o r the heats of formation of vanadium t e t r a f l u o r i d e and vanadium p e n t a f l u o r i d e g i v e s , f o r the heat of d i s s o c i a t i o n of vanadium p e n t a f l u o r i d e , A H d ± s s ( V F 5 ) = -332 - (-352) = +20 kcal./mole, i n d i c a t i n g t h a t the d i s s o c i a t i o n of vanadium p e n t a f l u o r i d e i n t o vanadium t e t r a f l u o r i d e and f l u o r i n e i s extremely u n l i k e l y . The r e v e r s e r e a c t i o n i s h i g h l y exothermic and thus spontaneous. The r e a c t i o n of vanadium t e t r a f l u o r i d e and f l u o r i n e has been found t o proceed q u i t e r a p i d l y and there i s no experimental evidence t o suggest t h a t vanadium p e n t a f l u o r i d e d i s s o c i a t e s i n t o vanadium t e t r a f l u o r i d e and f l u o r i n e . (v) Thermodynamic S t a b i T r t y of the F i r s t P e r i o d T e t r a f l u o r i d e s F o l l o w i n g the l i n e s of the d i s c u s s i o n of the thermo-dynamic s t a b i l i t y of vanadium f l u o r i d e s , i t i s of i n t e r e s t t o c o n s i d e r the thermodynamic s t a b i l i t y of the t e t r a f l u o r i d e s of the f i r s t l o n g p e r i o d i n g e n e r a l , p a r t i c u l a r l y w i t h r e s p e c t t o the d i s s o c i a t i o n : MF 4 MF, + 1/2 F 2 101 where the heat of r e a c t i o n ( A H) i s given by: A H = A H f 0 ( M F 3 ) - A H f Q ( M P 4 ) The r e q u i r e d heats of formation are taken from Table 11 and are shown w i t h the r e s u l t i n g heats of r e a c t i o n i n Table 12. U n f o r t u n a t e l y , the necessary i o n i z a t i o n p o t e n t i a l s and I o n i c r a d i i to c a l c u l a t e the heats of formation of MF,-compounds are not a v a i l a b l e f o r the f i r s t p e r i o d t r a n s i t i o n elements of group V I I I , t h e r e f o r e the s t a b i l i t y of these t e t r a f l u o r i d e s w i t h r e s p e c t to the d i s p r o p o r t i o n a t i o n >" MF,- + MF 3 could not be e v a l u a t e d . TABLE 12 THE STABILITY OF TETRAFLUORIDES TO THE DISSOCIATION MFj|—»» MF3 + 1/2 F 2 - A H ^ M F ^ -AH^MF^) A H kcal./mole (Table 11) kcal./mole (Table 11) kcal,/mole T i c a l c . e x p t l . 345 315 385 393 + 40 + 78 V c a l c . e x p t l . 319 315 332 - 4 + 13 Cr c a l c . e x p t l . 281 265 233 287 - 48 + 22 Mn c a l c . e x p t l . 261 238 ( c a l c . 165 T 105J l65n ) 105 J - 96-j -156 J - 53i - 33 J Fe c a l c . 288 246-) 167J - 42-j -121 J Co c a l c . e x p t l . 183 187 ( c a l c . 1801 79 J ) 18CK 79J - 1 0 4 J -108-1 the values i n bracke t s r e p r e s e n t upper and lower l i m i t s to the c a l c u l a t e d heat of fo r m a t i o n . 102 The r e s u l t s i n Table 12 are q u i t e s t r i k i n g . The heats of d i s s o c i a t i o n of the t e t r a f l u o r i d e s of t i t a n i u m , vanadium and chromium i n t o the t r i f l u o r i d e s of these elements are q u i t e endothermic, p a r t i c u l a r l y i f the a v a i l a b l e experimental v a l u e s are chosen i n p r e f e r e n c e to c a l c u l a t e d heats of form-a t i o n . Heats of r e a c t i o n obtained from c a l c u l a t e d heats of formation (AH^o) tend to overestimate the e x o t h e r m i c i t y of the d i s s o c i a t i o n and to suggest t h a t the d i s s o c i a t i o n i s more f e a s i b l e than i t a c t u a l l y i s . Beyond chromium t e t r a f l u o r i d e , the c a l c u l a t e d heat of r e a c t i o n ( AH) becomes h i g h l y exo-thermic, and even c o n s i d e r i n g the tendency to o v e r e s t i m a t i o n of e x o t h e r m i c i t y i t i s apparent t h a t manganese t e t r a f l u o r i d e and i r o n t e t r a f l u o r i d e w i l l show a s t r o n g tendency to d i s s o -c i a t e i n t o the t r i f l u o r i d e s and f l u o r i n e . Manganese t e t r a -f l u o r i d e has r e c e n t l y been prepared (92) and does indeed d i s s o c i a t e i n t o manganese t r i f l u o r i d e and f l u o r i n e (93). Iron and c o b a l t t e t r a f l u o r i d e s have not been prepared. ( v i ) Bond E n e r g i e s of Vanadium F l u o r i d e s I f the heat of d i s s o c i a t i o n of a compound i s d e f i n e d as the heat r e q u i r e d to d i s s o c i a t e the molecule i n t o i t s constituent 1' atoms i n the gas phase, i t can be shown t h a t t h i s i s r e l a t e d to the heat of formation of the compound by a simple Born-Haber c y c l e . The c y c l e i s i l l u s t r a t e d below f o r a compound of the type MF . 103 MX n(c) >- M(g) + nX(g) A H F 0 - ( L + f D ( X 2 ) ) M(s) + -g X 2(g) where A H^ Is the heat of d i s s o c i a t i o n of MX D n L i s the heat of s u b l i m a t i o n of M D(X 2) i s the heat of d i s s o c i a t i o n of the halogen X 2 and A i s the standard heat of formation of MX^. Since the sum of a l l terms i n the c y c l e must be zero, the heat of d i s s o c i a t i o n i s given by: A Hp = - A H f 0(MX n) + L, + | D(X 2) . (7-6) The bond energy i s d e f i n e d as the average d i s s o c i a t i o n energy per M-X bond, eg. =AH-p/n, and i s given by the ex p r e s s i o n : b n - A H f 0(MX n) + L + \ D(X 2) ( 7 - 7 ) The bond energi e s of vanadium and niobium f l u o r i d e s have been c a l c u l a t e d by s u b s t i t u t i n g the a p p r o p r i a t e thermochemical data i n t o equation ( 7 - 7 ) . The d i s s o c i a t i o n energy of f l u o r i n e ( D ( F 2 ) ) was taken as 38 kcal./mole ( l l ) . The r e s u l t s are shown i n Table 13. 104 TABLE 13 BOND ENERGIES OF SOME GROUP IV AND V FLUORIDES MF n V F 3 V F 4 VFV 5 NbFp TIF ZrF 4 4 L - A H f 0 ( e x p t l ) k c a l . _ k c a l . 122 319+30(calc) 122 332 122 352 185 432 112 393 125 457 A H D k c a l . E b k c a l . (498+30) (166+10) 530 569 712 581 677 132 114 142 145 169 Reference ( A H f 0 e x p t l . ) c a l c . ( P r e s e n t work) } Present work. 31 28 26 The bond energie s of the vanadium c h l o r i d e s , c a l c u l a t e d i n the same manner, u s i n g D(Clg) = 58 kcal./mole ( l l ) , are shown i n Table 14 f o r comparison. TABLE 14 BOND ENERGIES OF VANADIUM CHLORIDES MF n k c a l . VC1, 1 VC1 3 >122 VC1 4 A H f 0 k c a l . A H D k c a l . E b k c a l . Reference ( A H f 0 e x p t l ; ) 108 288 144 83 137 346 115 136 374 93 84 The r e s u l t s i n Tables 13 and 14 show t h a t the bond ene r g i e s of vanadium f l u o r i d e s are approximately kofo g r e a t e r than those of vanadium c h l o r i d e s w i t h the same v a l e n c e . T h i s 105 i n c r e a s e i n bond energy from c h l o r i d e to f l u o r i d e f o l l o w s the probable i n c r e a s e In i o n i c c h a r a c t e r of the metal-halogen bond, s i n c e i o n i c c h a r a c t e r w i l l tend to decrease w i t h i n c r e a s -i n g p o l a r i z a b i l i t y and s i z e of the halogen, i e . from c h l o r i d e t o f l u o r i d e . The bond energy w i t h i n the c h l o r i d e and f l u o r i d e s e r i e s decreases w i t h i n c r e a s i n g valence of the metal, thus the bond energy again decreases w i t h i o n i c c h a r a c t e r of the metal-halogen bond. The bond energy of the f l u o r i d e s a l s o i n c r e a s e s from vanadium p e n t a f l u o r i d e t o niobium p e n t a f l u o r i d e , and from t i t a n i u m t e t r a f l u o r i d e t o z i r c o n i u m t e t r a f l u o r i d e , t h a t i s the bond energy i n c r e a s e s w i t h i n c r e a s i n g i o n i c c h a r a c t e r of the bond. These l a s t two trends can be r e l a t e d t o the i n c r e a s e of i o n i c bond c h a r a c t e r w i t h i n c r e a s i n g i o n i c s i z e . Thus lower valence i o n s , w i t h a l a r g e r metal i o n s i z e , w i l l form bonds of g r e a t e r i o n i c c h a r a c t e r than h i g h valence i o n s , and elements i n the second long p e r i o d w i l l , due to t h e i r g r e a t e r i o n i c s i z e , form bonds of g r e a t e r i o n i c c h a r a c t e r than elements i n the f i r s t l o n g p e r i o d . The trends e x h i b i t e d by the bond e n e r g i e s i n Tables 13 and 14 are c o n s i s t e n t w i t h the g e n e r a l l y expected behaviour of metal h a l i d e bonds „ ( l05) . The r e l i a b i l i t y of the p r e c e d i n g thermodynamic c a l -c u l a t i o n s on vanadium f l u o r i d e s would be g r e a t l y i n c r e a s e d i f the c a l c u l a t e d heat of formation of vanadium t r i f l u o r i d e c o uld be r e p l a c e d by an experimental v a l u e . U n f o r t u n a t e l y the heat of h y d r o l y s i s cannot be measured i n the same manner 1 0 6 as f o r the t e t r a f l u o r i d e and p e n t a f l u o r i d e because of the extreme slowness of the d i s s o l u t i o n of vanadium t r i f l u o r i d e i n water. D i r e c t measurement of the heat of combination of the elements i n a bomb a c c o r d i n g t o the methods of the workers at the Argonne l a b o r a t o r y ( 2 6 ) would not be s t r a i g h t f o r w a r d as the high e r f l u o r i d e s are more l i k e l y t o be formed because of the l a r g e excess of f l u o r i n e present i n the bomb. Perhaps the r e a c t i o n of vanadium metal w i t h hydrogen f l u o r i d e i n a bomb c a l o r i m e t e r would p r o v i d e a means of o b t a i n i n g the heat of formation of vanadium t r i f l u o r i d e as M u e t t e r t i e s and C a s t l e ( 9 4 ) have r e p o r t e d t h a t t h i s r e a c t i o n a p p a r e n t l y l e a d s to q u a n t i t a t i v e formation of vanadium t r i f l u o r i d e . I f t h i s can be done the r e s u l t would be of i n t e r e s t as i t would complete the s e r i e s of heats of formation of the vanadium f l u o r i d e s , and pro v i d e a f u r t h e r check of the r e l i a b i l i t y of the heats of formation obtained from estimated l a t t i c e e n e r g i e s . CHAPTER 8 : EXPERIMENTAL GENERAL TECHNIQUES Most of the e a r l y l i t e r a t u r e c o n t a i n s r e p o r t s t h a t f l u o r i d e s a t t a c k g l a s s , thus manipulation of these f l u o r i d e s r e q u i r e d metal or f l u o r o c a r b o n p l a s t i c apparatus. Recent s t u d i e s (6,7) have shown, however, t h a t when f r e e d of a l l t r a c e s of moisture, v o l a t i l e f l u o r i d e s can be r e a d i l y mani-p u l a t e d i n g l a s s apparatus. The marked s u s c e p t i b i l i t y of v o l a t i l e f l u o r i d e s to moisture i n g l a s s apparatus a r i s e s from an extremely r a p i d , s e l f - p r o p a g a t i n g chain r e a c t i o n i n v o l v i n g the g l a s s i t s e l f . eg. MF + H„0 — * • MOF 0 + 2 HP & n 2 n-2 4HP + S i 0 2 — ^ S i P ^ + 2H20 MP^ + HgO • *- e t c . Thus even minute t r a c e s of moisture w i l l e v e n t u a l l y r e s u l t i n complete h y d r o l y s i s of the f l u o r i d e . Any study of the p r o p e r t i e s of v o l a t i l e f l u o r i d e s t h e r e f o r e r e q u i r e s tech-niques which permit the ready man i p u l a t i o n and p u r i f i c a t i o n of the f l u o r i d e s by simple t r a p - t o - t r a p d i s t i l l a t i o n s i n m o i s t u r e - f r e e vacuum systems. Hydrocarbon vacuum greases cannot be t o l e r a t e d as they are a t t a c k e d by most f l u o r i d e s , and f l u o r o c a r b o n greases, while w i t h s t a n d i n g chemical a t t a c k by v o l a t i l e f l u o r i d e s , o f t e n c o n t a i n t r a c e s of moisture and thus must a l s o be avoided. 108 Such c o n d i t i o n s can e a s i l y be achieved w i t h simple a l l - g l a s s systems c o n s i s t i n g of a number of t r a p s connected w i t h combinations of c a p i l l a r y s e a l - o f f c o n s t r i c t i o n s and b r e a k s e a l s i n s t e a d of the greased taps used i n c o n v e n t i o n a l vacuum systems. These a l l - g l a s s systems can be r e a d i l y d r i e d p r i o r to use by evacuation and h e a t i n g the g l a s s w i t h a blowtorch. A f t e r the system has been thoroughly d r i e d under vacuum, the c a p i l l a r y l e a d i n g to the vacuum manifold i s s e a l e d o f f l e a v i n g an evacuated, a l l - g l a s s , grease and m o i s t u r e - f r e e system i n which the f l u o r i d e can be r e a d i l y manipulated. M a t e r i a l s can be t r a n s f e r r e d from one system to another by s e a l i n g i n t o a t r a p equipped w i t h a b r e a k s e a l , which i s then connected t o the new system. A f t e r the new t r a p l i n e has been thoroughly d r i e d the b r e a k s e a l i s broken w i t h a g l a s s covered, m a g n e t i c a l l y operated breaker and the f l u o r i d e t r a n s f e r r e d by d i s t i l l a t i o n . Many simple p h y s i c a l measure-ments can a l s o be made w i t h standard g l a s s apparatus modi-f i e d t o p r o v i d e a completely dry and e n c l o s e d system. The disadvantage of such techniques i s that each experiment r e q u i r e s the c o n s t r u c t i o n of a new vacuum t r a p l i n e . In some cases t h i s disadvantage can be overcome w i t h the use of a standard vacuum system i n which greased stopcocks are r e p l a c e d by p a c k l e s s metal bellows v a l v e s (6) (eg. Hoke type 431) which are u n i t e d to the g l a s s w i t h m e t a l - g l a s s s e a l s , hard s o l d e r e d to the metal v a l v e . , Metal v a l v e s however, are not u n i v e r s a l l y a p p l i c a b l e t o the study of 109 f l u o r i d e s as the metals are sometimes s u b j e c t to a t t a c k , p a r t i c u l a r l y by the more a c t i v e f l u o r i d e s o l v e n t s . The v a l v e s are a l s o d i f f i c u l t t o c l e a n , e s p e c i a l l y I f the bellows becomes d i r t y , and the metal t o Pyrex s e a l s are a p o i n t of weakness being s u b j e c t to breakage and the development of p i n h o l e l e a k s . The c a r e f u l d r y i n g of the manipulative system i s of l i t t l e e f f e c t i f the v o l a t i l e f l u o r i d e to be s t u d i e d a l r e a d y c o n t a i n s t r a c e s of moisture. Every e f f o r t must be made to prepare the f l u o r i d e under s t r i c t l y anhydrous con-d i t i o n s , a p p l y i n g somewhat the same means as o u t l i n e d above f o r d r y i n g the p r e p a r a t i v e apparatus, w i t h a few changes i n procedure because p r e p a r a t i o n s are o f t e n most c o n v e n i e n t l y done at atmospheric pressure i n a flow system. C a r e f u l p r e l i m i n a r y d r y i n g of the apparatus and reagents under vacuum f o l l o w e d by admission of dry n i t r o g e n and opening the system to the atmosphere only through a s e r i e s of c o l d t r a p s i s u s u a l l y adequate to exclude moisture from even excep-t i o n a l l y s e n s i t i v e m a t e r i a l s such as vanadium p e n t a f l u o r i d e . H y d r o l y s i s can be minimised by s t o r i n g the f l u o r i d e i n contact w i t h dry sodium f l u o r i d e , which combines w i t h hydrogen f l u o r i d e to form the s t a b l e sodium b i f l u o r i d e , and thus i n t e r r u p t s the h y d r o l y s i s c y c l e o u t l i n e d above. Obviously, to be used i n t h i s manner the sodium f l u o r i d e must not form a s t a b l e complex w i t h the v o l a t i l e f l u o r i d e . 110 S t u d i e s on i n v o l a t i l e , s o l i d f l u o r i d e s , which are g e n e r a l l y l e s s r a p i d l y a t t a c k e d by moisture, r e q u i r e s i m i l a r p r e c a u t i o n s a g a i n s t h y d r o l y s i s . T r a n s f e r s are made i n a dry-box i n which a dry atmosphere i s maintained through the use of phosphoric oxide d e s i c c a n t . C a r e f u l l y d r i e d apparatus and c o n t a i n e r s should be used and the manipulations should be performed as q u i c k l y as p o s s i b l e . A l l reagents p l a c e d i n contact w i t h h y d r o l y s a b l e f l u o r i d e s should be r i g o r o u s l y d r i e d p r i o r to use. In t h i s i n v e s t i g a t i o n s o l i d f l u o r i d e s were g e n e r a l l y c h a r a c t e r i s e d by X-ray powder photography, magnetic s u s c e p t i -b i l i t y and a n a l y s i s . X-ray powder photographs were taken on a General E l e c t r i c X-ray u n i t u s i n g a copper t a r g e t and n i c k e l f i l t e r s . The camera, a l s o s u p p l i e d by General E l e c t r i c , was of con-v e n t i o n a l design w i t h a circumference of 45 cm. Samples were se a l e d i n dry, t h i n - w a l l e d , 0.5 nim. diameter Pyrex or q u a r t z c a p i l l a r i e s (Pantak Ltd.) to prevent h y d r o l y s i s d u r i n g the exposure. Magnetic s u s c e p t i b i l i t i e s were measured w i t h a Gouy balance i n c o r p o r a t i n g a V a r i a n #4004 electromagnet w i t h 2 i n c h diameter tapered p o l e f a c e s , which generated a f i e l d of approx-i m a t e l y 8,000 gauss. Most s u s c e p t i b i l i t y measurements were done at room temperature u s i n g m e r c u r i - c o b a l t i t e t r a t h i o -cyanate ( I I ) (96) as a c a l i b r a t i o n standard. Where i n d i c a t e d , the temperature dependence of the magnetic s u s c e p t i b i l i t y was measured i n the range 80° to 300°K u s i n g a thermostatted sample I l l compartment, adapted from the design of F i g g i s and Nyholm (97)• The p a r t i c u l a r s of t h i s apparatus have been d e s c r i b e d elsewhere (98). The analyses were done w i t h standard techniques. Vanadium was estimated by t i t r a t i o n of vanadium ( I V ) , p r e v i o u s l y reduced w i t h sulphur d i o x i d e i f necessary, w i t h standard (0.01N) potassium permanganate i n approximately 2N s u l p h u r i c a c i d s o l u t i o n . Sulphur d i o x i d e , i f used, was removed by b o i l i n g the s o l u t i o n p r i o r to t i t r a t i o n . R a t i o s of vanadium (IV) t o vanadium (V) were determined by t i t r a t i n g the sample immediately a f t e r h y d r o l y s i s , f o l l o w e d by r e d u c t i o n of the t i t r a t e d sample w i t h sulphur d i o x i d e and t i t r a t i o n as — — - 2 above. I n t e r f e r i n g anions (such as I~, 10^ ^eO^ > e t c . ) were removed by r e d u c i n g a l l the vanadium t o vanadium (IV) with sulphur d i o x i d e f o l l o w e d by a d s o r p t i o n of the vanadyl (VO ) c a t i o n on a Dowex 50W c a t i o n exchange r e s i n . The anions were washed out of the column w i t h d i s t i l l e d water; the vanadium was e l u t e d w i t h 6N s u l p h u r i c a c i d ; the vanadyl s o l u t i o n was then d i l u t e d and t i t r a t e d w i t h standard perman-ganate s o l u t i o n . F l u o r i d e was best estimated (56), by d i s t i l l a t i o n as f l u o r o s i l i c i c a c i d from a constant b o i l i n g mixture (135°) of p e r c h l o r i c a c i d (99), f o l l o w e d by p r e c i p i t a t i o n as l e a d chlorxde f l u o r i d e from an a l k a l i n e (sodium a c e t a t e b u f f e r ) s o l u t i o n , c o n t a i n i n g an excess of c h l o r i d e i o n and l e a d n i t r a t e . The p r e c i p i t a t e was d i g e s t e d at 95° f o r one hour, 112 f i l t e r e d on a t a r e d g l a s s c r u c i b l e and weighed. Selenium was determined by p r e c i p i t a t i o n of the element from concentrated (3N) h y d r o c h l o r i c a c i d s o l u t i o n and weighing on a t a r e d g l a s s c r u c i b l e . PREPARATION AND PHYSICAL PROPERTIES OF VANADIUM PENTAFLUORIDE ( i ) The P r e p a r a t i o n of Vanadium P e n t a f l u o r i d e Vanadium p e n t a f l u o r i d e was prepared by f l u o r i n a t i o n of vanadium metal as p r e v i o u s l y d e s c r i b e d (j,!^). F l u o r i n e was s u p p l i e d by A l l i e d Chemical Company i n a s t e e l gas c y l i n d e r which was permanently i n s t a l l e d i n a wal k - i n fume hood. The pressure r e g u l a t i n g and gas d e l i v e r y system has been d e s c r i b e d elsewhere (56). Vanadium metal was p l a c e d i n a n i c k e l boat and i n s e r t e d i n t o a one i n c h diameter n i c k e l tube, twelve inches long, which was p l a c e d m a tube furnace and connected to the f l u o r i n e supply l i n e . The e x i t of the n i c k e l r e a c t o r tube was connected to a g l a s s t r a p l i n e , c o n s i s t i n g of f o u r or f i v e t r a p s equipped w i t h b r e a k s e a l s and c a p i l l a r y c o n s t r i c t i o n s . The connection was made by means of a n i c k e l f l a n g e to which was welded one h a l f of the body of a 3«*/4 i n c h brass compression u n i o n . The f l a n g e was b o l t e d to the n i c k e l r e a c t o r , the s e a l being made w i t h a compressible l e a d washer. The t r a p l i n e was con-nected to the f l a n g e w i t h a neoprene gasket and the compression f i t t i n g nut. T h i s p r o v i d e d a very l a r g e diameter e x i t from the r e a c t o r tube, thus p r e v e n t i n g the blockages due to conden-s a t i o n of l e s s v o l a t i l e f l u o r i d e s which f r e q u e n t l y o c c u r r e d i n the p r e v i o u s apparatus (56) which had a narrow e x i t tube. The apparatus was evacuated and "flamed-out" to remove a l l t r a c e of moisture. Dry n i t r o g e n gas was admitted, the vanadium metal heated to 350-360°C, and f l u o r i n e passed over the heated metal. The v o l a t i l e p e n t a f l u o r i d e was condensed i n t r a p s cooled to -78° w i t h a l c o h o l - d r y - i c e baths. A f t e r the r e a c t i o n was completed the f l u o r i n e was shut o f f and n i t r o -gen passed to f l u s h any t r a c e s of f l u o r i n e out of the system. The system was evacuated and the t r a p s c o n t a i n i n g the product were se a l e d o f f . The impure product obtained from t h i s r e a c t i o n , l a r g e l y a mixture of vanadium t e t r a f l u o r i d e and vanadium p e n t a f l u o r i d e , was connected to a simple d i s t i l l a t i o n system c o n t a i n i n g sodium f l u o r i d e , and v o l a t i l e m a t e r i a l s were r e p e a t e d l y d i s -t i l l e d from sodium f l u o r i d e . The p u r i f i e d vanadium penta-f l u o r i d e was s t o r e d In s e a l e d g l a s s t r a p s equipped w i t h break-s e a l s . The p u r i f i e d v o l a t i l e m a t e r i a l was t y p i c a l l y a white s o l i d , m e l t i n g about room temperature (m.p. 19.5°(7)) to a straw c o l o u r e d l i q u i d . A t y p i c a l a n a l y s i s gave V, 35.3; F, 63.2$. C a l c . f o r V F 5 : V, 3^.95; F, 65.05$. The r e s i d u e remaining i n the t r a p i n which the products were f i r s t condensed was a l i g h t brown powder which was a mixture of vanadium t e t r a f l u o r i d e and vanadium 114 t r i f l u o r i d e , the former predominating, as shown by X-ray powder photography and a n a l y s i s (Pound: V, 42.1$. C a l c . f o r VP^: V, 40.3; f o r VP 3: V, 47.3$.) ( i i ) The D e n s i t y of Vanadium P e n t a f l u o r i d e A s i l i c a d i l a t o m e t e r , c o n s i s t i n g of a bulb a p p r o x i -mately 6 mm. diameter, 2 cm. long, and a 2 mm. diameter c a p i l l a r y 13 cm. long, w i t h a r e f e r e n c e mark engraved at the base of the c a p i l l a r y , was c a l i b r a t e d w i t h mercury. The volume of the d i l a t o m e t e r was given by the r e l a t i o n : V = 0.6889 + 0.02883h c c . where V i s the volume of l i q u i d i n c c . contained i n the d i l a t o m e t e r and h i s the height of the l i q u i d , i n cm. above the engraved r e f e r e n c e mark. The d i l a t o m e t e r was s e a l e d to a t r a p l i n e and d r i e d by h e a t i n g under vacuum. S u f f i c i e n t vanadium p e n t a f l u o r i d e to b r i n g the l e v e l of l i q u i d i n t o the c a p i l l a r y was d i s t i l l e d i n t o the d i l a t o m e t e r and i t was sealed o f f and weighed. The d i l a t o m e t e r was immersed i n a t h e r m o s t a t i c a l l y c o n t r o l l e d water bath (+ 0.05°* v a r i a t i o n ) and the height of l i q u i d i n the c a p i l l a r y measured w i t h a v e r t i c a l t r a v e l l i n g microscope out-side the bath. The h e i g h t of l i q u i d i n the c a p i l l a r y was measured at each temperature, w i t h both i n c r e a s i n g and d e c r e a s i n g temperature to check on r e p r o d u c i b i l i t y . The 115 temperature was v a r i e d from 20° t o 45°, t h i s being the l i q u i d range of vanadium p e n t a f l u o r i d e . A f t e r the s e r i e s of volume measurements was completed, the vanadium p e n t a f l u o r i d e was f r o z e n i n the d i l a t o m e t e r . The d i l a t o m e t e r was then c a r e f u l l y broken open and the contained vanadium p e n t a f l u o r i d e d i s t i l l e d out under vacuum. The p i e c e s of the d i l a t o m e t e r were weighed to o b t a i n the weight of the vanadium p e n t a f l u o r i d e by d i f f -erence. Two experiments were performed and the r e s u l t s are shown, i n c h r o n o l o g i c a l order, i n Table 15. TABLE 15 DENSITY OF VANADIUM PENTAFLUORIDE RUN I (2.1834 g.VF 5) RUN I I (2.0754 g.VF 5) Temperature Volume De n s i t y Temperature Volume Density °C cc. g/cc. °C cc . g/cc. 20.0 O.8706 2.508 20.6 0.8330 2.491 25.2 0.8772 2.489 25.3 0.8388 2.474 30.2 0.8811 2.478 30.9 0.8440 2.459 34.7 0.8887* 2.457* 35.1 0.8492 2.444 39.8 O.8965 2.435 40.4 O.8563 2.424 44.9 0.9043 2.414 45.0 0.8626 2.406 30.1 0 .8841 2.470 24.3 0.8771 2.489 37.7 0.8522 2.435 20.4 0.8733 2.500 30.3 0.8431 2.462 26.6 0.8797 2.482 25.0 0.8371 2.479 33.4 0.8890 2.456 20.2 0.8323 2.494 38.4 0 .8948 2 .440 43.0 0.9018 2.421 25.0 O.8780 2.485 average of two d e t e r m i n a t i o n s . 116 In the range 25° t o 40° the graphs of volume (V) and d e n s i t y ( ) a g a i n s t temperature ( t ) were l i n e a r and obeyed the equations: 1.215 X 10~3 cc/deg. -0.00341 g/cc/deg. 2.479 - 0.0034l(t-25°) The average e x p r e s s i o n f o r the d e n s i t y from the two e x p e r i -ments, i n c l u d i n g standard d e v i a t i o n s , i s : <^= 2.483 (+0.004) - 0.00349(t-25°)+0.00008 g./cc. and the average c o e f f i c i e n t of expansion i s : | ^ = 1.263(+0.05) x 10" 3 cc./deg. ( l i i ) The V i s c o s i t y of Vanadium P e n t a f l u o r i d e A viscometer based on the Ostwald design was c o n s t r u c t e d w i t h s p e c i a l p r o v i s i o n s f o r h a n d l i n g a v o l a t i l e , e a s i l y hydro-l y s e d l i q u i d . The Pyrex viscometer c o n s i s t e d of a 15 cm. l e n g t h of one mm. c a p i l l a r y between an upper bulb of approx-i m a t e l y one ml. c a p a c i t y and a lower bulb of about two ml. c a p a c i t y . C a l i b r a t i o n marks were p r o v i d e d above and below the upper b u l b . The c a p i l l a r y was bent i n t o a U-shape near to the l a r g e bulb so t h a t the l i q u i d flowed i n t o the lower r e s e r v o i r a t the bottom of t h i s r e s e r v o i r . An 8 mm. diameter tube, dV, _ o dV p = 1.312 X 10 j cc/deg. do d o p ^ = -0.00356 g/cc/deg. -g^-^ ± = 2.487 - 0.00356 (t-25°) ^ 2 117 j o i n e d t o the top of the l a r g e r e s e r v o i r , was p a r a l l e l t o the c a p i l l a r y tube. A metal tap (Hoke type 431) was j o i n e d t o the top of the 8 mm. tube by means of a Kovar-Pyrex graded s e a l . A second tap, below t h i s f i r s t tap and a f f i x e d i n the same manner, p r o v i d e d a connection between the two arms of the viscometer which p e r m i t t e d a v a r i a t i o n i n the r e l a t i v e pres-sure between the arms. The apparatus was s e a l e d t o a vacuum l i n e and "baked" under vacuum a t 100° f o r two days to ensure complete d r y i n g . Two ml. of vanadium p e n t a f l u o r i d e was d i s t i l l e d i n t o the viscometer, which was then s e a l e d o f f . The viscometer was seal e d t o a vacuum l i n e c o n s i s t i n g of an evacuated 5 l i t r e bulb, a 5 l i t r e bulb w i t h dry n i t r o g e n a t atmospheric p r e s s u r e and the necessary arrangement of metal taps, through the main tap on the viscome t e r . With the tap connecting the two arms of the viscometer c l o s e d , the a p p l i c a t i o n of pressure through the main tap f o r c e d the l i q u i d i n t o the upper chamber. Open-i n g the tap between the two arms of the viscometer e q u a l i s e d the pressure on both s i d e s of the c a p i l l a r y and allowed the l i q u i d t o flow under the i n f l u e n c e of i t s h y d r o s t a t i c p ressure i n t o the lower r e s e r v o i r . Reducing the pressure by opening the viscometer to the evacuated bulb allowed r e p e t i t i o n of t h i s procedure without r e q u i r i n g i n c r e a s i n g l y g r e a t e r p r e s s u r e s of i n e r t gas. The viscometer was immersed i n a thermostatted water bath and where p o s s i b l e s e v e r a l measurements of the flow time f o r each temperature were made. The r e s u l t s , shown i n Table 16, were f a i r l y r e p r o d u c i b l e and a reasonable s t r a i g h t 118 l i n e can be drawn (F i g u r e l a ) . Measurements were made w i t h ascending and descending temperature and are given i n chrono-l o g i c a l order i n Table 16. D e n s i t i e s were c a l c u l a t e d from the p r e c e d i n g equation. TABLE 16 VISCOSITY OF VANADIUM PENTAFLUORIDE Temperature No. of d e t n S of D e n s i t y V x s c o s i t y flow time (g./cc) ( c e n t i p o i s e ) 25.35 3 2.482 124.2 25.45 2 2.482 125.7 31.9 1 2.458 86.8* 32.0 3 2.458 76.4 27.3 1 2.475 110.4. 27.3 1 2.475 120.0 these p o i n t s d e v i a t e from the s t r a i g h t l i n e by about 10$. The v i s c o s i t y of vanadium p e n t a f l u o r i d e can be expressed by the equation: = 124 - 7.2(t-25°) c e n t i p o i s e . The o c c a s i o n a l p o i n t at each temperature except 25° d e v i a t e d c o n s i d e r a b l y (about 10$) from the s t r a i g h t l i n e , thus the r e p r o d u c i b i l i t y i s not e x c e p t i o n a l l y good. Considerable experimental d i f f i c u l t i e s were encountered through the v o l a -t i l i t y of vanadium p e n t a f l u o r i d e which s e r i o u s l y l i m i t e d the a c c e s s i b l e temperature range and a l s o caused some of the p e n t a f l u o r i d e to d i s t i l l out of the viscometer d u r i n g the pressure r e d u c t i o n s t e p s . Decomposition of the vanadium 119 p e n t a f l u o r i d e caused an i n c r e a s i n g darkening of the l i q u i d c o l o u r d u r i n g the experiment. Formation of s o l i d s d u r i n g t h i s decomposition may have been r e s p o n s i b l e f o r the d e v i a t i o n s from the s t r a i g h t l i n e p l o t . C a l i b r a t i o n of the viscometer i s i d e a l l y done w i t h a l i q u i d of s i m i l a r d e n s i t y and v i s c o s i t y . Concentrated s u l p h u r i c a c i d (94.3 weight-percent, d e n s i t y 1.8318 g./cc.) was the only l i q u i d of r e l i a b l y known v i s c o s i t y which was at a l l c l o s e to matching the d e n s i t y and s u r f a c e t e n s i o n of vanadium p e n t a f l u o r i d e . The necessary e x t r a p o l a t i o n from the v i s c o s i t y of s u l p h u r i c a c i d (19.8 c e n t i p o i s e ) to t h a t of vanadium p e n t a f l u o r i d e (<^ 100 c e n t i p o i s e ) i s another source of e r r o r . C o n s i d e r i n g the l a r g e e r r o r s a r i s i n g from these sources, the v i s c o s i t y of vanadium p e n t a f l u o r i d e given by the above equation i s probably r e l i a b l e w i t h i n +20$. ( i v ) The Surface Tension of Vanadium P e n t a f l u o r i d e Two types of c a p i l l a r y r i s e apparatus were used. In the f i r s t apparatus a bundle of f o u r c a p i l l a r i e s r a n g i n g i n s i z e from approximately 1.5 mm. to 0.5 mm. i n t e r n a l diameter were p l a c e d i n a s i l i c a bulb, and thoroughly d r i e d . About two ml. of vanadium p e n t a f l u o r i d e were d i s t i l l e d i n t o the b u l b . The bulb was s e a l e d o f f , immersed i n a t h e r m o s t a t i c a l l y con-t r o l l e d bath and the h e i g h t of l i q u i d i n the c a p i l l a r i e s above the s u r f a c e of l i q u i d i n the bulb was determined w i t h a v e r t i c a l t r a v e l l i n g microscope as b e f o r e . Only three of the c a p i l l a r i e s gave a measurable r i s e of the l i q u i d and at any 120 one temperature i t was not p o s s i b l e t o measure the r i s e i n a l l of these three c a p i l l a r i e s due to the formation of bubbles i n the c a p i l l a r i e s . The temperature was v a r i e d from 20° to 40° and readings taken w i t h ascending and descending temperature. A f t e r the c a p i l l a r y r i s e of vanadium p e n t a f l u o r i d e had been measured, the apparatus was emptied, cleaned and c a l i b r a t e d by measuring the c a p i l l a r y r i s e f o r d i s t i l l e d water i n the same manner. A second experiment was done w i t h a d i f f e r e n t i a l c a p i l l a r y tensiometer, s i m i l a r t o t h a t shown i n F i g u r e 156(a) of Dodd and Robinson (100), w i t h two d i f f e r e n t c a p i l l a r i e s , 1.0 and 0.5 mm. diameter, s e a l e d t o a bulb of about 2 ml. c a p a c i t y . A s i d e arm of 5 mm. t u b i n g was a l s o s e a l e d to the sample bulb to f a c i l i t a t e the condensation of the sample i n the bul b . The apparatus was s e a l e d t o a simple d i s t i l l a t i o n system, to which was a l s o s e a l e d a b r e a k s e a l b o t t l e c o n t a i n i n g vanadium p e n t a f l u o r i d e . The e n t i r e apparatus was evacuated and the tensiometer was heated to 100° f o r two days under vacuum to ensure complete d r y i n g of the c a p i l l a r i e s . The remainder of the apparatus was heated w i t h a t o r c h i n t e r m i t t e n t l y d u r i n g t h i s p e r i o d as u s u a l . S u f f i c i e n t vanadium p e n t a f l u o r i d e t o f i l l the sample bulb was d i s t i l l e d i n t o the tensiometer which was then s e a l e d o f f and immersed i n a thermostatted water bath. The d i f f e r -e n t i a l c a p i l l a r y r i s e was measured w i t h the t r a v e l l i n g micro-scope as b e f o r e . A f t e r the c a p i l l a r y r i s e of vanadium penta-f l u o r i d e had been measured, the vanadium p e n t a f l u o r i d e was 121 d i s t i l l e d out under vacuum and the apparatus was c a l i b r a t e d by measuring the c a p i l l a r y r i s e f o r d i s t i l l e d water and benzene as b e f o r e . The s u r f a c e t e n s i o n ("ft ) of vanadium p e n t a f l u o r i d e was c a l c u l a t e d from the e x p r e s s i o n : v = f A h ? where f i s the apparatus f a c t o r Ah i s the d i f f e r e n c e i n l e v e l of l i q u i d i n each of the two c a p i l l a r i e s or the h e i g h t of l i q u i d i n the c a p i l l a r y above the l e v e l of l i q u i d i n the r e s e r v o i r . and ^ i s the d e n s i t y of the l i q u i d . The apparatus f a c t o r ( f ) was determined f o r each apparatus by measuring the 6h of a l i q u i d of known d e n s i t y and s u r f a c e t e n s i o n such as water or benzene. The d e n s i t y of vanadium p e n t a f l u o r i d e a t each temperature was c a l c u l a t e d from the p r e v i o u s l y determined e x p r e s s i o n . The r e s u l t s of both su r f a c e t e n s i o n experiments are given i n Table 17 and the temperature v a r i a t i o n of the s u r f a c e t e n s i o n i s shown g r a p h i -c a l l y i n F i g u r e l b . 122 TABLE 17 SURFACE TENSION OF VANADIUM PENTAFLUORIDE Experiment A Experiment B Temperature Surface Tension o. 24.5 29.7 34.8 4o.o dynes/cm.'-I I I 17.96 16.75 12.35 18.35 I I I 17.75 Temperature Surface Tension o p dynes/cm'-24.75 29.5 37.0 18.10 17.15 16.49 Apparatus F a c t o r 11.12 27.47 46.45 17.65 L e a s t squares analyses of the data f o r both experiments f o r p o i n t s i n the range 24.5 to 37.0 g i v e s f o r the surface t e n s i o n of vanadium p e n t a f l u o r i d e : Y = 18.2: - 0.l42(t-25°) dynes/cm" w i t h a probable e r r o r of + 0.2 and a standard d e v i a t i o n of + 0.3. The molar s u r f a c e energy (V ) was c a l c u l a t e d from the ex p r e s s i o n : V = ( M v ) 2 / 3 where M i s the molecular weight v Is the s p e c i f i c volume which i s equal to the r e c i p r o c a l of the d e n s i t y and & i s the sur f a c e t e n s i o n . The val u e s of the s u r f a c e t e n s i o n and d e n s i t y of vanadium p e n t a f l u o r i d e were taken from the pr e v i o u s e x p r e s s i o n s . The r e s u l t s are shown i n Table 18. 123 TABLE 18 MOLAR SURFACE ENERGY OF VANADIUM PENTAFLUORIDE Temperature D e n s i t y ( c a l c . ) Surface Tension Molar Surface c a l c . f Energy ( C) (g/cc.) (dynes/cm--) (dynes) 20 2.500 18.92 284.5 25 2.483 18.22 275.4 30 2.466 17.50 265.9 35 2.448 16.77 256.4 A graph of the molar s u r f a c e energy ( " P ) a g a i n s t temperature y i e l d s a s t r a i g h t l i n e w i t h a slope: dt (v) The I n f r a r e d Spectrum of Vanadium P e n t a f l u o r i d e The i n f r a r e d s p e c t r a were measured on Perkin-Elmer model 21 i n f r a r e d r e c o r d i n g spectrometers, f i t t e d w i t h sodium c h l o r i d e o p t i c s f o r the 2 to 15 micron r e g i o n and caesium bromide o p t i c s f o r the 15 to 40 micron r e g i o n , u s i n g two d i f f e r e n t gas c e l l s . The metal c e l l was a two cm. diameter s t a i n l e s s s t e e l tube, nine cm. long, w i t h a l a r g e diameter (6 cm.), 0.5 cm. t h i c k s t a i n l e s s s t e e l f l a n g e welded to each end. The s u r f a c e of each f l a n g e was f l a t except f o r a c o n c e n t r i c r i d g e about 0.5 mm. h i g h about 0.5 cm. from the i n n e r edge of the f l a n g e . Polyethylene windows, 0.5 mm. t h i c k , were sealed to the c e l l by clamping them between the f l a n g e and a matching, 0.5 cm. t h i c k , f l a t compression p l a t e by means of heavy screws p a s s i n g through the compression p l a t e and screwing i n t o the f l a n g e s 124 welded to the body of the c e l l . S u f f i c i e n t p r essure was a p p l i e d by means of the screws to squeeze the p o l y e t h y l e n e i n t o the small r i d g e on the c e l l f l a n g e without c u t t i n g i t , thus p r o v i d i n g a vacuum t i g h t s e a l . A Kovar-Pyrex graded s e a l was welded to the body of the c e l l and a g l a s s c o l d f i n g e r , b r e a k s e a l and c a p i l l a r y c o n s t r i c t i o n s e a l e d to the g l a s s p o r t i o n of the graded s e a l . The c e l l was s e a l e d to a vacuum l i n e through the c a p i l l a r y c o n s t r i c t i o n , evacuated and d r i e d by h e a t i n g g e n t l y under vacuum. A small amount of vanadium p e n t a f l u o r i d e was d i s t i l l e d i n t o the c o l d f i n g e r and the c e l l was s e a l e d o f f at the c a p i l l a r y c o n s t r i c t i o n . The c o n c e n t r a t i o n of vanadium p e n t a f l u o r i d e vapour i n the c e l l was c o n t r o l l e d by v a r y i n g the temperature of the c o l d f i n g e r . With t h i s c e l l , the r e g i o n from 42 to 11 microns could be observed as p o l y e t h y l e n e has only two weak a b s o r p t i o n bands at 13.7 and 13.9 microns. The second c e l l was a 10 cm. l o n g g l a s s c e l l , 3 cm. i n t e r n a l diameter w i t h f l a t ground f l a n g e s on each end. Potassium bromide windows were sealed to the g l a s s w i t h A r a l d i t e (Ciba Ltd.) epoxy r e s i n g l u e . The i n n e r s u r f a c e s of the potassium bromide windows had been p r e v i o u s l y coated w i t h a t h i n f i l m of p a r a f f i n wax to prevent a t t a c k of the window m a t e r i a l by vanadium p e n t a f l u o r i d e . These windows were t r a n s p a r e n t from 30 t o 2.5 microns, except f o r the sharp p a r a f f i n a b s o r p t i o n bands at 3.45, 6.8, 6.85, 13.7 and 13.9 microns which were q u i t e weak, because of the t h i n n e s s of the 125 f i l m , and d i d not obscure a l a r g e r e g i o n of the spectrum. A g l a s s c o l d f i n g e r , b r e a k s e a l and c a p i l l a r y s e a l o f f were s e a l e d to the c e l l as b e f o r e . The c e l l was d r i e d , and the spectrum determined as d e s c r i b e d above. Spectra were measured on two d i f f e r e n t samples m each c e l l over the e n t i r e a c c e s s i b l e range f o r each c e l l , w i t h no a p p r e c i a b l e d i f f e r e n c e s being observed. The p o s i t i o n s of the ab s o r p t i o n bands and estimated r e l a t i v e i n t e n s i t i e s are shown i n Table 9 and Fi g u r e 2. CHEMICAL PROPERTIES OF VANADIUM PENTAFLUPRIDE ( i ) The Rea c t i o n of Vanadium P e n t a f l u o r i d e w i t h Selenium  T e t r a f l u o r i d e An excess of selenium t e t r a f l u o r i d e (prepared as des-c r i b e d by Aynsley, Peacock and Robinson (10)) was d i s t i l l e d i n t o a s i l i c a r e a c t i o n v e s s e l which contained about 2 g. of vanadium p e n t a f l u o r i d e . Removal of the v o l a t i l e s at 0° l e f t a white s o l i d which could be slowly sublimed a t room tempera-t u r e s . A n a l y s i s i n d i c a t e d a composition c l o s e to a 1:1 complex of selenium t e t r a f l u o r i d e w i t h vanadium p e n t a f l u o r i d e . Found: V, 17.05; F, 50.3; Se, 32.5. VF^.SeF^ r e q u i r e s : V, 16.95; F, 56.8; Se, 26.25. V F 5 . 2 S e F 4 r e q u i r e s : V, 11.2; F, 5^.61; Se, 3^.65. and VF^.0.5 SeF^ r e q u i r e s : V, 22 .9 ; F, 59.5; Se 17.7-However the i n s t a b i l i t y of the product made a n a l y t i c a l d e f i n -i t i o n d i f f i c u l t . 126 ( i i ) The Reaction of Vanadium P e n t a f l u o r i d e w i t h Sulphur  T e t r a f l u o r i d e An excess of sulphur t e t r a f l u o r i d e (Dupont Co.) was d i s t i l l e d i n t o a s i l i c a v e s s e l c o n t a i n i n g about 2 g. of vanadium p e n t a f l u o r i d e . Removal of the v o l a t i l e m a t e r i a l s at room temperature l e f t no r e s i d u e i n the r e a c t i o n v e s s e l . However removal of the v o l a t i l e m a t e r i a l s w i t h the c o n t a i n e r h e l d a t -78° l e f t a white s o l i d which r e a d i l y l o s t sulphur t e t r a f l u o r i d e on s t a n d i n g . A small amount of t h i s s o l i d was shaken i n t o a s i d e arm w hile the e n t i r e apparatus was main-t a i n e d a t -JQ°, the s i d e arm was s e a l e d o f f and the contents taken f o r a n a l y s e s . On m e l t i n g the s o l i d , a c l e a r white l i q u i d was formed. The composition of the s o l i d i n d i c a t e d by a n a l y s i s was somewhere between a 1:1 and a 2:1 r a t i o of vanadium p e n t a f l u o r i d e to selenium t e t r a f l u o r i d e . (Found: V, 23.7; F, 57.7. V F ^ S F ^ r e q u i r e s : V, 20.1; F, 66.6. VF^.1/2 SF^ r e q u i r e s : V, 25.5$.). The i n s t a b i l i t y of the product makes a n a l y t i c a l d e f i n i t i o n even more d i f f i -c u l t than i n the case of the selenium t e t r a f l u o r i d e complex. L i t t l e can be s a i d about the complex because a d e f i n i t e composition could not be e s t a b l i s h e d . PREPARATION AND PHYSICAL PROPERTIES OF VANADIUM TETRAFLUORIDE ( i ) The P r e p a r a t i o n of Vanadium T e t r a f l u o r i d e Vanadium t e t r a f l u o r i d e was prepared i n l a r g e q u a n t i -t i e s of 10 to 20 grams from vanadium t e t r a c h l o r i d e by the method of Ruff and L i c k f e t t (12) and i n s m a l l e r q u a n t i t i e s of 0.5 to 1 gram by f l u o r i n a t i o n of vanadium metal at low temperatures. Vanadium t e t r a c h l o r i d e was prepared by p a s s i n g c h l o r i n e gas (Matheson Co.) over vanadium metal heated to 300-350° (102) i n a Pyrex r e a c t i o n tube. The product was c o l l e c t e d i n a g l a s s t r a p cooled to -78° and t r a n s f e r r e d , w i t h a minimum exposure to the a i r , to an a l l - g l a s s s t i l l . The f r a c t i o n d i s t i l l i n g between 152° and 154° was c o l l e c t e d . A t y p i c a l a n a l y s i s gave: V, 26.4; CI, 73.5$. C a l c . f o r V C l ^ : V, 26.4; CI, 73.6$. (a) P r e p a r a t i o n from Vanadium T e t r a c h l o r i d e The procedure d e s c r i b e d by Ruff and L i c k f e t t (12) was used except f o r two m o d i f i c a t i o n s to p r o v i d e a s o l v e n t and continuous s t i r r i n g of the r e a c t i o n mixture to improve the p u r i t y of the p r o d u c t . About 10, g. of vanadium t e t r a -c h l o r i d e was p l a c e d i n a 250 ml. p o l y e t h y l e n e r e a c t o r v e s s e l c o n t a i n i n g a T e f l o n coated magnetic s t i r r i n g b a r . The r e a c t o r was connected to a p o l y e t h y l e n e condenser, and 20 g. t r i c h l o r o f l u o r o m e t h a n e (Preon-11, Dupont Co.) was added and the r e a c t o r c h i l l e d t o -78°. A vigorous stream of n i t r o g e n 128 was maintained throughout the o p e r a t i o n to f l u s h out moisture. The s o l u t i o n was s t i r r e d c o n t i n u a l l y by means of an e x t e r n a l magnetic s t i r r e r motor. Twenty f i v e to t h i r t y ml. of anhydrous hydrogen f l u o r i d e was condensed i n t o the r e a c t o r at -78 degrees. The v e s s e l was then p e r m i t t e d t o r e t u r n t o room temperature over a p e r i o d of three hours. At t h i s p o i n t the r e a c t i o n was complete and the t r i c h l o r o f l u o r o m e t h a n e (bp. 24°) and any excess hydrogen f l u o r i d e (bp. 19°) was b o i l e d o f f by warming the r e a c t o r to about 30° w i t h a bath of warm water. The product obtained i n t h i s manner was i n v a r i a b l y pure vanadium t e t r a f l u o r i d e (Pound: V, 40.0, 40.4; F, 59.0, 59.6. VP^ r e q u i r e s : V, 40.3; F, 59.7$.). The magnetic moment of vanadium t e t r a f l u o r i d e at room temperature c a l c u l a t e d from the r e l a t i o n s h i p : was 1.64 Bohr magnetons. A sample of the t e t r a f l u o r i d e prepared i n the above manner but without a s o l v e n t and continuous s t i r r i n g , (eg. e x a c t l y as d e s c r i b e d by Ruff and L i c k f e t t (12)) was contam-i n a t e d w i t h about 3.5$ of VP 3 (Found: V, 4l .2$). The magnetic moment ( c a l c u l a t e d as above) was 1.9 Bohr magnetons, t h i s h i g h value a l s o i n d i c a t i n g the presence of vanadium t r i f l u o r i d e . M T 129 (b) P r e p a r a t i o n from Vanadium Metal and F l u o r i n e Approximately 5 g. of vanadium metal i n a n i c k e l boat was p l a c e d i n a 1 i n c h diameter n i c k e l r e a c t o r , and a s e r i e s of g l a s s t r a p s was f i t t e d to the e x i t . A f t e r h e a t i n g the r e a c t o r t o 150-200°, f l u o r i n e d i l u t e d w i t h n i t r o g e n was passed f o r two hours. A small amount of white s o l i d sublimed i n t o the g l a s s t r a p s which were cooled to -78°. The r e a c t o r was s e a l e d o f f and dismantled i n the dry-box. Around the e x i t of the r e a c t o r was found a mass of dark green s o l i d which was i d e n t i f i e d as vanadium t e t r a f l u o r i d e (Found: V, 40.5; P, 57.2$.). The s o l i d d e l i q u e s c e d i n a i r to a blue paste although not as q u i c k l y as powdered vanadium t e t r a f l u o r i d e prepared from vanadium t e t r a c h l o r i d e and hydrogen f l u o r i d e . The X-ray photograph of the r e s i d u e remaining upon d i s t i l l a t i o n of vanadium p e n t a f l u o r i d e from the impure brown product obtained by f l u o r i n a t i o n of vanadium metal at 350-360° i n d i c a t e d t h a t t h i s l i g h t brown powder was impure vanadium t e t r a f l u o r i d e . The r e s i d u e showed an X-ray d i f f r a c t i o n p a t t e r n i d e n t i c a l t o t h a t of vanadium t e t r a -f l u o r i d e prepared by other methods, p l u s a weak l i n e a r i s i n g from small amounts of vanadium t r i f l u o r i d e . A n a l y s i s con-firm e d the i d e n t i f i c a t i o n of the r e s i d u e as impure vanadium t e t r a f l u o r i d e (Pound V, 42.1$). 130 ( i i ) General P r o p e r t i e s of Vanadium T e t r a f l u o r i d e Pure vanadium t e t r a f l u o r i d e i s a b r i g h t , lime green powder which becomes brown on exposure to small amounts of moisture i n the dry-box. On exposure to the atmosphere i t d e l i q u e s c e d to a blue p a s t e . I t d i s s o l v e d r a p i d l y i n water y i e l d i n g an a c i d i c s o l u t i o n w i t h the c h a r a c t e r i s t i c r i c h blue c o l o u r of vanadium (IV) i n aqueous s o l u t i o n . I t was i n s o l u b l e i n carbon t e t r a c h l o r i d e , t r i c h l o r o f l u o r o m e t h a n e , ether, ben-zene and nitrobenzene, and i n i n o r g a n i c s o l v e n t s such as sulphur d i o x i d e , ammonia and hydrogen f l u o r i d e . In p y r i d i n e , a c e t o n i t r i l e , t e t r a h y d r o f u r a n , and ethoxymethoxyethane, vanadium t e t r a f l u o r i d e formed a g r e e n i s h s o l u t i o n w i t h a brown r e s i d u e remaining, which was a p p a r e n t l y a r e a c t i o n product. Chlorobenzene, toluene and pentane converted vanadium t e t r a -f l u o r i d e t o a b l a c k s o l i d while the s o l v e n t remained u n c o l -oured. Two determinations of the s p e c i f i c g r a v i t y under carbon t e t r a c h l o r i d e gave 3.28 g/cc (at 28°) and 3.02 g/cc (at 19.5°). A d e t e r m i n a t i o n under toluene gave a s p e c i f i c g r a v i t y of 2.2 g / c c , however as vanadium t e t r a f l u o r i d e appears t o r e a c t w i t h toluene, t h i s r e s u l t i s not r e l i a b l e . The I n f r a r e d spectrum of vanadium t e t r a f l u o r i d e as a N u j o l mull was measured from 400 to 2000 cm" 1 and the r e s u l t s are shown i n Table 9 and F i g u r e 2. 131 ( i i l ) The Magnetic S u s c e p t i b i l i t y of Vanadium T e t r a f l u o r i d e The magnetic s u s c e p t i b i l i t y was measured from 8 l ° to 295°K w i t h the apparatus d e s c r i b e d p r e v i o u s l y . The sus-c e p t i b i l i t y obeys the Curie-Weiss law and the magnetic moment at 293°K, c a l c u l a t e d a c c o r d i n g t o the Curie-Weiss law i s 2.17 Bohr magnetons, and a c c o r d i n g t o the Curie law i s 1.68 Bohr magnetons. The r e s u l t s are given i n Table 19. TABLE 19 THE MAGNETIC SUSCEPTIBILITY OF VANADIUM TETRAFLUORIDE T°K ! 0 \ ^ e f f M-eff (Curie-Weiss) ( C u r i e ) 293 11.91 2.17 1.68 252 12.97 2.17 1.62 243 13.11 2.16 1.60 204 14.49 2.17 1.54 181 15.23 2.16 1.49 132 17.71 2.17 1.37 83 21.01 2.17 1.18 9 = 198 C = 0.5835 cgsu. ( i v ) Thermal D i s p r o p o r t i o n a t i o n of Vanadium T e t r a f l u o r i d e Approximately 4 g. of vanadium t e t r a f l u o r i d e was pl a c e d i n a s i l i c a p y r o l y s i s tube, 1 i n c h i n diameter and 12 inches long, surrounded by an e l e c t r i c furnace, and con-nected, through a spray t r a p , t o a g l a s s t r a p l i n e . One t r a p was c h i l l e d to -78°, the system evacuated and the temperature of the sample i n the s i l i c a tube slowly i n c r e a s e d . At 100° the e v o l u t i o n of gas was vigo r o u s and powdered vanadium t e t r a f l u o r i d e was c a r r i e d i n t o the spray t r a p . The 132 temperature was maintained between 100 and 120 f o r 20-30 h r . u n t i l gas e v o l u t i o n ceased. The p a l e y e l l o w s o l i d r e s i d u e was vanadium t r i f l u o r i d e . (Found: V, 47.4; F, 52.8. C a l c . f o r V F 3 : V, 47.3; F, 52.7$). The -78° t r a p contained vanadium p e n t a f l u o r i d e as a white s o l i d which melted to a p a l e yellow l i q u i d a t 20°. (Found: V, 33.5; F, 6l.9. R a t i o F/V, 4.97/1. C a l c . f o r VF^: V, 34.9; F, 65.1$. Mp. VF^: 19.5° (7)). High temperatures are not r e q u i r e d f o r thermal d i s -p r o p o r t i o n a t i o n of vanadium t e t r a f l u o r i d e . Samples of vanadium t e t r a f l u o r i d e s e a l e d i n dry g l a s s tubes i n i t i a l l y at atmospheric pressure r a p i d l y formed an excess i n t e r n a l pressure of gas. The b r i g h t green s o l i d became n o t i c e a b l y darker i n c o l o u r , e v e n t u a l l y becoming a dark brown powder. A n a l y s i s of a sample of vanadium t e t r a f l u o r i d e which had been kept f o r about 10 days at room temperature showed t h a t 7.5$ of the vanadium was present as V ( i l l ) (Found: V, 4l.6$). (v) The S u b l i m a t i o n of Vanadium T e t r a f l u o r i d e During one p y r o l y s i s i n vacuum at 100°, as d e s c r i b e d above, the c o o l p o r t i o n of the s i l i c a tube, i e . the narrow neck l e a d i n g to the spray t r a p and vacuum l i n e , became completely blocked w i t h a g l a s s y dark green s o l i d , which was I d e n t i f i e d as s l i g h t l y impure vanadium t e t r a f l u o r i d e (Found: V, 4l.5$). The appearance and l o c a t i o n of the green d e p o s i t i n d i c a t e d t h a t i t c o u l d only have been formed by s u b l i m a t i o n 133 of the vanadium t e t r a f l u o r i d e . T h i s sublimed s o l i d showed an X-ray powder p a t t e r n i d e n t i c a l t o t h a t of the lime-green powdered vanadium t e t r a f l u o r i d e obtained from the t e t r a c h l o r i d e . ( v i ) The X-ray D i f f r a c t i o n of Powdered Vanadium T e t r a f l u o r i d e The X-ray d i f f r a c t i o n diagrams of powdered vanadium t e t r a f l u o r i d e , s e a l e d i n t h i n w a l l e d g l a s s c a p i l l a r i e s , were taken on the u n i t d e s c r i b e d e a r l i e r . The d i f f r a c t i o n diagrams obtained (a) from s e v e r a l p r e p a r a t i o n s of vanadium t e t r a f l u o r i d e from the t e t r a c h l o r i d e and hydrogen f l u o r i d e (b) from the sublimed product mentioned above, (c) from two samples from the f l u o r i n a t i o n of vanadium metal a t 200°C and, f i n a l l y , (d) from the r e s i d u e from impure vanadium penta-f l u o r i d e were e s s e n t i a l l y i d e n t i c a l . In no case was the r e s o l u t i o n of l i n e s w i t h d i f f r a c t i o n angles g r e a t e r than 45° e x c e p t i o n a l l y good. In a l l p i c t u r e s c e r t a i n l i n e s were sharp, and c e r t a i n l i n e s were d i f f u s e . A t y p i c a l set of data f o r vanadium t e t r a f l u o r i d e i s given i n Table 20. The h k l i n d i c e s were assigned on the b a s i s of a hexagonal u n i t c e l l of dimensions a_ = 5.37 A, _c = 5.16 A, c o n t a i n i n g two molecules per u n i t c e l l (<j>calc. = 3.28 g/cc; ^ exp = 3.15,+ 0.15 g / c c . ) . Numerical i n t e n s i t y v a l u e s f o r the l i n e s on the photograph were measured w i t h a H i l g e r and Watts PA-17 photometer, equipped w i t h a Leeds and Northrup "Speedomax G" r e c o r d i n g potentiometer. Where the l i n e s were too weak to measure w i t h the photometer v i s u a l estimates of i n t e n s i t y were made. 134 TABLE 20 X-RAY DIFFRACTION DATA FOR VF^ h k l 001 100 101 110 002 200 102 201 112 003 202 211 103 300 220 113 212 302 311 104 213 303 204 214 a -Q, = ( c a l c ) 0.0376 0.0462 0.0838 0.1386 0.1504 0.1848 O.1962 0.2224 0.2890 0.3390 1 0.3352 » 0.3614 0.3852 0.4158 0.4624 0.4773 1 0.4738 I 0.5662 0.6382 0.6478 0.6624 0.7548 O.7860 O.9250 (Hexagonal System) l / d 2  (obs) 0.0475 0.0862 0.1433 0.1527 0.1867 0.1990 0.2252 O.2938 0.3393 O.3619 0.3708 0.3872 0.4167 0.4616 0.4790 0.5542 0.5675 0.6382 0.6515 0.6633 0.7101 0.7575 0.7890 0.9286 0.9843 Approximate R e l a t i v e I n t e n s i t y 38 100 32 w 45 64 92 185 144 314 (broad and d i f f u s e ) 118 19 vw. 126 7 30 |- 30 (broad and d i f f u s e ) 23 11 vw. W W . W W . W W . 4 \ 1 3 6.0462 ~ 5 # 3 7 A c = J Q 001 = J o ^ 3 7 o - = 5.16 A Volume of u n i t c e l l (V) a 2 c s i n 60° = 128.9 A 3 MZ x 1.66 x 10 -24 V x 10' 72W = 3.28 g./cc, f o r Z = 2, i e . 2 molecules per u n i t c e l l ; M = molecular wt. = 127. 135 CHEMICAL PROPERTIES OF VANADIUM TETRAFLUPRIDE ( i ) R e action of Vanadium T e t r a f l u o r i d e w i t h Selenium  T e t r a f l u o r i d e An excess of selenium t e t r a f l u o r i d e was d i s t i l l e d i n t o a s i l i c a v e s s e l c o n t a i n i n g vanadium t e t r a f l u o r i d e (0.88 g., 6.9 m. mole) and r e f l u x e d at atmospheric p r e s s u r e (b.p. 1P6° ( l P l ) ) f o r 15 minutes. An orange-brown s o l i d was formed which was p a r t i a l l y s o l u b l e i n selenium t e t r a f l u o r i d e . C o o l i n g the t r a p t o room temperature and removing unreacted selenium t e t r a f l u o r i d e i n vacuum l e f t 1.9P3 g. of a l i g h t brown s o l i d which was i d e n t i f i e d as the 1:1 adduct SeF^.VF^ (Found: Se, 28.5; V, 18.2; F, 50.6. SeVFg r e q u i r e s : Se, 28.P; V, 18.1; F, 53.9$. Weight expected f o r 1:1 adduct 1.95 g-i) • The magnetic s u s c e p t i b i l i t y of the VF^.SeF^ adduct was determined over the range 88° to 295°K as f o r vanadium t e t r a f l u o r i d e . The r e s u l t s , given i n Table 21, show t h a t the s u s c e p t i b i l i t y obeys the Curie-Weiss law. The magnetic moment of the adduct at 295°K, c a l c u l a t e d from the Curie law, i s 1.86 Bohr magnetons, and from the Curie-Weiss law, i s 2.3 Bohr magnetons. TABLE 21 1 MAGNETIC SUSCEPTIBILITY OP THE VF^.SeP^ ADDUCT T°K M-eft M-eff (Curie-Weiss) (Curi e ) 294.5 15.59 2.32 1.92 273 16.32 2.31 1 .90 2 4 5 17.65 2.32 1.87 2 1 5 18.70 2.29 1.80 1 9 3 20.18 2.31 1.77 1 6 5 21.87 2.29 1.71 138 24.30 2.31 1 .64 124 25.88 2.32 1.61 88 30.15 2.32 1 .46 9 = 1 3 4 ° c = O.665 cgsu. ( i i ) R e a ction of Vanadium T e t r a f l u o r i d e w i t h Ammonia A l a r g e excess of anhydrous ammonia (Matheson Co.) was condensed on to vanadium t e t r a f l u o r i d e . On m e l t i n g the ammonia an orange-brown s l u r r y was formed. Removal of excess ammonia and h e a t i n g the remaining s o l i d t o 100° i n vacuum y i e l d e d a b u f f s o l i d , amminotetrafluorovanadium (TV) (60) (Pound: V, 35.5. C a l c . f o r NR^VF^: V, 35.4$.). The compound r e a d i l y gave a crown aqueous s o l u t i o n which, when a c i d i f i e d , c o u l d be t i t r a t e d f o r vanadium (IV) w i t h standard potassium permanganate. The magnetic moment of ammino-t e t r a f luorovanadium (IV) was 1.83 Bohr magnetons at 293°K. 137 ( i l l ) R e a ction of Vanadium T e t r a f l u o r i d e w i t h P y r i d i n e Excess dry p y r i d i n e was condensed on to 0.628 g. (4.94 m. mole) of vanadium t e t r a f l u o r i d e . R eaction o c c u r r e d immediately on m e l t i n g the p y r i d i n e , forming a brown s o l i d . Removal of excess p y r i d i n e under vacuum l e f t 1.036 g . ( C a l c . f o r C^H^NVP^ 1.02 g.) of a grey-pink powder which was i d e n t i f i e d as VF^Py (Pound: V, 24.7; C a l c . f o r C 5H 5NVP 4: V, 24.7$.). P y r i d i n e t e t r a f l u o r o v a n a d i u m (IV) (60) d i s s o l v e d r e a d i l y i n water t o form a green s o l u t i o n which, a f t e r a c i d i f i c a t i o n , was t i t r a t e d w i t h standard permanganate s o l u t i o n . Comparison of the t i t r a t i o n w i t h permanganate immediately a f t e r d i s s o l u t i o n w i t h the vanadium a n a l y s i s showed t h a t a l l the vanadium was presen t i n the t e t r a v a l e n t s t a t e . The magnetic moment of p y r i d i n e t e t r a f l u o r o v a n a d i u m (TV) was 1.73 Bohr magnetons a t 293°K. ( i v ) R e a c t i o n of Vanadium T e t r a f l u o r i d e w i t h Bromine T r i f l u o r i d e Excess bromine t r i f l u o r i d e (Matheson Co.) was condensed on t o 0.55 g.(4.3 m. mole) of vanadium t e t r a f l u o r i d e i n a s i l i c a v e s s e l . The vanadium t e t r a f l u o r i d e d i s s o l v e d immed-i a t e l y i n molten bromine t r i f l u o r i d e and bromine was l i b e r a t e d . The contents of the r e a c t i o n v e s s e l , which were completely v o l a t i l e , were d i s t i l l e d on to 0.324 g.(4.3 m. mole) potassium c h l o r i d e . Removal of v o l a t i l e m a t e r i a l under vacuum l e f t O.83 g. of potassium hexafluorovanadate (v) (l4). (Pound: V, 25.6; F, 5+.0. C a l c . f o r KVFg: V, 25.0; F, 55.9$. 4.3 m. mole VF^ r e q u i r e s 0.88 g. KVFg). (v) R e a c t i o n of Vanadium T e t r a f l u o r i d e w i t h F l u o r i n e A mixture of f l u o r i n e and n i t r o g e n was passed over vanadium t e t r a f l u o r i d e heated to 100° i n a n i c k e l r e a c t o r . Reaction began immediately on admission of f l u o r i n e and a white s o l i d , i d e n t i f i e d as vanadium p e n t a f l u o r i d e , was con-densed i n a -78° t r a p connected to the r e a c t o r e x i t . When e v o l u t i o n of vanadium p e n t a f l u o r i d e ceased and the r e a c t o r was dismantled, no r e s i d u e remained i n the boat. ( v i ) R e a c t i o n of Vanadium T e t r a f l u o r i d e w i t h N i t r y l F l u o r i d e An excess of n i t r y l f l u o r i d e , prepared as d e s c r i b e d b Aynsley, Hetherington and Robinson (103), was condensed on t vanadium t e t r a f l u o r i d e . The v e s s e l was s e a l e d and kept a t -78° to maintain l i q u i d n i t r y l f l u o r i d e i n contact w i t h vanadium t e t r a f l u o r i d e . No r e a c t i o n was e v i d e n t . Removal of the v o l a t i l e m a t e r i a l s a f t e r a p e r i o d of 47 days l e f t unchanged vanadium t e t r a f l u o r i d e , a c c o r d i n g to the X-ray powder photograph, p l u s a small amount of ( N 0 2 ) 2 S i F g r e s u l t i n g from a t t a c k of n i t r y l f l u o r i d e on the g l a s s . In a second experiment i o d i n e p e n t a f l u o r i d e was d i s t i l l e d on to vanadium t e t r a f l u o r i d e In a s i l i c a v e s s e l , f o l l o w e d by n i t r y l f l u o r i d e . Upon me l t i n g , a brown-red suspension formed and vigorous e v o l u t i o n of a gas o c c u r r e d 139 which condensed to a brownish s o l i d . The mixture was maintained at room temperature f o r one hour, then the excess N02F which had escaped was d i s t i l l e d back and h e l d f o r 30 minutes to ensure as complete a r e a c t i o n as p o s s i b l e . Removal of the v o l a t i l e m a t e r i a l s l e f t NOgVFg (l4) (Found: V, 23.8; C a l c . f o r NOgVFg: V, 24.2$.). - However, the product had a magnetic moment of 0.4 Bohr magnetons at 293°K, i n d i c a t i n g contamination of the s a l t w i t h vanadium t r i f l u o r i d e or t e t r a f l u o r i d e . ( v i i ) R e a c t i o n of Vanadium T e t r a f l u o r i d e w i t h Sulphur T e t r a f l u o r i d e A l a r g e excess of sulphur t e t r a f l u o r i d e (Dupont Co.) was d i s t i l l e d i n t o a s i l i c a v e s s e l c o n t a i n i n g vanadium t e t r a f l u o r i d e . Vanadium t e t r a f l u o r i d e d i d not appear to be s o l u b l e i n sulphur t e t r a f l u o r i d e and removal of the v o l a t i l e m a t e r i a l i n vacuum l e f t unchanged vanadium t e t r a f l u o r i d e . (Found: V, 40.4$). ( v i i i ) R e a ction of Vanadium T e t r a f l u o r i d e w i t h Iodine  P e n t a f l u o r i d e A l a r g e excess of i o d i n e p e n t a f l u o r i d e (Matheson Co.) was condensed onto vanadium t e t r a f l u o r i d e i n a s i l i c a v e s s e l . On warming and r e f l u x i n g the i o d i n e p e n t a f l u o r i d e at atmos-p h e r i c p r e s s u r e (b.p. 98° (100)), some vanadium t e t r a f l u o r i d e d i s s o l v e d y i e l d i n g a red brown s o l u t i o n . Removal of i o d i n e p e n t a f l u o r i d e under vacuum l e f t a dark brown s o l i d which was l40 shown by a n a l y s i s (Pound: V, 40.5; 40.7$) and X-ray to be unchanged vanadium t e t r a f l u o r i d e . ( i x ) R e a c t i o n of Vanadium T e t r a f l u o r i d e w i t h Sulphur  T r i o x i d e Pure, dry ot-sulphur t r i o x i d e , prepared by dehydrating fuming s u l p h u r i c a c i d w i t h phosphorus,, pentoxide, was condensed on to vanadium t e t r a f l u o r i d e . On warming the mixture the sulphur t r i o x i d e polymerised, but no r e a c t i o n o c c u r r e d . Complete removal of the sulphur t r i o x i d e under vacuum was d i f f i c u l t and the vanadium t e t r a f l u o r i d e remaining was con-taminated w i t h a n o n s t o i c h i o m e t r i c amount of sulphur t r i o x i d e . (x) Reaction of Vanadium T e t r a f l u o r i d e w i t h Sulphur D i o x i d e An excess of anhydrous sulphur d i o x i d e (Matheson Co.) was d i s t i l l e d on to vanadium t e t r a f l u o r i d e and allowed to melt and warm up to i t s b o i l i n g p o i n t (-10°). No r e a c t i o n occurred and no evidence of s o l u t i o n was observed. Removal of sulphur d i o x i d e l e f t unchanged vanadium t e t r a f l u o r i d e (Pound: V, 40.4$). An excess of potassium f l u o r i d e , mixed w i t h vanadium t e t r a f l u o r i d e , was allowed to stand i n b o i l i n g sulphur d i o x i d e f o r one-half hour. On removal of the sulphur d i o x i d e only a mixture of unreacted potassium f l u o r i d e and vanadium t e t r a -f l u o r i d e remained, as confirmed by an X-ray d i f f r a c t i o n powder photograph. 141 THE PREPARATION AND PROPERTIES OF HEXAFLUOROVANADATE (IV) SALTS ( l ) P r e p a r a t i o n of Potassium Hexafluorovanadate (IV) from  Vanadium T e t r a f l u o r i d e Potassium f l u o r i d e and vanadium t e t r a f l u o r i d e (2:1 molar r a t i o ) were mixed i n a s i l i c a r e a c t i o n v e s s e l , together w i t h 10 ml. of selenium t e t r a f l u o r i d e . A dark r e d s o l u t i o n was formed above the mixed s o l i d r e a c t a n t s . A f t e r the mixture had stood f o r 30 minutes at room temperature, the SeF^ was removed under vacuum and the s a l t was heated to 150° f o r s e v e r a l hours. The r e s i d u e , a p a l e p i n k powder, was potassium hexafluorovanadate (IV) (Pound: V, 21.25; P, 47 . 3 . C a l c . f o r KpVFg: V, 21.0; P, 46.9$). T i t r a t i o n of a f r e s h l y d i s s o l v e d sample, without p r i o r treatment w i t h sulphur d i o x i d e confirmed t h a t a l l vanadium was prese n t as vanadium ( I V ) . ( i i ) P r o p e r t i e s of Potassium Hexafluorovanadate (IV) The magnetic s u s c e p t i b i l i t y of potassium h e x a f l u o r o -vanadate obeys the Curie-Weiss law i n the range 105° to 296°K. The magnetic moment a t 296°K, c a l c u l a t e d from the Curie-Weiss law, i s 2.05 Bohr magnetons, and from the Curie law i s 1.71 Bohr magnetons. The prese n t r e s u l t s , g iven i n Table 22, are s i m i l a r t o those r e c e n t l y r e p o r t e d by Li e b e e_t. al. (17) i n c l u d i n g the 8l°K measurement, which i n the prese n t case does not f a l l on the s t r a i g h t l i n e which can be drawn through the p o i n t s i n the range 105° to 296°K. 142 TABLE 22 THE MAGNETIC SUSCEPTIBILITY OF POTASSIUM HEXAFLUOROVANADATE (IV) T°K JA-eff (Curie-Weiss) ( C u r i e ) 296 12.56 2.05 1.74 266 13.42 2.04 1.70 241 14.16 2.03 1.66 226 15.01 2.04 1.66 209 15.47 2.02 1.6l 189 17.03 2.05 1.61 186 16.32 2.00 1.57 150 17.65 1.95 1.46 128 21.22 2.05 1.48 105 22.25 2.00 1.37 83 28.96 2.17 1.39 0 = 118° C = 0.515 cgsu. The X-ray powder photograph of KgVFg was indexed on the b a s i s of a t r i g o n a l u n i t c e l l of a_ = 5.68, _c = 4.66, c o n t a i n i n g one molecule per u n i t c e l l . The data, which are given i n Table 23 are i n agreement w i t h those of Liebe e_t. _ a l . (17). The s p e c i f i c g r a v i t y under carbon t e t r a c h l o r i d e was found t o be 2.5 g . / c c , not i n very good agreement w i t h the X-ray d e n s i t y of 3.09 g./cc. 143 a = TABLE 23 X-RAY DIFFRACTION DATA FOR TRIGONAL K 0VBV 2 6 hkl Q = l / d 2 hkl Q - l/d2 ( c a l c . ) (obs.) ( c a l c . ) (obs.) 100 0.0413 o.o4i4 321 0.8307 0.8317 001 0.6460 0.0464 402 0.8448 0.8448 101 0.6873 0.0882 4io 0.8673 0.8680 110 0.1239 0.1250 223 0.9096) n Q 1 P n 200 0.1652 0.1661 4 l l 0.9133' U.^ J-^ H-111 0.1699 0.1726 322 O.9687 0.9694 002 0.1840 0.1852 214 1.0251 1.0286 201 0.2112 0.2126 403 1.0748? , n 7 f i Q 102 0.2253 0.2279 501 I.O785J J-.ufoy 211 0.3351 0.3373 330 1.1151 1.1127 202 0.3494 0.3520 323 1.19877 n P O P n 300 0.3717 0.3729 421 1.2024-3 J-.^u^u 0.4070 224 1.2316 1.2351 003 0.41401 0 , I Q , 510 1.2803? •, 2826 301 0.4177J °-4195 4 l 3 1.2813* !.2S26 212 0.4731 0.4758 ? 1.3250 220 0.4956 0.4968 511 1.3263 1.3297 221 0.5416 0.5423 422 1.3404 1.3397 311 0.5829 0.5848 215 1.4391 1.4412 400 O.6608 abs. 512 1.4543 1.4648 222 0.6796 0.6832 600 1.4868 1.4852 213 0.7031) 0 7 o 8 o 401 0.7068J o.foao 312 0.7209 0.7249 320 0.7847 0.7884 h 1 (h 2 + nk + k 2) _ fH: (3) 5 6 8 A J 3 , \ k o ~ / 3 ; 0 . 1 2 3 9 - A * c = %oi = 4-66 A Volume = a 2 c 2 s i n 120° = 130.3 A 3 ~ 1.6604 X M x Z " ~> / j r . r r n . 1 -1 -1 P = —- ^ = 3.09 g./cc. f o r Z = 1, i . e . 1 molecule per u n i t c e l l . M = Molecular weight = 243. 144 ( i i i ) Attempted P r e p a r a t i o n of Other Hexafluorovanadate(IV)  S a l t s R e a c t i o n of caesium f l u o r i d e w i t h vanadium t e t r a -f l u o r i d e (2:1 molar r a t i o ) i n selenium t e t r a f l u o r i d e as above y i e l d e d a b r i g h t p i n k powder which t e n a c i o u s l y r e t a i n e d selenium t e t r a f l u o r i d e . A f t e r prolonged evacuation w i t h h e a t i n g , a pale p i n k powder, which was impure caesium hexa-fluorovanadate (IV) was obtained (Pound: V, 11.6. C a l c . f o r CSpVP^: V, 11.8$.). D i r e c t t i t r a t i o n of a f r e s h l y d i s s o l v e d sample i n d i c a t e d t h a t not a l l the vanadium was present as vanadium ( l V ) . The magnetic moment was 2.48 Bohr magnetons a t 293°K. The X-ray powder photograph i n d i c a t e d hexagonal Cs 2VPg, p l u s i m p u r i t y l i n e s which c o u l d have been due to CsVPg and p o s s i b l y Cs^VPg or Cs^VP^. Potassium f l u o r i d e and vanadium t e t r a f l u o r i d e (2:1 molar r a t i o ) were r e a c t e d i n i o d i n e p e n t a f l u o r i d e (Matheson Co.) s o l u t i o n . The p a l e p i n k product contained much f r e e i o d i n e and was a mixture of KpVFg and KVPg (Found: V, 20.6. C a l c . f o r K 2VP 6: V, 21.0. C a l c . f o r KVFg: V, 25.0$.). T i t r a t i o n of a f r e s h l y d i s s o l v e d sample i n d i c a t e d t h a t about 20$ of vanadium was pres e n t as vanadium (V) (Found: V, 15.8, R a t i o V(V)/V ( t o t a l ) : 0.77/1.). The magnetic moment was 1.15 Bohr magnetons a t 296°K. The X-ray powder photograph showed l i n e s which could have been due to both K^VF^ and KVFg. Caesium f l u o r i d e and vanadium t e t r a f l u o r i d e (2:1 molar r a t i o ) were a l s o r e a c t e d i n IF,-. The p i n k product was a 145 mixture of CSgVFg and CsVFg (Found: V, 11.4. C a l c . f o r C s 2 V F 6 : V, 11.8. C a l c . f o r CSgVFg: V, 17.1$.). T i t r a t i o n of a f r e s h l y d i s s o l v e d sample i n d i c a t e d a pe n t a v a l e n t vanadium content of about 35$ (Found: V ( l V ) , 7.65. R a t i o V ( l V ) / V ( t o t a l ) : O.67/I.). The magnetic moment was 1.27 Bohr magnetons a t 2y3°K. When cal c i u m or barium f l u o r i d e were mixed w i t h vanadium t e t r a f l u o r i d e ( l : l molar r a t i o ) i n i o d i n e p e n t a f l u o r i d e s o l u t i o n , the green s o l i d product was found by X-ray powder photography to c o n s i s t of the unchanged r e a c t a n t s . The r e a c t i o n of an equimolar mixture of potassium f l u o -r i d e and vanadium t e t r a f l u o r i d e i n i o d i n e p e n t a f l u o r i d e gave a non-homogeneous product which, a c c o r d i n g t o the X-ray powder photograph, appeared to be a mixture of K^VF^ and unreacted vanadium t e t r a f l u o r i d e . THERMOCHEMISTRY OF VANADIUM FLUORIDES ( i ) The C a l o r i m e t e r A s o l u t i o n c a l o r i m e t e r was c o n s t r u c t e d from a simple Parr bomb c a l o r i m e t e r . The metal i n t e r n a l v e s s e l and bomb assembly were removed and r e p l a c e d w i t h a one l i t r e p o l y e t h y -lene v e s s e l and a hollow s h a f t brass s t i r r e r . The brass s t i r r e r s h a f t vpassed through a hole i n the l i d of the c a l o r i m e t e r and a l a r g e aluminum wheel was a f f i x e d t o the s t i r r e r s h a f t , where i t emerged from the c a l o r i m e t e r , by means of a set screw i n the 146 wheel. The depth of immersion of the s t i r r e r c o u l d be v a r i e d by a d j u s t i n g the p o s i t i o n of the aluminum wheel on the s h a f t . P o l y e t h y l e n e bearings were p r o v i d e d i n the l i d of the c a l o r i -meter to hold the s t i r r e r s h a f t f i r m l y i n p o s i t i o n so as to o b t a i n smooth s t i r r i n g w i t h a minimum of f r i c t i o n . Since independent adjustment of the p o s i t i o n of the s t i r r e r blades on the s h a f t was a l s o d e s i r a b l e , the blades were f i x e d to a c o l l a r which was h e l d i n p o s i t i o n on the s h a f t by means of a set screw. A small e l e c t r i c motor, f i x e d to the outer w a l l of the c a l o r i m e t e r w i t h an i n s u l a t i n g bracket was used to d r i v e the s t i r r e r . A b e l t d r i v e was used between the motor and the aluminum wheel at the top of the s t i r r e r s h a f t to minimize heat t r a n s f e r to the s t i r r e r . The o r i g i n a l b a k e l i t e double-w a l l e d c a l o r i m e t e r j a c k e t was unchanged although a d d i t i o n a l i n s u l a t i o n i n the form of cotton wool was p r o v i d e d around the i n n e r v e s s e l because i t was somewhat sm a l l e r than the o r i g i n a l v e s s e l . An e i g h t ohm heater, c o n s i s t i n g of nichrome wire wound about a p o l y e t h y l e n e tube was immersed i n the p l a s t i c c a l o r i -meter v e s s e l . In determining the water e q u i v a l e n t of the system the bare wires could be exposed to the water i n the v e s s e l without e f f e c t . In vanadium f l u o r i d e s o l u t i o n however, passage of c u r r e n t r e s u l t e d i n e l e c t r o c h e m i c a l c o r r o s i o n of the nichrome wire which produced changes i n the r e s i s t a n c e of the h e a t e r . To combat t h i s d i f f i c u l t y a very t h i n c o a t i n g of p a r a f f i n was a p p l i e d to the heater w i r e . T h i s was s u f f i c i e n t 147 to prevent c o r r o s i o n of the wire but had l i t t l e e f f e c t on the heat t r a n s f e r to the s o l u t i o n . The c u r r e n t was s u p p l i e d t o the heater from a f r e s h l y charged 12 v o l t l e a d storage c e l l and t h i s c u r r e n t was measured by determining w i t h a p o t e n t i o -meter the p o t e n t i a l d i f f e r e n c e a c r o s s an a c c u r a t e l y measured standard r e s i s t o r of manganin w i r e . A stopwatch graduated to 0 . 2 sec. was used f o r time measurements. The temperature was measured w i t h a standard Beckmann thermometer, graduated to 0 . 0 1°C. Since the presence of g l a s s i n the s o l u t i o n was u n d e s i r a b l e , because of the p o s s i b i l i t y of a si d e r e a c t i o n between the h y d r o f l u o r i c a c i d produced i n the h y d r o l y s i s r e a c t i o n and the g l a s s , the p a r t of the Beckmann thermometer which was immersed i n the s o l u t i o n was coated w i t h a t h i n f i l m of p a r a f f i n , which was s u f f i c i e n t to prevent a t t a c k on the g l a s s but w i t h l i t t l e e f f e c t on the thermal response of t he the rmome t e r . Because a s l i g h t l y d i f f e r e n t sample d e l i v e r y system was used f o r each compound s t u d i e d , i t was necessary to determine the water e q u i v a l e n t f o r each system. T y p i c a l l y , a known amount of heat (Q), c a l c u l a t e d from time and e l e c t r i c a l measure-ments a c c o r d i n g to the e x p r e s s i o n Q = O . 2 3 8 9 R I t (where R i s the heater r e s i s t a n c e i n ohms, I i s the c u r r e n t i n amps and t i s the time i n seconds) was d e l i v e r e d t o the c a l o r i m e t e r by means of the heater c o i l , and the i n c r e a s e i n temperature measured by means of the Beckmann thermometer. A graph of temperature ( i n a r b i t r a r y degrees C) a g a i n s t time was prepared f o r each measurement and used to c o r r e c t the temperature i n c r e a s e f o r heat l o s s e s d u r i n g h e a t i n g by e x t r a p o l a t i n g the c o o l i n g curves and measuring on the graph, the temperature i n c r e a s e at the mid-point of the h e a t i n g p e r i o d (104). The water e q u i v a l e n t (WE) was c a l c u l a t e d from the e x p r e s s i o n : WE = Q ~ ( W X ^ x ^ T ) (where Q i s the heat a p p l i e d i n c a l o r i e s , W i s the weight of water i n grams, s i s the s p e c i f i c heat of the s o l u t i o n and A T i s the temperature change i n degrees). A constant weight of water, measured on the same balance, was used f o r every measurement of the water e q u i v a l e n t , heat of h y d r o l y s i s and s p e c i f i c heat of the r e s u l t a n t s o l u t i o n . The s p e c i f i c heat was determined f o r each s o l u t i o n immediately a f t e r the h y d r o l y s i s of the vanadium compound, by the same procedure as o u t l i n e d above f o r the water e q u i v a l e n t . Time-temperature graphs were used to determine AT as b e f o r e . Two measurements were made on each s o l u t i o n and the s p e c i f i c heat was c a l c u l a t e d from the e x p r e s s i o n s = Q-~ x ^ T) ^ Wt x A T where s, Q, WE, and AT are as before and Wt i s now the weight of s o l u t i o n r e s u l t i n g from the h y d r o l y s i s . ( i i ) The Heat of H y d r o l y s i s of Vanadium T e t r a f l u o r i d e The sample h o l d e r designed f o r powdered s o l i d s such as vanadium t e t r a f l u o r i d e i s shown i n F i g u r e 3. I t c o n s i s t e d of a 13 mm.,diameter brass tube approximately 6 cm. long, the top of which could be screwed i n t o the base of the hollow s t i r r e r s h a f t . A 0.01 mm. P a r a f i l m 'M1 (Marathon Chemical Co.) diaphragm was sealed t o the bottom of the c o n t a i n e r by means to f o l l o w p. 148 FIG. 3 SAMPLE HOLDERS for V | and BREAKSEAL ,10 mm— PYREX TUBE 5cm long 5 mm thin walled 316 stainless steel) 135 cm long J 10/30 CONE v 1 CAPILLARY SEAL OFF 5 mm tube KOVAR- PYREX SEAL FOR VEj RER SHAFT PIERCING DEVICE .-—SPIKE-15mm long FRAME 30 mm dia. -LIQUID CONTAINER 3 cm long ^PLATINUM DIAPHRAGM 0.002 mm thick Hffl-POLYETHYLENE PLUNGER-II mm did. POWDER SAMPLE HOLDER 60 mm long SPIKE PARAFILM DIAPHRAGM 0010 mm thick 149 of a wide threaded f l a n g e and compression nut. A c l o s e f i t t i n g p o l y e t h y l e n e p l u g c o n t a i n i n g a sharp spike f o r p i e r c i n g the diaphragm was i n s e r t e d i n t o the top of the sample tube. With the p o l y e t h y l e n e p l u g and the P a r a f i l m diaphragm i n p l a c e a completely enclosed volume of about 3 ml. was a v a i l a b l e f o r the s o l i d sample. A t y p i c a l d e t e r m i n a t i o n of the heat of h y d r o l y s i s of vanadium t e t r a f l u o r i d e was done as f o l l o w s . The sample ho l d e r was c a r e f u l l y d r i e d and weighed w i t h the p o l y e t h y l e n e plunger and P a r a f i l m diaphragm i n p l a c e . In the dry-box, the P a r a f i l m diaphragm was removed and about two grams of vanadium t e t r a f l u o r i d e were loaded i n t o the t a r e d sample h o l d e r . The diaphragm was then r e p l a c e d and the sample hol d e r reweighed. The sample c o n t a i n e r was then screwed to the base of the s t i r r e r s h a f t ; and immersed i n t o the c a l o r i -meter v e s s e l , which a l r e a d y contained a weighed amount of water. The system was then allowed to reach thermal e q u i l i b r i u m . At the a p p r o p r i a t e moment the p o l y e t h y l e n e p l u g was pushed through the sample ho l d e r by means of a long f i b r e rod which reached down the hollow s t i r r e r s h a f t . The spike h e l d i n the p l u g p i e r c e d the diaphragm and the p l u g was f o r c e d i n t o the s o l u t i o n pushing a l l of the s o l i d i n t o the water. When the p l u g l e f t the h o l d e r , the s o l u t i o n entered and washed the i n n e r w a l l s f r e e of any t r a c e s of vanadium t e t r a f l u o r i d e t h a t may have adhered to them. In t h i s manner complete s o l u t i o n of the s o l i d was ensured. Temperature readings were taken a t one minute i n t e r v a l s before and a f t e r the vanadium t e t r a f l u o r i d e 150 was hydrolyzed and the temperature Increase upon h y d r o l y s i s was c o r r e c t e d f o r heat l o s s e s t o the surroundings by the g r a p h i c a l procedure used above. Pour measurements of the heat of h y d r o l y s i s of vanadium t e t r a f l u o r i d e were made and the r e s u l t s are given i n Table 24. The s p e c i f i c heat was determined w i t h two runs on each r e s u l t a n t s o l u t i o n , and the average value f o r a l l experiments, -1 -1 0.985+0.002 c a l . g . deg. , was used i n the c a l c u l a t i o n s . The s o l u t i o n s were t i t r a t e d f o r vanadium w i t h standard 0.01N potassium permanganate s o l u t i o n a f t e r each experiment and the r e s u l t s I n d i c a t e d t h a t a l l the vanadium had d i s s o l v e d as vanadium ( I V ) . The vanadium analyses were w i t h i n 0.5 of the t h e o r e t i c a l value of 40.3$ vanadium f o r vanadium t e t r a f l u o r i d e . TABLE 24 HEAT OP HYDROLYSIS OP VANADIUM TETRAFLUORIDE 1. , 2. 3. 4. Wt. of VP 4(g.) 2.7763 2.3875 2.1476 2.3377 Wt. of water(g.) 956.2 956.25 956.2 956.2 % V (by t i t r a t i o n ) 39.7 40.0 40.5 40.0 Water e q u i v a l e n t of / / \ 4 Q S + 0 5 c a l o r i m e t e r ( c a l . / d e g . ) — A T ( g r a p h i c a l ) ( d e g r e e s ) 0.601 0.522 N 0.471 0.509 Heat l i b e r a t e d by wt. of VP4(cal.) 597.45 518.74 467.92 505.77 Heat l i b e r a t e d per mole(cal.) 27,330 27,593 27,671 27,476 151 The average heat of h y d r o l y s i s of vanadium t e t r a f l u o r i d e i s : A H h y d = -27,520 c a l . The standard d e v i a t i o n shown by the r e s u l t s i s _+120 cal . The r e s u l t s were q u i t e r e p r o d u c i b l e , as i n d i c a t e d by the low value of +120 c a l f o r the standard d e v i a t i o n . C o n s i d e r i n g the p o s s i b i l i t y t h a t unknown e r r o r s may a r i s e i n the experiment the r e s u l t s are to be considered r e l i a b l e t o w i t h i n +0.5 k c a l or about 1.5$. ( i i i ) The Heat of H y d r o l y s i s of Vanadium P e n t a f l u o r i d e Since vanadium p e n t a f l u o r i d e i s a l i q u i d at 25°, the sample h o l d i n g device d e s c r i b e d f o r vanadium t e t r a f l u o r i d e could not be used. The sample h o l d e r designed f o r l i q u i d vanadium p e n t a f l u o r i d e i s shown i n F i g u r e 3. The sample was contained i n an approximately 2 ml. c y l i n d r i c a l s t a i n l e s s s t e e l chamber, 6 mm. i n t e r n a l diameter and 30 mm. long which was attached to a 13 cm. long, 4 mm. diameter t h i n - w a l l e d s t a i n l e s s s t e e l tube. A p l a t i n u m diaphragm, 0.002 mm. t h i c k , was welded to the bottom of the sample chamber to provide a vacuum t i g h t s e a l . The top of the long s t a i n l e s s s t e e l tube was welded to a Kovar-Pyrex graded s e a l . Glass was used at the top of the sample h o l d e r to s i m p l i f y the manipulations r e q u i r e d to i n t r o d u c e vanadium p e n t a f l u o r i d e i n t o the sample h o l d e r . There was no danger of an e t c h i n g r e a c t i o n o c c u r r i n g i n the presence of the g l a s s d u r i n g the h y d r o l y s i s because the g l a s s p o r t i o n s of the apparatus were not i n contact w i t h the r e s u l t i n g s o l u t i o n . 152 The g l a s s end of the graded s e a l was sealed t o a break-s e a l and a c a p i l l a r y c o n s t r i c t i o n i n l e t . The sample h o l d e r was connected t o the vanadium p e n t a f l u o r i d e supply system by means of a B-10 j o i n t a t t a c h e d t o the arm of the h o l d e r w i t h the c a p i l l a r y c o n s t r i c t i o n . The B-10 j o i n t was s e a l e d w i t h a T e f l o n s l e e v e , l i g h t l y smeared w i t h f l u o r o c a r b o n grease. An inne r tube passed through the j o i n t to minimize co n t a c t of the vanadium p e n t a f l u o r i d e w i t h the greased j o i n t . The weighed sample h o l d e r was evacuated and c a r e f u l l y d r i e d . About one gram of vanadium p e n t a f l u o r i d e was d i s t i l l e d i n t o the sample ho l d e r which was then s e a l e d o f f at the c a p i l l a r y c o n s t r i c t i o n and reweighed. The sample h o l d e r con-t a i n i n g the vanadium p e n t a f l u o r i d e was then suspended i n the c a l o r i m e t e r v e s s e l , through the hollow s h a f t of the s t i r r e r , and the system was pe r m i t t e d t o come to thermal e q u i l i b r i u m w i t h the standard sodium hydroxide s o l u t i o n contained i n the c a l o r i m e t e r v e s s e l . Below the diaphragm a smal l sharp spike was h e l d i n a v e r t i c a l p o s i t i o n i n a nichrome frame, as shown i n F i g u r e 3. The frame was f i x e d t o the bottom of the s t i r r e r s h a f t by means of a threaded brass c o l l a r so t h a t the spike was h e l d c o n c e n t r i c w i t h the s t i r r e r s h a f t , and thus w i t h the sample holder which was h e l d i n the hollow s t i r r e r s h a f t . When thermal e q u i l i b r i u m had been reached, a s l i g h t p r e ssure of dry n i t r o g e n was a p p l i e d a t the top of the sample ho l d e r and the b r e a k s e a l was broken. The sample ho l d e r was then g e n t l y f o r c e d down onto the sharp s p i k e , r u p t u r i n g the p l a t i n u m diaphragm and a l l o w i n g the n i t r o g e n t o f o r c e the vanadium p e n t a f l u o r i d e i n t o the sodium hydroxide s o l u t i o n . The n i t r o g e n pressure was then r e l e a s e d and a l i t t l e s u c t i o n a p p l i e d to p u l l the s o l u t i o n up to the top of the metal p a r t of the c o n t a i n e r and thus hydrolyze any vanadium p e n t a f l u o r i d e adhering to the w a l l s of the c o n t a i n e r . The sample system was then opened to the atmosphere and the water l e v e l i n the h o l d e r was allowed to come to e q u i l i b r i u m w i t h the l e v e l i n the c a l o r i m e t e r v e s s e l . Temperature measurements were taken at one minute i n t e r v a l s f o r s e v e r a l minutes before and a f t e r h y d r o l y s i s , and the temperature r i s e on h y d r o l y s i s was c o r r e c t e d f o r heat l o s s e s to the surroundings by the g r a p h i c a l method used above. Pour determinations of the heat of h y d r o l y s i s of vanadium p e n t a f l u o r i d e were done i n the above manner. The r e s u l t s are shown i n Table 25. S p e c i f i c heat measurements were made on each of the r e s u l t a n t s o l u t i o n s and the average r e s u l t of s i x experiments -1 -1 on the f o u r s o l u t i o n s , 0.978+0.005 c a l g. deg. was used i n subsequent c a l c u l a t i o n s of the heat of h y d r o l y s i s of vanadium p e n t a f l u o r i d e . The n o r m a l i t y of the r e s u l t a n t a l k a l i n e vanadate s o l u t i o n s was determined by t i t r a t i o n s w i t h standard 0.1N h y d r o c h l o r i c a c i d s o l u t i o n and the amount of vanadium p e n t a f l u o r i d e d i s s o l v e d was c a l c u l a t e d from the d i f f e r e n c e between the i n i t i a l and f i n a l n o r m a l i t i e s a c c o r d i n g to the h y d r o l y s i s scheme: V P 5 ( l ) + 60H"(aq) — > VO~(aq) + 5F~(aq) + 3H"20 The s o l u t i o n s were a l s o analysed f o r vanadium. The r e s u l t s f o r two s o l u t i o n s were about 2$ h i g h e r than the 35.0$ vanadium expected f o r pure vanadium p e n t a f l u o r i d e . Two other s o l u t i o n s agreed w e l l w i t h the expected r e s u l t f o r vanadium p e n t a f l u o r i d e . The a n a l y t i c a l f i g u r e s f o r vanadium were a l s o used to c a l c u l a t e the weight of vanadium p e n t a f l u o r i d e which had been d i s s o l v e d . In a l l cases the weight of vanadium p e n t a f l u o r i d e c a l c u l a t e d from the vanadium a n a l y s i s agreed w i t h the weight of vanadium p e n t a f l u o r i d e c a l c u l a t e d from the n e u t r a l i z a t i o n of sodium hydroxide as above, however only i n experiments 3 and 4 d i d t h i s agree w i t h the weight of vanadium p e n t a f l u o r i d e determined by the gross and t a r e weights of the sample h o l d e r s both before and a f t e r h y d r o l y s i s . The reason f o r t h i s d i s c r e p a n c y i s not known, but i t does not appear to a f f e c t the heat of h y d r o l y s i s i n a systematic manner, because s o l u t i o n s w i t h h i g h e r vanadium c o n c e n t r a t i o n s than expected gave heats of h y d r o l y s i s both hi g h e r and lower than those s o l u t i o n s which gave the expected c o n c e n t r a t i o n s of vanadium. Ther e f o r e the e r r o r r e s u l t i n g from t h i s source can not be estimated, and i s probably Included i n the standard d e v i a t i o n of the r e s u l t s . T i t r a t i o n of the vanadate s o l u t i o n s immediately a f t e r the h y d r o l y s i s experiment i n d i c a t e d no d e t e c t a b l e amounts of reduced s p e c i e s such as t e t r a v a l e n t vanadium. TABLE 25 THE HEAT OP HYDROLYSIS OP VANADIUM PENTAPLUORIDE 1. 2. 3. 4. Wt. NaOH S o l u t i o n (g) 900.0 900.0 897.0 900.0 Wt VBV i n sample c o n t a i n e r (g) 0.9993 0.9971 I.0891 1.1552 I n i t i a l n o r m a l i t y NaOH S o l u t i o n (N) 0.1845 0.1845 0.1115 0.1115 P i n a l n o r m a l i t y NaOH S o l u t i o n (N) 0.1370 0.1370 0.0610 0.0595 Wt. VFfc d i s s o l v e d ( c a l c . from n o r m a l i t y change)(g) 1.05 1.07 1.12 1.15 %Y by t i t r a t i o n w i t h KMnOi| 37.0 37.2 35.7 35.0 Water e q u i v a l e n t (cal/deg) - 79+1 A T ( g r a p h i c a l ) (too) 1.000 1.014 1.110 1.145 Heat evolved by wt. of V P 5 ( c a l . ) 960.18 973.62 1062.65 1099.58 Heat evolved per mole of V P 5 ( c a l . ) 140,284 142,562 142,454 138,970 The average heat of h y d r o l y s i s of vanadium p e n t a f l u o r i d e i s : A H h y d =-141,100 c a l . The standard d e v i a t i o n of the r e s u l t s i s + 1,750 c a l . The standard d e v i a t i o n i n the h y d r o l y s i s of vanadium p e n t a f l u o r i d e appears to be much high e r than t h a t f o r vanadium t e t r a f l u o r i d e , but the g r e a t e r magnitude of the heat of h y d r o l y s i s of the former r e s u l t s i n the r e l a t i v e e r r o r s being 156 comparable. The standard d e v i a t i o n of +1.8 k c a l i n l4 l k c a l observed i n the heat of h y d r o l y s i s of vanadium p e n t a f l u o r i d e r e p r e s e n t s a r e l a t i v e e r r o r of about 1$, whereas the e r r o r s i n the vanadium t e t r a f l u o r i d e h y d r o l y s i s were +0.12 k c a l . i n 27 k c a l . or about 0.5$. The l a r g e r e r r o r i n the h y d r o l y s i s of vanadium p e n t a f l u o r i d e r e f l e c t s the g r e a t e r d i f f i c u l t i e s encountered i n d i s s o l v i n g a r e a c t i v e v o l a t i l e l i q u i d compared w i t h a s o l i d . I t i s estimated as before t h a t unknown sources of e r r o r w i l l p o s s i b l y c o n t r i b u t e a d d i t i o n a l e r r o r s of compar-able magnitude to the e r r o r a l r e a d y appearing i n the standard d e v i a t i o n , so the r e s u l t s are probably r e l i a b l e to +3 k c a l or 2$. ( i i i ) The Heat of H y d r o l y s i s of Vanadium T e t r a c h l o r i d e Since there i s no danger of a t t a c k of g l a s s i n the h y d r o l y s i s of a c h l o r i d e , a g l a s s sample ho l d e r was used. The sample was contained In a bulb of about 3 ml. c a p a c i t y w i t h a f r a g i l e g l a s s t i p a t the bottom and a b r e a k s e a l at the top. The b r e a k s e a l was s e a l e d to a 25 cm. l e n g t h of 8 mm. t u b i n g so t h a t i t could be suspended i n the c a l o r i m e t e r through the hollow s t i r r e r s h a f t . Vanadium t e t r a c h l o r i d e , prepared as d e s c r i b e d p r e v i o u s l y , was d i s t i l l e d i n t o t a r e d sample h o l d e r s through the f r a g i l e g l a s s t i p , the f r a c t i o n s d i s t i l l i n g between 152° and 154° being taken. The sample c o n t a i n e r s were sea l e d a t atmospheric p r e s s u r e , weighed and immersed i n the c a l o r i m e t e r , through the hollow s t i r r e r s h a f t , 157 w i t h the top end of the long 8 mm. tube emerging from the hollow s h a f t . The bulb c o n t a i n i n g the vanadium t e t r a c h l o r i d e was then s i t t i n g i n the c a l o r i m e t e r v e s s e l which contained a t a r e d amount of water. The system was allowed to re a c h thermal e q u i l i b r i u m . A s l i g h t p r e ssure of n i t r o g e n gas was a p p l i e d through the tube emerging from the c a l o r i m e t e r , and the b r e a k s e a l was broken t o admit the n i t r o g e n t o the sample v e s s e l . The f r a g i l e t i p of the sample bulb was broken by j e r k i n g the h o l d e r upwards a g a i n s t the end of the s t i r r e r s h a f t . The e x t r a p ressure of ni t r o g e n i n the sample c o n t a i n e r f o r c e d the vanadium t e t r a -c h l o r i d e i n t o the water and prevented the h y d r o l y s i s products from b l o c k i n g the e x i t of the sample c o n t a i n e r . The n i t r o g e n pressure was r e l e a s e d and the water allowed to flow i n t o the co n t a i n e r t o remove any l a s t t r a c e s of vanadium t e t r a c h l o r i d e . T h i s technique i s s i m i l a r to t h a t used by Ruff and P r e i d r i c h (85) i n t h e i r s t u d i e s of the heat of h y d r o l y s i s of vanadium t e t r a c h l o r i d e i n a l k a l i n e peroxide s o l u t i o n . Pour determinations of the heat of h y d r o l y s i s were made and the r e s u l t s are shown i n Table 26. Temperatures were c o r r e c t e d g r a p h i c a l l y f o r heat l o s s t o the surroundings as b e f o r e . The s p e c i f i c heat of each r e s u l t a n t vanadyl c h l o r i d e s o l u t i o n was determined immediately a f t e r the h y d r o l y s i s experiment and the average value f o r s i x d e t e r m i n a t i o n s , O.98O+O.OO5 cal.g'^degT" 1", was used i n subsequent c a l c u l a t i o n s of the heat of h y d r o l y s i s . The s o l u t i o n s were analyzed a f t e r each experiment and the r e s u l t s were found t o be w i t h i n 0.6 158 of the t h e o r e t i c a l value of 26.4$ vanadium expected f o r vanadium t e t r a c h l o r i d e . A l l the r e s u l t s were s l i g h t l y h i g h which may i n d i c a t e t h a t the weight of water taken, while i t i s constant and r e p r o d u c i b l e throughout, i s not e x a c t l y known. Th i s w i l l not a f f e c t the c a l o r i m e t r i c values because the apparatus was c a l i b r a t e d w i t h the same amount of water as i s used i n the h y d r o l y s i s experiments, hence any d e v i a t i o n of the measured weight of water from the t r u e weight w i l l be i n c o r -porated i n t o the water e q u i v a l e n t of the system. TABLE 26 HEAT OP HYDROLYSIS OF VANADIUM TETRACHLORIDE Wt. of V C l 4 ( g . ) 2.5098 2.3686 2.5000 2.0053 Wt. of H 20(g.) 900.0 900.0 900.0 900.0 $V 26.6 27.0 26.7 26.9 Water e q u i v a l e n t ( c a l . deg.) 99 95.3 95.3 95.3 A.T(graphical) (deg.) 0.899 O.858 0.903 0.740 Heat l i b e r a t e d by wt. of V C l ^ . ( c a l . ) 884.1 840.5 884.7 724.9 Heat l i b e r a t e d per • mole VC14 ( c a l . ) 67,994 68,488 68,302 69,650 The average heat of h y d r o l y s i s of vanadium t e t r a c h l o r i d e i s : A H h y d =-68,630 c a l . 159 The standard d e v i a t i o n of the r e s u l t s i s +440 c a l . or ap p r o x i -mately 0.6$. The f a i r l y low standard d e v i a t i o n i n d i c a t e s t h a t the use of n i t r o g e n gas to f o r c e the vanadium t e t r a c h l o r i d e from the sample c o n t a i n e r i n t o the s o l u t i o n does not i n t r o d u c e e x c e s s i v e l y l a r g e e r r o r s . Unevaluated e r r o r s w i l l probably double t h i s e r r o r as before, hence the r e s u l t s are probably r e l i a b l e t o + l k c a l or about 1.3$. 160 APPENDIX: THE THERMOCHEMISTRY OP THE VANADYL ION The measured heat of h y d r o l y s i s of vanadium t e t r a -c h l o r i d e ( r e a c t i o n a) i n water was combined w i t h the standard heat of formation of vanadium t e t r a c h l o r i d e ( r e a c t i o n b) and the thermochemical c y c l e completed by r e a c t i o n s (c) to (e) to give a r e a c t i o n (g) r e p r e s e n t i n g the p r e v i o u s l y unknown standard heat of formation of the V 0 + + ( a q ) i o n . (a) V C l ^ ( l ) + H 2 0 ( l ) 5>V0 + +(aq) + 2H +(ag) + 4C1~ (b) V(c) + 2Cl 2(g) > V C 1 4 ( 1 ) (c) 4Cl~(aq) + 4 H + ( a q ) ^ 2Cl 2(g) + 2H 2(g) (d) H 2(g) + 1/2 0 2(g) 5^H 20(1) (e) H 2(g) > 2 H + ( a q ) The a d d i t i o n of r e a c t i o n s (a) to (e) together r e s u l t s i n r e a c t i o n ( g ) : (g) V(c) + 1/2 0 2(g) • >.V0 + +(aq) , which i s the formation r e a c t i o n f o r the aqueous vanadyl i o n from i t s elements i n t h e i r standard s t a t e s . The heat of r e a c t i o n (g) which i s the heat of formation of V 0 + + ( a q ) , i s given by the e x p r e s s i o n : H = H +K +H +RL+H . g a D c d e The r e q u i r e d data f o r the c a l c u l a t i o n of H are shown i n Table S 27. 161 TABLE 27 STANDARD HEAT OP FORMATION OP THE V O + + ( a q ) ION Heat of r e a c t i o n H a D e s c r i p t i o n Numerical Values kcal/mole kcal/mole H •b H H d H. heat of h y d r o l y s i s of V C l ^ A H f 0 ( v c i 4 ) -4( A H f 0 ( H C l ) ( a q ) ) A H f O ( H 2 0 ( l ) ) A H f 0 ( H + ( a q ) ) -4(-40.02) -68.6 -136 160 - 68.32 0 Reference Present work. (Table 26) 84 83 T h e r e f o r e : H = -113 kcal./mole to As before the only e r r o r s estimated are those a r i s i n g from the h y d r o l y s i s r e a c t i o n , which i n the case of vanadium t e t r a -c h l o r i d e are probably about +1 kcal./mole. The heats of form-a t i o n of each of the ions are the values given at i n f i n i t e d i l u t i o n as b e f o r e . A l l numerical v a l u e s are those given f o r 298°K. Therefore the heat of formation of the vanadyl i o n at 298°K i s : A H f , o ( V 0 + + ( a q ) ) = -113 + 1 kcal./mole. The standard f r e e energy of formation of V O + + ( a q ) , obtained from measurements on e l e c t r o c h e m i c a l c e l l p o t e n t i a l s i s ; A P f 0 ( V O + 4 " ( a q ) ) = -109 kcal./mole (95). 162 S u b s t i t u t i n g t h i s value f o r A P f 0 a n d t n e P r e s e r ] t value f o r the heat of formation of the vanadyl i o n i n t o the equation, A F f Q = A H f 0 - T A S f 0 g i v e s a value of -13.4 entropy u n i t s f o r the entropy change of formation, A S^ 0» The corresponding value f o r the vanadate (VO^) i o n i s not known (95) t h e r e f o r e comparisons are not p o s s i b l e . R u f f and F r e i d r i c h (85) measured the heat of the r e a c t i o n (h) VC1^(1) + | OH"(aq) + \ 0 2H~(aq) VO~(aq) + -| H 20 + 4Cl"(aq) and found a value of -l6l kcal./mole. T h i s can be added to the r e v e r s e of the prese n t h y d r o l y s i s r e a c t i o n of vanadium t e t r a f l u o r i d e , (eg. r e a c t i o n a ), (a') V O + + ( a q ) + 4Cl"(aq) + 2H +(aq) < — V C l ^ ( l ) + HpO(l) to y i e l d r e a c t i o n ( j ) ; ( j ) V O + + ( a q ) + 5/2 OH~(aq) + 1/2 Q^T VO~(aq) + 7/2 BLOU), which r e p r e s e n t s the o x i d a t i o n of t e t r a v a l e n t vanadium to penta-v a l e n t vanadium i n a l k a l i n e peroxide s o l u t i o n . Since the heat of r e a c t i o n (a') i s the negative of the measured heat of h y d r o l y s i s of vanadium t e t r a c h l o r i d e , t h a t i s H i = 68.6 kcal./mole, the heat of r e a c t i o n ( j ) i s given by H. = H + H i = -161 + 68.6 = -92.4 kcal./mo j n a Thus the heat of o x i d a t i o n o f t e t r a v a l e n t vanadium t o penta-v a l e n t vanadium i n a l k a l i n e peroxide s o l u t i o n i s about -92 kcal./mole. 164 REFERENCES: 1. Weinstock, B. and Malm, J.G., J.A.C.S. (1958), 80, 4466. 2. Sharpe, A.G., J.C.S. (1949), 2901. 3. B a r t l e t t , N. and Ma i t l a n d , R., A c t a . C r y s t . (1958),11,747. 4. B a r t l e t t , N. and Robinson, P.L., J.C.S. (l96l), 3417. 5. Kemmitt, R.D.W. and Sharp, D.W.A., J.C.S. (1961), 2496. 6. O'Donnell, T.A., J.C.S. (1956), 4681. 7. C l a r k , H.C. and Emeleus, H.J., J.C.S. (1957), 2119. 8. C l a r k , H.C. and Emeleus, H.J., J.C.S. (1958), 190. 9. Trevorrow, L.E., F i s h e r , J . and Steunenberg, R.K., J.A.C.S. (1957), 79, 5165. 10. Sharpe, A.G., Adv. F l u o r i n e . Chem. (i960), 1, 29. 11. Sharpe, A.G., Q. Revs. (1957), 11, 49. 12. Ruff, 0. and L i c k f e t t , H., Ber. (1911), 44, 2539. 13. Simons, J.H. and Powell, M.G., J.A.C.S. (1945), 67, 75. 14. Gutmann, V. and Emeleus, H.J., J.C.S. (1949), 2979. 15. Huss, E. and Klemm, W., Z. anorg. chem. (1950), 262, 25. 16. Hoppe, R., R e c u e i l . (1956), 75, 569. 17. L i e b e , ¥., Weise, E. and Klemm, W., Z. anorg. chem. (1961), 311, 281. 18. F a i r b r o t h e r , P. and F r i t h , W.C., J.C.S. (1951), 3051. 19. C l a r k , H.C, Chem. Revs. (1958), 869. 20. M u e t t e r t i e s , E.L. and P h i l l i p s , W.D., J.A.C.S. (1957), 79, 322. 21. Rogers, M.T. and Katz, J . J . , J.A.C.S. (1952), 74, 1375. 22. Gutmann, V., and Jack, K.H., A c t a . C r y s t . (1951), 4, 246. 23. Peacock, R.D., Prog. Inorg. Chem. (i960), 2, 193. 24. Gaunt, J . , Trans. F a r a . Soc. (1953), 49, 1122. 25. von Wartenberg, H., Z. anorg. chem. (1939)* 242, 406. 26. Greenberg,E., S e t t l e , J.L., Feder, H.M. and Hubbard, W.N. J . Phys. Chem. (l96l), 65, 1168. 27. S e t t l e , J.L., Feder, H.M. and Hubbard, W.N., J . Phys. Chem. (1961), 65, 1337. 28. Gross, P., A b s t r a c t s of the F i r s t I n t l . Symposium on F l u o r i n e Chemistry, Birmingham, J u l y (1959), P-15-29. D a r n e l l , A.J., J . Inorg. Nucl. Chem. (i960), 15, 359. 30. Woolf, A.A., J.C.S. (1951), 231. 31. Meyers, O.E. and Brady, A.P., J . Phys. Chem. (i960), 64,591. 32. Waddington, T.C., Adv. Inorg. Chem. Radiochem. (1959), 1, 157. 33. M o e l l e r , T. "Inorganic Chemistry", Wiley, New York (1952). 34. George, J.W., Prog. Inorg. Chem. (i960), 2_, 33. 35. Katz, J . J . and Sheft, I., Adv. Inorg. Chem. Radiochem. (I960), 2, 195. 36. Wyckoff, R.G., " C r y s t a l S t r u c t u r e s " , v o l . I, I n t e r s c i e n c e , New York (1958). 37. G i l l e s p i e , R.J. and Nyholm, R.S., Q. Revs. (1957), 11, 339. 38. Dodd, R.E., Woodward, L.A. and Roberts, H.L. Trans. F a r a . Soc. (1955), 52, 1052. 39. Cotton, F.A., George, J.W. and Waugh, J.S., J . Chem. Phys. (1958), 28, 994. 40. R o l f , J.A., Woodward, L.A. and Long, D.A., Trans. F a r a . Soc. (1953), 49, 1388. 41. M u e t t e r t i e s , E.L. and P h i l l i p s , W.D., J.A.C.S. (1959), 81, 1084. 42. Burbank, R.D. and Bensey, F.N., J . Chem.Phys. (1953), 21, 602. 43. Hoffmann, C.J., Holder, C.E. and J o l l y , W.L., J . Phys. Chem. (1958), 62, 364. 44. Woolf, A.A. and Greenwood, N.N., J.C.S. (1950), 2200. 166 45. Hub, D.R. and Robinson, P.L., J.C.S. (1954), 2640. 46. Rogers, M.T. and Garver, E.E., J . Phys. Chem. (1958), 62, 952. 47. Brown, P. and Robinson, P.L., J.C.S. (1955), 3147. 48. Peacock, R.D., J.C.S. (1953), 3617. 49. F a i r b r o t h e r , P., F r i t h , W.C. and Woolf, A.A. J.C.S. (1954), 1031. 50. Glasstone, S. "Textbook of P h y s i c a l Chemistry", 2nd ed. Van Nostrand, New York, (1946). 51. Huckel, W., " S t r u c t u r a l Chemistry of Inorganic Compounds" ( E n g l i s h e d i t i o n t r a n s l a t e d by L.H. Long) Elsevier,-Amsterdam, (1951), volume I I , pp. 459-474. 52. von Wartenberg, H., Z. anorg. chem. ( l 9 4 l ) , 247, 135. 53. Zachariasen, W.H., Ac t a . C r y s t . (1949), 2, 388. 54. E u l e r , R.D. and Westrum, E.P., J . Phys. Chem. (1961), 65, 132. 55. B a r t l e t t , N. and Lohmann, D.H., unpublished o b s e r v a t i o n s . 56. C a v e l l , R.G., M.Sc. T h e s i s , U.B.C. ( i960) . 57. D'Eye, R.W.M. and Wait, E., "X-ray Powder Photography i n Inorganic Chemistry""^ Butterworths, London, (I960), p. 186. 58. P a u l i n g , L., "The Nature of the Chemical Bond", 3rd e d i t i o n , C o r n e l l , Ithaca, N.Y., ( I 9 6 0 ) . 59. Sharpe, A.G. and Woolf, A.A., J.C.S. (1951), 798. 60. C a v e l l , R.G. and C l a r k , H.C, J . Inorg. N u c l . -Chem. (1961), 17, 257. 61. M u e t t e r t i e s , E.L., J.A.C.S. ( i960) , 82, 1082. 62. B a r t l e t t , N. and Q u a i l , J.W., J.C.S. (1961), 3728. 63. F i g g i s , B.N. and Lewis, J . , "Modern C o o r d i n a t i o n Chemistry" e d i t e d by Lewis, J . and W i l k i n s , R.G. I n t e r s c i e n c e , New York, ( i960) . Chapter 6. 64. Selwood, P.W., "Magnetochemistry", 2nd e d i t i o n , I n t e r s c i e n c e , New York, (1956). 167 65. Machin, D.J., Personal communication. 66. Perakins, N., J . Phys. Radium (1927), 8, 473. 67. Klemm, W. and Hoschek, E., Z. anorg. chem. (1936), 226,359. 68. Fleming, D.G., B.Sc. T h e s i s , U.B.C. (1961). 69. Van V l e c k , J.H., " E l e c t r i c and Magnetic S u s c e p t i b i l i t i e s " , Oxford, (1932). 70. G r i f f i t h , J.S., "Theory of T r a n s i t i o n Metal Ions", Cambridge, (1961), pp. 276-8. 71. K o t a n i , M., J . Phys. Soc. (Japan), (1949), 4, 293. 72. Hargreaves, G.B. and Peacock, R.D., J.C.S. (1958), 3776. 73. G r i f f i t h s , J.H.E., Owen, J . , Park, J.G. and P a r t i d g e , M.F., Proc. Roy. Soc. (1959), A, 250, 84. 74. Anderson, P.W., Phys. Rev. (1950), 79, 350. 75. Wilmshurst, J.K. and B e r n s t e i n , H.J.,- J . Chem. Phys. (1957), 27, 661. 76. Wilmshurst, J.K., Mol. Spect. (i960), 5, 343. 77. Gaunt, J . and Ainscough, J.B., Spect. Acta (1957), 1Q» 57. 78. Gutowsky, H.S. and L i e h r , A.D., J . Chem. Phys. (1952), 20,1652. 79. Burke, T.G. and Jones, E.A., J . Chem. Phys. (1961),19,l6ll. 80. Badger, R.M. and Zumwalt, L.R., J . Chem. Phys. (1938), 6, 711. 81. Herzberg, G., " I n f r a r e d and Raman Spectra", Van Nostrand, Princeton,- N.J. (1945). 82. Peacock, R.D., and Sharp, D.W.A., J.C.S. (1959), 2762. 83. " S e l e c t e d Values of Chemical Thermodynamic P r o p e r t i e s " , N a t i o n a l Bureau of Standards C i r c u l a r 500, Washington, (1952). 84. Gross, P., Personal Communication. 85. Ruff, 0. and F r e i d r i c h , L., Z. anorg. chem. (1914), 89, 279. 86. K a p u s t i n s k i i , A.F., Q. Revs. (1956), 10, 283. 168 87. C o t t r e l l , T.L., "The Strengths of Chemical Bonds", Butterworths, (1958). 88. "Atomic Energy L e v e l s " , N a t i o n a l Bureau of Standards C i r c u l a r 467, Washington, (1949). 89. Barber, M., L i n n e t t , J.W. and T a y l o r , N.H., J.C.S. (1961), 3323. 90. von Wartenberg, H., Z. anorg. chem. (1942), 249 100. 91. Kubaschewski, 0. and Evans, E.L., " M e t a l l u r g i c a l Thermochemistry", 3rd. e d i t i o n , Pergamon, London, (1958). 92. Hoppe, R., Dahne, W. and Klemm, W., Naturwiss. (1961), 48, 429. 93. Hoppe, R., Personal communication to N. B a r t l e t t , U.B.C. 94. M u e t t e r t i e s , E.L. and C a s t l e , J.E., J . Inorg. N u c l . Chem. (1961), 18,149. 95. Latimer, W.M., "Oxidation P o t e n t i a l s " , 2nd. e d i t i o n , P r e n t i c e - H a l l , New York, (1952). 96. F i g g i s , B.N. and Nyholm, R.S., J.C.S. (1958), 4190. 97. F i g g i s , B.N., and Nyholm, R.S., J.C.S. (1959), 331. 98. C l a r k , H.C. and O'Brien, R.J., Can. J . Chem. (1961), 39, 1030. 99. W i l l a r d , H.H. and Winter, O.B., Ind. Eng. Chem. (1933), 5, 7. 100. Dodd, R.E. and Robinson, P.L., "Experimental Inorganic Chemistry", E l s e v i e r , Amsterdam, (T957)] 101. Aynsley, E.E., Peacock, R.D. and Robinson, P.L., J.C.S. (1952), 1231. 102. Mertes, A.T., J.A.C.S. (1913), 35, 671. 103. Aynsley, E.E., Hetherington, G. and Robinson, P.L., J.C.S. (1954), 1119. 104. D a n i e l s , F., Matthews, J.H., W i l l i a m s , J.W., Bender, P. and A l b e r t y , R.A., "Experimental P h y s i c a l Chemistry", McGraw-Hill, New York, (1956). 105. Orgel, L.E., " I n t r o d u c t i o n t o T r a n s i t i o n Metal Chemistry", Methuen, London, (I960). 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.831.1-0062265/manifest

Comment

Related Items