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Chromium tetrafluoride and related compounds. Sadana, Yoginder Nath 1963

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CHROMIUM TETRAFLUORIDE AND RELATED COMPOUNDS by IOGINDER NATH SADANA M.Sc., The University of Agra, 1953 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in the Department of Chemistry We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA April, 1963 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Chemistry  The University of British Columbia, Vancouver 8, Canada. Date April. 1 9 6 3 GRADUATE STUDIES F i e l d of Study: Inorganic Chemistry Topics i n Inorganic Chemistry Topics i n Organic Chemistry Advanced Inorganic Chemistry Seminar Related Studies: H.C. Clark N. Bart l e t t W.R. Cullen D.E. McGreer J.P. Kutney R. Bonnet N. B a r t l e t t H.C. Clark D.E. McGreer Structure of Metals I I Physical Metallurgy Applied Chemical Metallurgy E. Teghtsoonian J.A.H. Lund E. Peters Mrs. A.M. Armstrong The University 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 YOGINDER N. SADANA B.Sc, University of Agra, 1951 M.Sc, University of Agra, 1953 SATURDAY, MAY 4th, 1963, at 9:30 A.M. IN ROOM 261, CHEMISTRY BUIDING COMMITTEE IN CHARGE Chairman:. F.H. Soward K.B. Harvey C.A. McDowell E. Teghtsoonian. . J , Trotter External Examiner:. A.G. Sharpe Department of Inorganic Chemistry University o f Cambridge Cambridge, England N. B a r t l e t t H.C. Clark F. W. Dalby G. Dutton CHROMIUM TETRAFLUORIDE AND RELATED COMPOUNDS ABSTRACT The present investigation was concerned with a study, of the preparation and properties of chromium tet r a -fluoride and i t s complexes; magnetic properties and X-ray structures were also studied. Attempts to prepare the pentafluoride of chromium using fl u o r i n a t i n g agents other than fluorine were unsuccessful. Chromium t e t r a f luoride was prepared by the, action of fluorine on the heated metal, the.products being invola-t i l e chromium t r i f l u o r i d e , v o l a t i l e chromium t e t r a -f l u o r i d e , and more v o l a t i l e fluorides which were con-densed at "78°C. The best yields of chromium tet r a -fluoride were obtained at 3.50°C and with a moderate flow rate of f l u o r i n e . Chromium t e t r a f l u o r i d e i s a glassy s o l i d which forms blue vapours when heated. I t does not dissolve i n the usual organic solvents and i s hydrolysed readily in moist a i r . I t i s paramagnetic, the moment corresponding to two unpaired electrons, and has a Weiss Constant of -70°. Chemically, chromium tetrafluoride i s s u r p r i s i n g l y inert and at room temperature does not react with ammonia, pyridine, sulphur dioxide, sulphur t r i o x i d e , iodine pentafluoride,bromine t r i f l u o r i d e , bromine penta-fl u o r i d e , chlorine t r i f l u o r i d e , or selenium te t r a -f l u o r i d e . However, i t reacts instantaneously with water giving C r + 3 and Cr04~2 ions i n solution. When heated with bromine t r i f l u o r i d e for longer periods, reaction occurs to give CrFg'O.5BrF3> On heating with a mixture of bromine t r i f l u o r i d e and bromine pentafluoride, a wine-coloured solution was obtained, and the residue after d i s t i l l a t i o n of v o l a t i l e materials was CrOF3'0.25 BrF3. The reaction of selenium tetrafluoride with chromium t e t r a f l u o r i d e at 120°C yielded two compounds, CrF3.SeF4 and CrF2.SeF4. Complexes of the type ACrF^, where A = K, Rb, or Cs, were prepared for the f i r s t time, by heating the respec-t i v e a l k a l i metal chloride and chromium t e t r a f l u o r i d e i n a 1:1 molar r a t i o i n bromine t r i f l u o r i d e . The resulting complex compounds contained half a molecule of bromine t r i f l u o r i d e per molecule of the com-plex, i f the removal of excess bromine t r i f l u o r i d e , after the reaction, was carried out at 100°C-. How-ever, i f removal was carried out at 160°C, the pure . complexes were obtained. Complexes of the type A2CrF^, where A =;.K, or Cs, were prepared i n bromine t r i f l u o r i d e solutions„ The presence of extra bromine t r i f l u o r i d e i n the molecule of the complex compound affected the structure seriously in potassium complexes but had no effect on cesium complexes. Thus K^CirF^O,, SErF^ was tetragonal and on heating at 160°C in vacuo yielded the cubic modification of pure K 2CrFg; C s ' 0 0 5 B r F 3 ' atl<* Cs 2CrF 6 were, both cubic. Reactions of chromium t r i o x i d e with bromine t r i f l u o r -ide, bromine pentafluoride, and chlorine t r i f l u o r i d e were also investi.gate .de With bromine, fluorides, the products obtained were the corresponding adducts of CrOFg. These are very reactive and sensitive to moist a i r . The compounds are paramagnetic. The reaction with chlorine t r i f l u o r i d e was very i n t e r -esting because the appearance of the product depended on the experimental conditions. I f the reaction was carried out at ordinary temperature, the product was a buff-coloured powdery mass, but i f the reaction was performed by passing chlorine t r i f l u o r i d e vapour over heated chromium t r i o x i d e (100--120°C), the product was a brick-red substance. Chemical analyses and magnetic measure-ments indicated that both these products were i d e n t i c a l and had the composition CrOFg'0.25C1F3.- The brick-red product could be converted into the l i g h t buff-coloured powdery mass by heating at 70°C i n vacuo. . Reactions of potassium dichromate with bromine t r i -f l u o r i d e , bromine pentafluoride, and chlorine t r i -f l u oride were also investigated. Potassium dichromate reacted with these smoothly at room temperature and the product of reaction i n each case was KCrOF^. i i ABSTRACT The present investigation was concerned with a study of the preparation and properties of chromium tetrafluoride and i t s complexes; magnetic properties and X-ray structures were also studied. Attempts to prepare the pentafluoride of chromium using fluorinating agents other than fluorine were unsuccessful. Chromium tetrafluoride was prepared by the action of fluorine on the heated metal, the products being involatile chromium trifluoride, volatile chromium tetrafluoride, and more volatile fluorides which were condensed at -7&°C. The best yields of chromium tetrafluoride were obtained at 350°C and with a moderate flow-rate of fluorine. Chromium tetrafluoride i s a glassy solid which forms blue vapours when heated. It does not dissolve in the usual organic solvents and i s hydrolysed readily in moist a i r . It is paramagnetic, the moment corresponding to two unpaired electrons, and has a Weiss Constant of - 7 0 ° . Chemically, chromium tetrafluoride i s surprisingly inert and at room temperature does not react with ammonia, pyridine, sulphur dioxide, sulphur trioxide, iodine penta-" fluoride, bromine trifluoride, bromine pentafluoride, chlorine trifluoride, or selenium tetrafluoride. However, i t reacts instantaneously with water, giving Cr +^ and CrO^ ""^  ions in solution. When heated with bromine trifluoride for longer periods, reaction occurs to give CrF^•0.5BrF-D)• On i i i heating with a mixture of bromine t r i f l u o r i d e and bromine pentafluoride, a wine-coloured solution was obtained, and the residue af t e r d i s t i l l a t i o n of v o l a t i l e materials was C r O F ^ O ^ B r F ^ . The reaction of selenium t e t r a f luoride with chromium t e t r a f l u o r i d e at 120°C yielded two compounds, CrFySeF^ and CrF 2»SeF / f. Complexes of the type ICrF^, where A = K, Rb, or Cs, were prepared f o r the f i r s t time, by heating the respective a l k a l i metal chloride and chromium t e t r a f l u o r i d e i n a 1:1 molar r a t i o i n bromine t r i f l u o r i d e . The r e s u l t i n g complex compounds contained h a l f a molecule of bromine t r i f l u o r i d e per molecule of the complex, i f the removal of excess bromine t r i f l u o r i d e , a f t e r the reaction, was carried out at 100°C. However, i f removal was carried out at 160°C, the pure complexes were obtained. Complexes of the type A 2CrF6, where A = K or Cs, were prepared i n bromine t r i f l u o r i d e solutions. The presence of extra bromine t r i f l u o r i d e i n the molecule of the complex compound affected the structure ser i o u s l y i n potassium com-plexes but had no effect on cesium complexes. Thus K 2CrF^'0.5BrF^ was tetragonal and on heating at 160°C in, vacuo yielded the cubic modification of pure KgCrF^; Cs 2CrF6'0.5BrF2 and Cs 2CrF£ were both cubic. Reactions of chromium t r i o x i d e with bromine t r i -f l u o r i d e , bromine pentafluoride, and chlorine t r i f l u o r i d e were also investigated. With bromine f l u o r i d e s , the iv products obtained were the corresponding adducts of CrOF^. These are very reactive and sensitive to moist a i r . The compounds are paramagnetic. The reaction with chlorine trifluoride was very-interesting because the appearance of the product depended on/ the experimental conditions. If the reaction was carried out at ordinary temperature, the product was a buff-coloured powdery mass, but i f the reaction was performed by passing chlorine trifluoride vapour over heated chromium trioxide (100—120°C), the product was a brick-red substance. Chemical analyses and magnetic measurements indicated that both these products were identical and had the composition CrOF^'O^SClFo,. The brick-red product could be converted into the light buff-coloured powdery mass by heating at 70°C i n vaqup. Reactions of potassium dichromate with bromine trifl u o r i d e , bromine pentafluoride, and chlorine trifluoride were also investigated. Potassium dichromate reacted with these smoothly at room temperature and the product of reaction in each case was KCrOF^. V The compounds investigated i n the present research are tabulated below. Magnetic Moment Compound M-29L {& ) Crystal Structure 7 (B.M.) CrF 2.SeF 4 5.34 complex CrF^-SeF^ • • • • • • • CrF3«0.5BrF3 3.96 • • * • CrF 3.BrF 3 3.67 • • • • CrF 4 3.02 (-•70°) • • • • KCrF^ 3-15 hexagonal RbCrFc 5 3.17 (-•32°) hexagonal CsCrF^ 3.20 cubic KCrF5«0.5BrF3 3.22 (-•BCP) tetragonal CsCrF5.0.5BrF3 • • • cubic K 2CrF 6-0.5BrF 3 3.06 tetragonal K 2CrF 6 3.06 cubic Cs 2CrF 6»0.5BrF 3 3.14 cubic CrOF3-0.2$BrF3 2.02 • • • • CrOF3-0.25BrF5 1.85 • • • • CrOF3-0.25ClF3 1.83 • • • • KCrOF^»0.5BrF3 1.73 ( +4°) orthorhombic KCrOF, *0.5BrFf-4 5 1.75 complex ix ACKNOWLEDGEMENTS The work described in this thesis was carried out in the Laboratories of the Chemistry Department, University of British Columbia, under the supervision of the Head of the Department, Dr. C.A. McDowell, and the direction of Dr. H.C. Clark. The writer wishes to express his sincere thanks to Dr. McDowell and his deep appreciation of a l l that Dr. Clark has done for him during the progress of this work. He also wishes to express his thanks to Dr. N. .Bartlett for helpful discussions. The writer gratefully acknowledges the financial support given him by the National Research Council (Canada) in the form of a Studentship. v i TABLE OF CONTENTS Page ABSTRACT i i LIST OF TABLES v i i LIST OF FIGURES v i i i ACKNOWLEDGEMENTS ix Chapter I. INTRODUCTION 1 I I . THE PREPARATION AND PHYSICAL PROPERTIES OF CHROMIUM TETRAFLUORIDE 8 III. CHEMICAL PROPERTIES OF CHROMIUM TETRAFLUORIDE . . 19 IV. REACTIONS OF CHROMIUM TETRAFLUORIDE WITH HALOGEN FLUORIDES AND SELENIUM TETRAFLUORIDE . . 24 V. COMPLEXES DERIVED FROM CHROMIUM TETRAFLUORIDE . . 33 VI. MAGNETIC PROPERTIES 45 VII. REACTION OF CHROMIUM TRIOXIDE AND POTASSIUM DICHROMATE WITH HALOGEN FLUORIDES 62 VIII. THE ANALYTICAL DETERMINATION OF FLUORINE . . . . 72 IX. TETRAFLUORIDES OF THE FIRST TRANSITION SERIES . . 89 EXPERIMENTAL X. GENERAL TECHNIQUES 95 XI. ANALYTICAL TECHNIQUES 103 XII. MAGNETIC MEASUREMENTS AND X-RAY INVESTIGATIONS. . 114 X I I l . PREPARATION AND REACTIONS OF CHROMIUM TETRAFLUORIDE '. . 120 XIV. COMPLEXES DERIVED FROM CHROMIUM TETRAFLUORIDE . . 134 XV. REACTIONS OF CHROMIUM TRIOXIDE AND POTASSIUM DICHROMATE 154 REFERENCES 168 v i i . LIST OF TABLES Page 1. A comparison of the chemical properties of vanadium tetrafluoride and chromium tetrafluoride. . „93 2. Magnetic susceptibility measurements of chromium tetrafluoride 117 3 . Magnetic susceptibility measurements of KCrF^.O.^BrF^. . 135 4. Calculated and observed s i n 2 0 values for KCrF^ • 0 . 5BrF^ 137 5. Calculated and observed s i n 2 ^ values for KCrF^ 139 6. Magnetic susceptibility measurements of RbCrF^ 140 7. Calculated and observed s l n ^ O values for RbCrF^ 142 8. Calculated and observed s i n 2 0 values for CsCrF^ 143 9. Calculated and observed s i n % values for KMnF^  146 10. Magnetic susceptibility measurements of K 2CrF 6 .0.5BrF 3 147 11. Calculated and observed sin 2£ values for K 2CrF o .0.5BrF 3 148 12. Calculated and observed s l n ^ O values for K 2CrF D 150 13. Calculated and observed sin 2# values for Cs 2CrF D * 0 . 5 B r F 3 152 14. Magnetic susceptibility measurements of KCrOFif«0.5BrF3 163 v i i i LIST OF FIGURES page I. Stark Patterns for Cr+Z)" and C r + 5 in octahedral and tetrahedral environment 59 II. Reaction assembly for the preparation of KCrF^ 101 III. Plot o f J ^ ~ versus T for chromium tetraf luoride 118 1 I . INTRODUCTION The element f l u o r i n e , the most r e a c t i v e halogen, was i s o l a t e d i n 1886 by Moissan (1). Because o f i t s h i g h l y r e a c t i v e nature, i t s i s o l a t i o n was delayed, and i t s chemistry i s l e s s w e l l known than those o f o t h e r halogens, s i n c e u n l e s s c o n s i d e r a b l e care i s taken a study o f i t s behaviour w i l l always l e a d t o f l u o r i n a t i o n of the apparatus. However, wi t h the development o f many f l u o r i n e - r e s i s t a n t m a t e r i a l s and w i t h the a v a i l a b i l i t y o f f l u o r i n e commercially, r a p i d progress has been made i n r e c e n t y e a r s , and r e s e a r c h i n t h i s f i e l d i s c u r r e n t l y v e r y i n t e n s e . The p r e s e n t a c t i v i t y encompasses a l l aspects o f f l u o r i n e chemistry and has r e s u l t e d not o n l y i n the p r e p a r a t i o n of many new, simple and complex compounds, some o f which are indeed unique, such as 0 2 P t F ^ ( 2 ) , XeF^ (3), and XePtF^ (4)» but a l s o i n the re-examination of p r e v i o u s l y r e p o r t e d compounds, e.g. a compound r e p o r t e d t o be osmium o c t a f l u o r i d e has been shown t o be, i n r e a l i t y , osmium h e x a f l u o r i d e (5). E x c e l l e n t reviews (6,7,8) have a l s o appeared i n r e c e n t y e a r s . F l u o r i n e i s the l i g h t e s t halogen and l i k e f i r s t members of other p e r i o d i c groups, i t possesses unusual p r o p e r t i e s . The outermost s h e l l of the f l u o r i n e atom (n=2) c o n t a i n s o n l y p e l e c t r o n s and no d e l e c t r o n s , the 2 2 *5 e l e c t r o n i c c o n f i g u r a t i o n being I s 2s 2p-?. T h i s s h e l l can be completed by forming one tT^bond, ( i . e . c o v a l e n t bond 2 f o r m a t i o n ) , or by t r a n s f e r o f an e l e c t r o n t o i t r e s u l t i n g i n the f o r m a t i o n o f a f l u o r i d e i o n . The chemistry o f f l u o r i n e , t h e r e f o r e , i s l i m i t e d t o the r e a c t i o n s o f f l u o r i d e i ons and o f i t s (T^bonded co v a l e n t compounds. The outermost s h e l l s o f the o t h e r halogen atoms, on the o t h e r hand, c o n t a i n d o r b i t a l s and can form dTT'bonds. Another important f e a t u r e o f f l u o r i n e chemistry o r i g i n a t e s i n f l u o r i n e f s s m a l l s i z e (R = 1.36 1) and h i g h e l e c t r o n e g a t i v i t y . I t s s m a l l s i z e makes i t s t e r i c a l l y p o s s i b l e t o pack r e l a t i v e l y l a r g e numbers o f f l u o r i n e atoms around a g i v e n atom. T h i s t o g e t h e r w i t h the h i g h e l e c t r o n e g a t i v i t y of f l u o r i n e , r e s u l t s i n the f o r m a t i o n of many f l u o r i d e s i n which the elements show h i g h (though not n e c e s s a r i l y the h i g h e s t ) o x i d a t i o n s t a t e s . I t a l s o r e s u l t s i n many i n o r g a n i c f l u o r i d e s having s t r u c t u r e s q u i t e d i f f e r e n t from those o f the corresponding c h l o r i d e s , bromides, and i o d i d e s . On the o t h e r hand, the r a d i i o f the oxide and h y d r i d e ions are c l o s e t o t h a t o f the f l u o r i d e i o n ; c r y s t a l l o g r a p h i c resemblances between f l u o r i d e s and oxides have l o n g been known and those between f l u o r i d e s and h y d r i d e s have a l s o been p o i n t e d out ( 9 ) . One o f the most important achievements i n f l u o r i n e chemistry has been the p r e p a r a t i o n and study o f the halogen f l u o r i d e s . T h e i r p h y s i c a l p r o p e r t i e s and g e n e r a l chemical behaviour have been reviewed r e c e n t l y (10). The halogen f l u o r i d e s have found numerous uses as f l u o r i n a t i n g agents, 3 and i n t h i s respect they complement the behaviour of elemental f l u o r i n e , inasmuch as f l u o r i n a t i o n s involving elemental f l u o r i n e usually are vigorous and r e s u l t i n oxidation to the higher oxidation states of the elements, while the f l u o r i n a t i n g powers of the halogen f l u o r i d e s cover a range of r e a c t i v i t y and need not always r e s u l t i n oxidations of lower states. In addition, f l u o r i n a t i o n s i n the l i q u i d phase are possible with halogen f l u o r i d e s . An order of t h e i r r e a c t i v i t y can be derived from the magnitude of the p a r t i a l negative charge on the f l u o r i n e atom i n these compounds. Since the d r i v i n g force f o r f l u o r i n a t i n g reactions i s to increase t h i s p a r t i a l negative charge, the compounds with low numerical value of p a r t i a l negative charge w i l l be the strong f l u o r i n a t i n g agents. The value of the p a r t i a l negative charge (11) on the f l u o r i n e atom i n each compound i s as follows: Compound CIF^ BrF^ IFy BrF^ IF^ C1F BrF P a r t i a l negative charge on f l u o r i n e atom 0.04 0.05 0.06 0.07 0.08 0.09 0.13 One would expect CIF^ with the lowest p a r t i a l negative charge on f l u o r i n e to be the strongest f l u o r i n a t i n g agent and t h i s i s indeed observed; chlorine t r i f l u o r i d e i s as strong a f l u o r i n a t i n g agent as elemental f l u o r i n e . Chlorine monofluoride and bromine monofluoride have not been used f o r f l u o r i n a t i n g purposes; the former has a 4 very low freezing point and boiling point and the latter i s highly unstable (10). In addition to the fluorinating : properties of the: halogen fluorides, their use as non-aqueous solvents and hence as media for the preparation of complex fluorides has been much studied (12). Bromine trifluoride has been mostly used for this purpose. The remarkable solvent properties of bromine trifluoride are related to i t s association and ionization and are consistent with the high value of Trouton's constant (13) and i t s other physical properties. In this respect bromine trifluoride resembles ammonia and water. 2H20 > H 30 + + 0H~ 2 N H > m + + m -j 4 & 2BrF 3 > B r F 2 + + BrF^~ The specific conductivity of bromine trifluoride at 25°C *"*3 "*1 *1 —6 is 8.0 x 10 ohm cm. in contrast to 10 for chlorine trifluoride and 2 x 10 for iodine pentafluoride (10). Base analogues in this solvent system and the corresponding acids containing B r F 2 + have been prepared. Bromine trifluoride may thus act as a fluoride ion donor or acceptor i n an acid-base type solvent system in which fluoride ion plays a role similar to that of the proton in water. 5 Another important f i e l d of f l u o r i n e chemistry i n which rapid progress has been made i n recent years i s i n the study of f l u o r i n e derivatives of t r a n s i t i o n metals. These may be i o n i c , covalent, or polymeric. Ionic f l u o r i d e s usually contain elements i n t h e i r lower oxidation states and are characterized by high melting points, e.g. t r i f l u o r i d e s of the f i r s t t r a n s i t i o n metal s e r i e s . Fluorides containing metals i n high oxidation states are usually gases or v o l a t i l e l i q u i d s or s o l i d s , resembling the covalent f l u o r i d e s of non-metals. In a few f l u o r i d e s , macromolecular structures seem to occur, e.g. TIF^, BeF2« The element chromium forms a number of f l u o r i d e s i n which the oxidation number of chromium may be +2, +3, +4, or +5. Chromium (II) f l u o r i d e and chromium (III) f l u o r i d e have been known since 1894 (14) and are prepared by the action of hydrogen f l u o r i d e on the respective chlorides, but t h e i r c r y s t a l structures have been deter-mined only recently. Chromium (II) f l u o r i d e (15) has a distorted r u t i l e type structure with four neighbours at 2.01 1 and two neighbours at 2.43 &. The green chromium (III) f l u o r i d e (16) possesses a vanadium t r i - . f l u o r i d e structure. Many complexes derived from both of these f l u o r i d e s are known. The +4 oxidation state f o r chromium i s not a common one; rather, i t i s exhibited i n only a few cases. The known compounds are chromium tetra-t-butoxide, 6 chromium t e t r a f l u o r i d e , chromium tetr a c h l o r i d e , and chromium dioxide. Chromium tetra-t-butoxide was obtained (17) by heating bis-benzene chromium with d i - t - b u t y l peroxide i n a sealed tube. Chromium dioxide was f i r s t described by Mauss (18) who maintained that i t could not be regarded as an oxide of t e t r a p o s i t i v e chromium. Its magnetic properties have been studied (19) and found dependent on the mode of preparation ( 2 0 ) . The + 4 oxidation state i n the dioxide has recently been questioned ( 2 1 ) and i t has been suggested that i t i s an acid chromium (III) compound, ( C r 0 ) 2 C r Q ^ . Chromium tetrac h l o r i d e has been reported by von Wartenberg ( 2 2 ) and probably exists only i n the gaseous state. Chromium t e t r a f l u o r i d e was prepared by von Wartenberg ( 2 3 ) by the action of f l u o r i n e at 3 0 0 — 5 0 0 ° C on chromium, chromium t r i c h l o r i d e , or chromium t r i f l u o r i d e . He also observed that chromium t e t r a f l u o r i d e formed steel-blue vapours. Apart from t h i s , no work has previously been reported on the properties of chromium t e t r a f l u o r i d e . However, complexes containing t e t r a p o s i t i v e chromium have recently been prepared: K^CrF^ by Klemm and Huss ( 2 4 ) , and Rb 2CrF^ and Cs 2CrF£ by Bode and Voss ( 2 5 ) . A f i r e - r e d pentafluoride of chromium has been reported to form i n small quantities simultaneously with chromium t e t r a f l u o r i d e ( 2 3 ) , or to be formed by thermal decomposition of hexafluorochroraates ( 2 5 ) . Recently a 7 s e r i e s o f o x y f l u o r i d e complexes c o n t a i n i n g chromium i n the +5 o x i d a t i o n s t a t e has been d e s c r i b e d (26), e.g. KCrQF^, AgCrOF,. Thus, the work r e p o r t e d t o date on the h i g h e r f l u o r i d e s o f chromium i s scanty and no s y s t e m a t i c i n v e s t i g a t i o n seems to have been c a r r i e d out. This was n o t i c e d by Sharpe (27) when he remarked t h a t the h i g h e r f l u o r i d e s o f chromium m e r i t f u r t h e r i n v e s t i g a t i o n . I t was t h e r e f o r e decided t o i n v e s t i g a t e f u r t h e r the f l u o r i n a t i o n o f chromium metal and t o study the p r e p a r a t i o n and p r o p e r t i e s o f these h i g h e r f l u o r i d e s ; i n the f o l l o w i n g pages a r e p o r t i s g i v e n o f the p r e p a r a t i o n and p r o p e r t i e s o f chromium t e t r a f l u o r i d e and o f complexes d e r i v e d from i t . 8 I I . THE PREPARATION AND PHYSICAL PROPERTIES OF CHROMIUM TETRAFLUORIDE A» P R E P A M T M The +4 o x i d a t i o n s t a t e o f chromium i n chromium t e t r a f l u o r i d e suggests two methods o f p r e p a r a t i o n — o x i d a t i v e and r e d u c t i v e . O x i d a t i v e methods would i n v o l v e the f l u o r i n a t i o n o f compounds c o n t a i n i n g chromium i n 0 , + 2 , o r +3 o x i d a t i o n s t a t e s . The r e d u c t i v e methods would demand a compound c o n t a i n i n g chromium i n an o x i d a t i o n s t a t e h i g h e r than +4 and then r e d u c i n g i t t o the d e s i r e d o x i d a t i o n s t a t e and f l u o r i n a t i n g i t s i m u l t a n e o u s l y . F o r r e d u c t i v e methods chromium t r i o x i d e would be a very s u i t a b l e compound and i s e a s i l y a v a i l a b l e i n a v e r y pure grade. I t s f l u o r i n a t i o n r e a c t i o n s have been s t u d i e d and are d e s c r i b e d i n a l a t e r s e c t i o n . In o x i d a t i v e methods chromium metal would be an i d e a l s t a r t i n g m a t e r i a l s i n c e i t i s r e a d i l y a v a i l a b l e i n a v e r y pure form ( 9 9 . 9 T O), and can be handled e a s i l y . Compounds of +2 and +3 chromium would have to be obtained anhydrous and are more d i f f i c u l t t o handle than the m e t a l . . F l u o r i n a t i o n of chromium o r chromium ( I I I ) t r i f l u o r i d e has been r e p o r t e d by von Wartenberg ( 2 3 ) t o y i e l d chromium t e t r a f l u o r i d e and s m a l l amounts o f chromium p e n t a f l u o r i d e . I n the p r e s e n t work, the t e t r a f l u o r i d e o f chromium was always prepared by f l u o r i n a t i o n o f the metal at a temperature o f 3 5 0°C; the d e t a i l s are g i v e n i n the experimental s e c t i o n o f t h i s t h e s i s . 9 The d i r e c t f l u o r i n a t i o n of chromium metal has been observed to give i n v o l a t i l e chromium (III) t r i f l u o r i d e , v o l a t i l e chromium t e t r a f l u o r i d e , and more v o l a t i l e f l u o r i d e s which are condensed at -78°C. The best y i e l d s of chromium t e t r a f l u o r i d e have been obtained at a temperature of approximately 350°C and with a moderate flow-rate of f l u o r i n e . At lower temperatures or with a low flow-rate of f l u o r i n e , the y i e l d i s very poor while on the other hand, at higher temperatures or with a high flow-rate, f l u o r i n a t i o n occurs too vigorously and so much heat i s l i b e r a t e d that the n i c k e l reactor tube becomes d u l l - r e d and i s attacked by the stream of f l u o r i n e , leading to contamination of the product. During the preparation i t was always observed that dense yellow vapours were produced which condensed i n the trap cooled to -78°C. The s o l i d was chromyl f l u o r i d e as indicated by the chromium analysis, and by the f a c t that a small amount of the compound was s u f f i c i e n t to i g n i t e benzene, a property previously described f o r chromyl f l u o r i d e (28). The formation of chromyl f l u o r i d e r e s u l t s from the a v a i l a b i l i t y of oxygen during the f l u o r i n a t i o n reaction. Oxygen may be available from the metal which i s usually covered with an oxide laye r , or from the f l u o r i n e i t s e l f . In order to reduce the y i e l d of chromyl f l u o r i d e 10 d u r i n g f l u o r i n a t i o n , i t was decided t o pass hydrogen over the heated metal p r i o r t o f l u o r i n a t i o n t o reduce any oxide l a y e r p r e s e n t . A few f l u o r i n a t i o n s were c a r r i e d out i n which the metal was heated i n a stream o f hydrogen gas and then f l u o r i n a t e d . T h i s r e s u l t e d i n a decreased y i e l d o f chromyl f l u o r i d e . However, the f o r m a t i o n of chromyl f l u o r i d e could not be a l t o g e t h e r stopped due to the f a c t t h a t a s m a l l amount o f oxygen was always present as i m p u r i t y i n the f l u o r i n e i t s e l f . Much care had to be e x e r c i s e d w h i l e h a n d l i n g chromium t e t r a f l u o r i d e . Because o f i t s ready decomposition w i t h moisture, i t needed to be handled i n a dry-box. I t could be s t o r e d i n w e l l d r i e d g l a s s apparatus without any r e a c t i o n . A sample was s e a l e d ia vacuo i n a s m a l l pyrex g l a s s tube and recovered unchanged a f t e r s e v e r a l months, i n d i c a t i n g t h a t no r e a c t i o n o c c u r r e d w i t h g l a s s . B, PTOICAIt PROPERTIES The p r e p a r a t i o n and some p h y s i c a l p r o p e r t i e s o f chromium t e t r a f l u o r i d e were r e p o r t e d i n 1941 by von Wartenberg (22). He observed t h i s compound to be cinnamon-coloured, and amorphous w i t h a d e n s i t y equal t o 2.9 g/cc. He a l s o n o t i c e d t h a t i t formed blue vapours and a t t a c k e d g l a s s when heated to 150°C. Chromium t e t r a f l u o r i d e , as o b t a i n e d i n the p r e s e n t study, i s a brown-greenish s o l i d , i n s o l u b l e i n the u s u a l o r g a n i c s o l v e n t s . When heated i n a dry g l a s s v e s s e l 11 i n vacuo, i t sublimes at 150°C, condensing i n the c o o l e r p a r t o f the tube as a deep-green d e p o s i t . At h i g h e r temperatures, i n a i r , i t forms blue vapours and decomposition occurs w i t h the f o r m a t i o n of chromyl f l u o r i d e , and r e a c t i o n w i t h g l a s s a l s o o c c u r s . In a i r i t becomes covered v e r y q u i c k l y w i t h a brown c r u s t and t h e r e f o r e must be handled under dry-box c o n d i t i o n s . The X-ray p i c t u r e o f the powder showed no l i n e s d e s p i t e many attempts t o o b t a i n a c r y s t a l l i n e p roduct. The samples were then taken i n g l a s s o r s i l i c a tubes and s e a l e d under vacuum. The tubes were heated f o r d i f f e r e n t i n t e r v a l s o f time at d i f f e r e n t temperatures. Temperatures i n t he r e g i o n o f 140—170°C caused r e a c t i o n w i t h the g l a s s c o n t a i n e r and decomposition products were o b t a i n e d . Long i n t e r v a l s o f time at lower temperatures a l s o r e s u l t e d i n r e a c t i o n w i t h the tubes. Annealing at temperatures i n the r e g i o n o f 80°C d i d not g i v e products which showed d i f f r a c t i o n l i n e s . However, i n one case a sample heated at 100°C f o r seventy-two hours gave a few broad l i n e s which could be indexed on a t e t r a g o n a l l a t t i c e (a = 9*350 1; c = 13.34 1). I t was not p o s s i b l e t o decide i f these l i n e s were due to some product o f decomposition o r due to the t e t r a f l u o r i d e . Htickel (29) has c l a s s i f i e d halogen compounds as being v o l a t i l e , n o n - v o l a t i l e , ( o r o f i n t e r m e d i a t e t y p e ) , depending on whether t h e i r b o i l i n g p o i n t s l i e below 300°C 12 or are very h i g h or i n the neighbourhood of 300°C. V o l a t i l e h a l i d e s are c h a r a c t e r i z e d by forming m o l e c u l a r l a t t i c e s w h i l e the i n v o l a t i l e h a l i d e s possess c o o r d i n a t i o n l a t t i c e s . A molecular l a t t i c e i s formed i f the c o o r d i n a t i o n number o f the c e n t r a l atom i n the i s o l a t e d molecule i s the same as i n the l a t t i c e , w h i l e i n a c o o r d i n a t i o n l a t t i c e the c o o r d i n a t i o n number of the c e n t r a l atom i n the l a t t i c e i s a m u l t i p l e o f t h a t o f the c e n t r a l atom i n the i s o l a t e d molecule. M o l e c u l a r l a t t i c e s are formed i f : ( i ) the c e n t r a l atom i s w e l l s h i e l d e d by the surrounding l i g a n d s , i . e . the c o o r d i n a t i o n number of the c e n t r a l atom i s h i g h ( i i ) the bonds to the c e n t r a l atom are weakly p o l a r i z a b l e and the i n t e r a c t i o n between molecules w i l l not a f f e c t the bonding i n the i n d i v i d u a l molecule ( i i i ) the bonds are weakly p o l a r In t r a n s i t i o n metal f l u o r i d e s o f the f i r s t s e r i e s which c o n t a i n l a r g e f l u o r i d e i o n s (Rp = 1 . 3 6 A) surrounding t r a n s i t i o n metal c a t i o n s which are q u i t e s m a l l , the c a t i o n w i l l be w e l l s h i e l d e d and the compounds might be expected t o be v o l a t i l e having m o l e c u l a r l a t t i c e s , e s p e c i a l l y i f the molecule contains more than t h r e e atoms o f f l u o r i n e per atom o f the metal. Thus we might expect the f l u o r i d e s o f the compositions MF, , MF,-, MF A, and s t i l l h i g h e r f l u o r i d e s t o 13 form m o l e c u l a r l a t t i c e s and possess h i g h v o l a t i l i t y . T h i s i s indeed t r u e as the m a j o r i t y of h e x a f l u o r i d e s are v o l a t i l e ( b o i l i n g below 60°C) (7), so a l s o are p e n t a f l u o r i d e s (7), b o i l i n g below 300°C. The t r i f l u o r i d e s possess i o n i c l a t t i c e s as i s shown by t h e i r h i g h m e l t i n g p o i n t s . T e t r a f l u o r i d e s deserve s p e c i a l mention s i n c e they occupy a unique p o s i t i o n between the two extreme ty p e s . Formation o f an i o n i c c o o r d i n a t i o n l a t t i c e i n t h e i r case w i l l r e q u i r e a c o o r d i n a t i o n number o f a t l e a s t e i g h t f o r the c e n t r a l atom. I f c o n s i d e r a t i o n i s g i v e n t o the f i r s t t r a n s i t i o n s e r i e s o n l y , i t can s a f e l y be s a i d t h a t the p o s s i b i l i t y o f a c o o r d i n a t i o n number o f e i g h t f o r the c e n t r a l atom i s u n l i k e l y . The maximum p o s s i b l e c o o r d i n a t i o n number i s s i x , f o r which t h e r e i s ample evidence s i n c e a n i o n i c complexes o f these elements i n which the c e n t r a l atom has a c o o r d i n a t i o n number o f s i x are w e l l known. Thus, from a c o n s i d e r a t i o n of c o o r d i n a t i o n numbers, the f o r m a t i o n o f a p u r e l y i o n i c l a t t i c e i n t e t r a f l u o r i d e s i s h i g h l y u n l i k e l y , e s p e c i a l l y f o r those o f the f i r s t t r a n s i t i o n s e r i e s — T i , V, Cr, Mn, Fe, Co, N i , and Cu. I n t h i s s e r i e s , o n l y the elements T i , V, Cr, and Mn form t e t r a f l u o r i d e s , of which the f i r s t t h r e e are b e t t e r known (7) w h i l e the l a s t one has been i s o l a t e d v e r y r e c e n t l y and l i t t l e i s y e t known about i t (30). That the t e t r a f l u o r i d e s o f t i t a n i u m and t i n a l s o do not form m o l e c u l a r l a t t i c e s was suggested by Htickel (29) who observed t h a t the d e n s i t i e s o f these two f l u o r i d e s were 14 g r e a t e r than those of the corresponding t e t r a c h l o r i d e s . T h i s should not be the case i f both types o f h a l i d e s form m o l e c u l a r l a t t i c e s . I n order t o account f o r t h i s d i s c r e p a n c y i n d e n s i t y , he suggested a ch a i n s t r u c t u r e f o r t i t a n i u m t e t r a f l u o r i d e , formed from condensed T i F ^ octahedra i n which each t i t a n i u m atom has a c q u i r e d a c o o r d i n a t i o n number o f s i x , except the one a t the end o f the c h a i n which has the c o o r d i n a t i o n number f o u r o n l y . T h i s w i l l make the c h a i n p o l a r but i f the neighbouring chains are arranged such t h a t one chain has the t i t a n i u m atom w i t h c o o r d i n a t i o n number f o u r a t one end and i t s neighbour has t h i s atom at the o t h e r end, the c r y s t a l w i l l not possess a p o l a r a x i s . The s i m i l a r v o l a t i l i t i e s o f t i t a n i u m , vanadium, and chromium t e t r a f l u o r i d e s and the tendency o f these elements t o achieve a maximum c o o r d i n a t i o n number o f s i x suggest t h a t the t e t r a f l u o r i d e s o f these elements are polymers o f condensed octahedra making contact along one edge. Such a s t r u c t u r e f o r vanadium t e t r a f l u o r i d e has a l r e a d y been suggested (31)• T h i s i s supported by the f a c t t h a t the d e n s i t y o f vanadium t e t r a f l u o r i d e i s h i g h e r than t h a t o f vanadium t e t r a c h l o r i d e : f o r VF, d = 3.15 g/cc, f o r VC1. 4 4 d = I.84 g/cc. Chromium t e t r a f l u o r i d e may a l s o be assumed to have formed from condensed CrF^ octahedra. The suggested c h a i n s t r u c t u r e f o r t i t a n i u m t e t r a -f l u o r i d e has been supported by r e c e n t s t u d i e s o f the s p e c i f i c heat o f t i t a n i u m t e t r a f l u o r i d e ( 3 2 ) . The attainment o f 15 coordination number six in vanadium tetrafluoride and chromium tetrafluoride i s supported by their magnetic moments which are higher than the spin-only values, suggesting an orbital contribution characteristic of ions in an octahedral environment. This i s discussed in detail later. The chain structure of tetrafluorides i s also in harmony with the ease of sublimation of these compounds. The proposed structure i s : Cr C r F I F F Cr Cr. Cr Cr Cr / \ / \ F X F F F F C. COORDINATION NUMBER OF CHROMIUM (IV) The coordination number of an atom or ion i s a very important property of the ion as i t gives an insight into the structure of the compounds and the arrangement of atoms within the structure. The coordination number of an atom is a function of both electrostatic interactions and of size factors. 1. Electrostatic Considerations Kossel (33) used electrostatic concepts to explain 16 the phenomenon o f complex f o r m a t i o n and showed t h a t t h e systems a r i s i n g from the a t t r a c t i o n between o p p o s i t e l y charged i o n s and p o s s e s s i n g minimum p o t e n t i a l energy d i d not as a r u l e correspond t o the a d d i t i o n o f n monovalent anions to an i o n o f +n charge. I n an a n i o n i c complex t h e r e are two types o f f o r c e s a t work, f o r c e s o f a t t r a c t i o n between c a t i o n s and anions, and f o r c e s o f r e p u l s i o n between anio n s . Magnus (34) computed the c o e f f i c i e n t s e x p r e s s i n g the r a t i o between the f o r c e s o f a t t r a c t i o n and r e p u l s i o n f o r a s e r i e s o f complex i o n s w i t h v a r y i n g numbers o f c o o r d i n a t e d anions and t h e i r v a r y i n g g e o m e t r i c a l arrange-ments. These c o e f f i c i e n t s are c a l l e d s c r e e n i n g c o n s t a n t s . The g e n e r a l e x p r e s s i o n (35) f o r the energy o f f o r m a t i o n (U) of the complex i o n , w i l l take the form U = p(n - sp)-^ where p - number o f c o o r d i n a t e d anions n = charge on the c e n t r a l metal i o n r = r a d i u s o f the metal i o n sp = s c r e e n i n g constant, i t s v a l u e depending on p F o r a constant r , the v a l u e o f U w i l l be p r o p o r t i o n a l t o p(n - s p ) . I f we compute the v a l u e o f U f o r v a r y i n g p ( f o r constant charge n and r a d i u s r ) of the c e n t r a l i o n , we f i n d t h a t U a t f i r s t i n c r e a s e s , passes through a maximum, and then d e c r e a s e s . F o r an i o n i c charge of +4 on the c e n t r a l i o n , the v a l u e s o f p(n - sp) f o r d i f f e r e n t v a l u e s o f p. are 1 7 g i v e n below. n ~ 4 P 4 5 6 7 8 p(n - sp) 12^32 13.12 14.04 11.90 12.24 I t i s seen i n the above t h a t the most l i k e l y -c o o r d i n a t i o n number f o r an i o n i n a +4 o x i d a t i o n s t a t e surrounded by ne g a t i v e i o n s each o f u n i t charge, i s s i x . S i m i l a r e l e c t r o s t a t i c c o n s i d e r a t i o n s l e a d t o the f o l l o w i n g maximum c o o r d i n a t i o n numbers (36) f o r d i f f e r e n t charges on the c e n t r a l i o n and s i n g l y charged n e g a t i v e i o n s . charge on c e n t r a l i o n 1 2 3 4 5 6 7 8 maximum c o o r d i n a t i o n number f o r s i n g l y charged n e g a t i v e i o n s 2 4 5 6 8 8 8 12 The above c o n s i d e r a t i o n s a r e , o f course, based on the assumption t h a t f o u r groups are arranged t e t r a h e d r a l l y and s i x , o c t a h e d r a l l y . 2. S i z e F a c t o r I n a d d i t i o n t o e l e c t r o s t a t i c c o n s i d e r a t i o n s , the othe r important f a c t o r governing the c o o r d i n a t i o n number and hence the s t e r e o c h e m i c a l arrangement i s the r e l a t i v e magnitude o f the s i z e s o f the c a t i o n and the anion s . The values o f maximum c o o r d i n a t i o n numbers produced from e l e c t r o s t a t i c i n t e r a c t i o n s are v a l i d i n s o f a r as s p a t i a l f a c t o r s do not p l a y a r o l e — f o r example a l a r g e c e n t r a l i o n . I n c r e a s i n g the number o f c o o r d i n a t i o n i o n s w i l l b r i n g them 18 i n c ontact w i t h one another and w i l l a t one stage r e s u l t i n no contact w i t h the c e n t r a l i o n . A c t u a l l y a c r i t i c a l r a d i u s r a t i o can be c a l c u l a t e d f o r each c o o r d i n a t i o n number. The c r i t i c a l r a d i u s r a t i o i s t h a t v a l u e o f the r a t i o o f the r a d i i o f the c e n t r a l i o n and c o o r d i n a t e d i o n s f o r which the c o o r d i n a t e d i o n s w i l l touch each o t h e r as w e l l as the c e n t r a l i o n . In these c a l c u l a t i o n s the i o n s are assumed to be s p h e r i c a l . The r a t i o s f o r d i f f e r e n t c o o r d i n a t i o n numbers are g i v e n below (36). maximum c o o r d i n a t i o n number 2 3 4 6 8 10 r + / r ~ - 0.15 0.22 0.41 0.73 1.00 The r a d i u s o f C r + / f i s O.56 A* (37) which g i v e s a v a l u e o f r a d i u s r a t i o C r + ^ : F ~ equal t o 0.41, equal to the minimum c h a r a c t e r i s t i c o f o c t a h e d r a l c o n f i g u r a t i o n . Thus, from the c o n s i d e r a t i o n o f e l e c t r o s t a t i c and s i z e f a c t o r s , an o c t a h e d r a l c o o r d i n a t i o n i s suggested f o r chromium i n the +4 o x i d a t i o n s t a t e , and i s c o n s i s t e n t w i t h the proposed c h a i n s t r u c t u r e . 19 III. CHEMICAL PROPERTIES OF CHROMIUM TETRAFLUORIDE Chromium, li k e other transition metals, shows many-oxidation states, of which + 2 , + 3 , and +6 are well known. The oxidation state of +4 is not characteristic for chromium and i s exhibited in only a few cases, e.g. chromium tetra-t-butoxide, the oxide C r 0 2 , and chromium tetrafluoride. Chromium in the +4 oxidation state has the electronic configuration l s 2 2 s 2 2 p ^ 3 s 2 3 p ^ 3 d 2 containing two unpaired electrons in the d orbitals. It has, therefore, vacant d orbitals and may be expected to form coordination compounds by accepting electrons from donor molecules. Chromium in the +3 state has an extraordinarily great tendency to form complex compounds, a tendency exceeded by no other element and equalled perhaps only by tripositive cobalt and tetrapositive platinum. Dipositive chromium also forms a variety of complex compounds but the great majority of these are strong reducing agents. The maximum coordination number of chromium in these complexes is six, as i s also the case for the other members of the f i r s t transition metal series. The coordinated groups may be neutral molecules, negative ions, or a combination of these. Neutral groups which have been used as ligands include nitrogen bases— ammonia, pyridine, etc.—and oxides such as sulphur dioxide, n i t r i c oxide, nitrogen dioxide, and carbon monoxide. The ligands vary considerably in their strength to form coordination compounds. 20 A. REACTION WITH AMMONIA Ammonia, the simplest and most easily available electron-donor molecule, i s a base in the Lewis sense. Besides combining with protonic acids, ammonia forms addition compounds with nonprotonic electronically unsaturated molecules like boron trifluoride and sulphur trioxide and with many other metal ions. In both cases the reaction may be regarded as neutralization in the Lewis sense. Inter-action with electronically unsaturated molecules leads to the formation of molecular addition compounds, e.g. H^NiBF , and reaction with metal ions to ammoniated cations, e.g. Cr(m^)^t PttNH^)^"*4", etc. In addition to being an electron-donor, ammonia i s also well known as an acid-base solvent system. It ionizes to form acidic and basic ionic species, according to the equation 2NH-v N^H,+ + NH ~ 3 4 2 K i o n = 1-9x10-33 at -50°C (38) Thus, solvolysis can occur in liquid ammonia in addition to the formation of donor-acceptor complexes. Compounds which are soluble in liquid ammonia are often solvolysed existing as hypothetical acids or bases. However, i t may be pointed out that fluorides have a low tendency to solvolyse. For example, vanadium tetrachloride i s solvolysed in liquid ammonia (39) but vanadium tetrafluoride forms an adduct (31)• Similarly, tungsten hexachloride (40) and niobium penta-21 c h l o r i d e (41) are s o l v o l y s e d , but t h e i r r e s p e c t i v e f l u o r i d e s form c o o r d i n a t i o n compounds ( 4 2 , 4 3 ) . When excess dr y l i q u i d ammonia was r e a c t e d w i t h chromium t e t r a f l u o r i d e , the s t a r t i n g m a t e r i a l s were recovered unchanged, i n d i c a t i n g t h a t no r e a c t i o n had taken p l a c e . Another w e l l known n i t r o g e n base used to form c o o r d i n a t i o n compounds i s p y r i d i n e , which has a l s o r e c e i v e d some a t t e n t i o n as a s o l v e n t (44) and which possesses a l o n e e l e c t r o n p a i r on n i t r o g e n which can be donated t o a c c e p t o r molecules. Many c o o r d i n a t i o n compounds w i t h p y r i d i n e as donor molecule are known. However, chromium t e t r a f l u o r i d e d i d not show any r e a c t i o n w i t h p y r i d i n e e i t h e r . The p o s s i b i l i t y o f forming an ammine can be f o l l o w e d from a c o n s i d e r a t i o n of the l a t t i c e energies ( 4 5 ) . The f o r m a t i o n of an ammine i n v o l v e s a change i n the d i s t a n c e between the c a t i o n and the anion i n the o r i g i n a l compound to the v a l u e c h a r a c t e r i s t i c of the l a t t i c e of the ammine. The process o f s e p a r a t i n g the i o n s i s accompanied by an expenditure o f energy (E) and t h i s w i l l be g r e a t e r the l a r g e r the l a t t i c e energy o f the o r i g i n a l compound. On the o t h e r hand, the process o f adding the ammonia molecule t o the c a t i o n w i l l evolve energy, ( A ) . Thus the heat of f o r m a t i o n (Q) o f the ammine i s g i v e n by Q = A - E . The p r o b a b i l i t y of ammine f o r m a t i o n i s determined by the r e l a t i v e v a l u e s of A and E. I n the case where E i s l a r g e , 22 the energy (A) evolved by the a d d i t i o n o f ammonia t o the c a t i o n i s i n s u f f i c i e n t t o make up f o r E and the ammine i s not formed. L a t t i c e energies o f t e t r a f l u o r i d e s i n c r e a s e from t i t a n i u m t o chromium ( 4 6 ) and t h e r e f o r e the tendency towards ammine f o r m a t i o n w i l l decrease. A l s o , i t f o l l o w s t h a t the tendency towards ammine f o r m a t i o n w i l l be l e a s t i n f l u o r i d e s and oxides f o r which the l a t t i c e e nergies are l a r g e compared t o the v a l u e s f o r other h a l i d e s . B. REACTIONS WITH SULPHUR DIOXIDE AND SULPHUR TRIOXIDE The r e a c t i o n s o f chromium 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 and t r i o x i d e were s t u d i e d f i r s t l y t o examine the r e a c t i v i t y of chromium t e t r a f l u o r i d e towards simple o x i d e s , and secondly because many t r a n s i t i o n metal f l u o r i d e s r e a c t i n a very i n t e r e s t i n g manner w i t h these o x i d e s . I n a d d i t i o n to i t s donor p r o p e r t i e s , s u l p h u r d i o x i d e has a l s o been used as a s o l v e n t system f o r chemical r e a c t i o n s . Sulphur d i o x i d e was condensed on t o a sample of chromium t e t r a f l u o r i d e and the r e a c t a n t s were allowed t o mix at room temperature. There were no s i g n s o f any r e a c t i o n t a k i n g p l a c e and a f t e r the removal o f excess s u l p h u r d i o x i d e , the r e s i d u e was found t o be unchanged chromium t e t r a f l u o r i d e . In another experiment sulphur t r i o x i d e was allowed to melt i n the presence o f chromium t e t r a f l u o r i d e , 23 and the r e s i d u e , a f t e r removal o f the excess o f su l p h u r t r i o x i d e , was found t o be unchanged chromium t e t r a f l u o r i d e . Thus chromium 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 u l phur d i o x i d e o r su l p h u r t r i o x i d e , and resembles vanadium t e t r a f l u o r i d e i n t h i s r e s p e c t ( 3 1 ) . c t REACTION: mm WATER The r e a c t i o n ; o f chromium t e t r a f l u o r i d e w i t h water has p r e v i o u s l y been s t u d i e d by von Wartenberg (22) who was a b l e t o c h a r a c t e r i z e the f l u o r i n a t i o n product o f chromium as chromium t e t r a f l u o r i d e by u s i n g t h i s h y d r o l y s i s r e a c t i o n . He assumed the r e a c t i o n , 3CrF. » 2 C r F 0 + H CrO, + 6HF 4 3 2 4 +6 to occur and confirmed t h a t the r a t i o o f Cr t o the t o t a l chromium corresponded t o the above eq u a t i o n . I n the present i n v e s t i g a t i o n , t h i s h y d r o l y s i s has a l s o been confirmed. D i s s o l u t i o n o f chromium t e t r a f l u o r i d e i n water gave a y e l l o w i s h - g r e e n s o l u t i o n c o n t a i n i n g chromium i n the +6 and +3 o x i d a t i o n s t a t e s . The r a t i o o f C r + ^ : C r + 3 was found t o be 1:2, as i n d i c a t e d by the above equation f o r h y d r o l y s i s . Many t e t r a h a l i d e s can be regarded as s a l t s o f v e r y weak bases and are expected t o be e x t e n s i v e l y h y d r o l y s e d . Thus chromium t e t r a f l u o r i d e and a l l the complexes d e r i v e d from i t are immediately h y d r o l y s e d by water; the same i s t r u e f o r vanadium t e t r a f l u o r i d e and i t s complexes. 24 IV. REACTIONS OF CHROMIUM TETRAFLUORIDE WITH HALOGEN FLUORIDES AND SELENIUM TETRAFLUORIDE The study o f the r e a c t i o n s o f halogen f l u o r i d e s w i t h m e t a l l i c f l u o r i d e s has l e d to the p r e p a r a t i o n o f many new compounds. Bromine 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 , i o d i n e p e n t a f l u o r i d e , and c h l o r i n e t r i f l u o r i d e have been most f r e q u e n t l y used f o r t h i s purpose. Of t h e s e , bromine t r i f l u o r i d e and i o d i n e p e n t a f l u o r i d e have been the most w i d e l y used because o f t h e i r l a r g e l i q u i d ranges and t h e i r h i g h molar energies o f v a p o r i z a t i o n (47). In a d d i t i o n , they are not as r e a c t i v e as bromine p e n t a f l u o r i d e and c h l o r i n e t r i f l u o r i d e and can be handled more s a f e l y i n g l a s s apparatus. D u r i n g r e a c t i o n s w i t h bromine p e n t a f l u o r i d e , r e a c t i o n w i t h the connecting g l a s s tubes was f r e q u e n t l y observed. These c o v a l e n t f l u o r i d e s can r e a c t w i t h the metal f l u o r i d e s i n the f o l l o w i n g two ways: ( i ) d i s s o l u t i o n and/or adduct f o r m a t i o n ( i i ) o x i d a t i o n and f l u o r i n a t i o n The f o r m a t i o n of adducts o r complexes w i t h metal f l u o r i d e s i s w e l l known and has been observed f o r a l l o f these halogen f l u o r i d e s . The tendency to form such adducts decreases i n the o r d e r Thus, the adducts formed from bromine t r i f l u o r i d e are v e r y s t a b l e , as a l s o are those formed from i o d i n e p e n t a f l u o r i d e (7). The t r a n s i t i o n metal f l u o r i d e adducts are u n s t a b l e BrF C1F 3 25 as compared t o those o f a l k a l i metal f l u o r i d e s ; i n f a c t i o d i n e p e n t a f l u o r i d e does not appear t o form any t r a n s i t i o n metal f l u o r i d e adducts. S i n c e the o r d e r o f s t a b i l i t y o f the adducts i s the same as the apparent o r d e r of the s t r e n g t h o f i n t e r m o l e c u l a r f l u o r i n e bonding i n the l i q u i d s , BrF^y I F ^ } BrF^ ) C1F , i t has been suggested t h a t f l u o r i d e i o n s p l a y a r o l e analogous t o t h a t o f the pr o t o n i n hydrogen bonding, and the f o r m a t i o n o f a complex would r e q u i r e the presence o f a s t a b l e vacant o r b i t a l on the c e n t r a l atom (48). The s t a b i l i t y o f the adduct w i l l a l s o depend on the nature o f the metal atom and i n c r e a s e s w i t h an i n c r e a s e i n the r a d i u s o f the metal. A wide range o f bromine t r i f l u o r i d e adducts has been prepared and these f a l l i n t o two c a t e g o r i e s — o n e i n which two molecules of bromine t r i f l u o r i d e a re prese n t per molecule of the metal f l u o r i d e , and the ot h e r i n which the metal f l u o r i d e and bromine t r i f l u o r i d e a re i n a 1:1 molar r a t i o . F o r example, a u r i c f l u o r i d e forms a 1:1 adduct, A u F y B r F ^ , w h i l e p l a t i n u m t e t r a f l u o r i d e forms a 1:2 adduct, P t F ^ » ( B r F ^ ) 2 , which decomposes a t 200°C to g i v e pure P t F ^ (49). In c o n t r a s t t o the s t a b l e adduct formed by pl a t i n u m t e t r a f l u o r i d e , the t i t a n i u m t e t r a f l u o r i d e adduct, a l s o a 1:2 type, i s h i g h l y u n s t a b l e and decomposes in, vacuo a t room temperature (50). Rhodium t e t r a f l u o r i d e d i s s o l v e s i n bromine t r i f l u o r i d e and can be r e c o v e r e d 2 6 a f t e r d i s t i l l a t i o n of bromine t r i f l u o r i d e , and forms no adduct. Thus i t i s observed t h a t t h e t e t r a f l u o r i d e s r e a c t w i t h bromine t r i f l u o r i d e a p p a r e n t l y by simple d i s s o l u t i o n , as w e l l as by forming adducts of both t y p e s , namely 1 : 1 and 1 : 2 adducts. The p o s s i b i l i t y o f the second type o f r e a c t i o n , t h a t i s , the o x i d a t i o n and f l u o r i n a t i o n o f the metal f l u o r i d e t o a h i g h e r f l u o r i d e , w i l l depend on the nature and r e a c t i v i t y o f the halogen f l u o r i d e and a l s o on the o x i d a t i o n s t a t e of the metal i n t h e metal f l u o r i d e . I f the metal f l u o r i d e c o n t a i n s a metal t h a t can e x i s t i n a h i g h e r ox-i d a t i o n s t a t e , then t h e r e i s a g r e a t p o s s i b i l i t y o f t h i s type o f r e a c t i o n o c c u r r i n g . T h i s p o s s i b i l i t y w i l l be ^ g r e a t e r the more r e a c t i v e the halogen f l u o r i d e ; thus, i o d i n e p e n t a f l u o r i d e , which i s the l e a s t r e a c t i v e o f the halogen f l u o r i d e s , v e r y o f t e n l e a v e s the o x i d a t i o n s t a t e o f the metal unchanged, w h i l e r e a c t i v e bromine t r i f l u o r i d e u s u a l l y y i e l d s products c o n t a i n i n g the metal i n the h i g h e s t o x i d a t i o n s t a t e . The r e a c t i o n s o f chromium t e t r a f l u o r i d e w i t h these f l u o r i d e s were s t u d i e d i n the hope o f p r e p a r i n g a h i g h e r f l u o r i d e o f chromium, and o f determining the r e a c t i v i t y of chromium t e t r a f l u o r i d e towards these f l u o r i n a t i n g agents. Chromium 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 any o f t h e s e f l u o r i d e s a t room temperature, and t h e r e f o r e the r e a c t i o n s a t h i g h temperatures ( i . e . the b o i l i n g p o i n t s o f the halogen 27 f l u o r i d e s ) were i n v e s t i g a t e d . When chromium t e t r a f l u o r i d e was r e a c t e d w i t h b o i l i n g bromine t r i f l u o r i d e , the c o l o u r of the r e s i d u e i n the s i l i c a r e a c t i o n v e s s e l was green, resembling t h a t o f chromic compounds. Magnetic s u s c e p t i b i l i t y measurements were c a r r i e d out a t room temperature and the computed v a l u e o f the magnetic moment corresponded to the presence o f t h r e e u n p a i r e d e l e c t r o n s , i n d i c a t i n g t h a t chromium i n the product had an o x i d a t i o n number of +3. Chemical a n a l y s i s and e q u i v a l e n t weight d e t e r m i n a t i o n showed t h a t the green r e a c t i o n product was a bromine t r i f l u o r i d e adduct, w i t h the formula C r F ^ ' O ^ B r F ^ . On the o t h e r hand, no r e a c t i o n o c c u r r e d between chromium t e t r a f l u o r i d e and e i t h e r bromine p e n t a f l u o r i d e o r c h l o r i n e t r i f l u o r i d e . Because of the low b o i l i n g p o i n t s o f these two and a l s o t h e i r low heats o f v a p o r i z a t i o n , when they were heated w i t h chromium t e t r a f l u o r i d e t h e i r r a t e s o f e v a p o r a t i o n were v e r y r a p i d and unchanged chromium t e t r a f l u o r i d e remained a t the end o f each experiment. S i n c e i n none o f the p r e c e d i n g r e a c t i o n s a product c o n t a i n i n g chromium i n an o x i d a t i o n s t a t e h i g h e r than f o u r was o b t a i n e d , i t was d e c i d e d to r e a c t chromium t e t r a f l u o r i d e w i t h a mixture o f bromine t r i f l u o r i d e and bromine penta-f l u o r i d e . T h i s mixture would a l l o w the r e a c t a n t s t o be heated t o a h i g h e r temperature than i s p o s s i b l e w i t h bromine p e n t a f l u o r i d e and would a c t as a s t r o n g e r f l u o r i n a t i n g agent than pure bromine t r i f l u o r i d e . The r e a c t i o n was 28 c a r r i e d out i n a s i l i c a tube and a f t e r t he removal o f excess bromine f l u o r i d e s a t 100°C, a reddish-brown product was o b t a i n e d . The.product was paramagnetic and the c a l c u l a t i o n o f the magnetic moment gave a v a l u e f o r the moment/* = 2.01 B.M., i n d i c a t i n g the presence o f one unpa i r e d e l e c t r o n . The percentage o f chromium was 34.2% and an e q u i v a l e n t weight d e t e r m i n a t i o n was c a r r i e d out t o determine whether the compound was an o x y f l u o r i d e o r a pure f l u o r i d e . The compound r e a c t e d v i g o r o u s l y w i t h water and a s o l u t i o n c o n t a i n i n g Cr and Cr ^ was o b t a i n e d . An e q u i v a l e n t weight d e t e r m i n a t i o n showed t h a t the product was an o x y f l u o r i d e and the magnetic measurement and chemical a n a l y s i s , suggested the formula CrOF^*Gt25BrF^. Chromium 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 e i t h e r i o d i n e p e n t a f l u o r i d e o r c h l o r i n e t r i f l u o r i d e on h e a t i n g . The r e a c t i o n s o f chromium t e t r a f l u o r i d e w i t h the halogen f l u o r i d e s can t h e r e f o r e be summarized as f o l l o w s : CrF. + BrF_ »no r e a c t i o n 4 3 C r F ^ + BrF^ 125°C } C r F «0.5BrF CrF, + B r F c ( o r C1F„ or I F j ( i ) r 0 ° m t e m p * ^ r e a c t i o n 4 5 3 5 ( i i ) h e a t t o b o i l i n g CrF, + BrF, + BrF_ heat t o b o i l i n g > C r Q F . Q ^ B r F , 4 3 5 3 3 I t i s not c l e a r why chromium t e t r a f l u o r i d e does not form adducts. Titanium t e t r a f l u o r i d e , as mentioned e a r l i e r , forms adducts w h i l e vanadium t e t r a f l u o r i d e on 29 reacting with bromine t r i f l u o r i d e immediately y i e l d s vanadium pentafluoride ( 3 1 ) . On the other hand, the reaction product of chromium t e t r a f l u o r i d e with bromine t r i f l u o r i d e at 125°C was CrF^'O^BrF^, containing chromium i n an oxidation state lower than i n the s t a r t i n g material. I t appears highly u n l i k e l y that chromium t e t r a f l u o r i d e w i l l be able to oxidize bromine t r i f l u o r i d e and be i t s e l f reduced to chromium t r i f l u o r i d e . The alt e r n a t i v e explanation would require just the thermal decomposition of chromium t e t r a f l u o r i d e to chromium t r i f l u o r i d e , followed by adduct formation by the l a t t e r . The decomposition of chromium t e t r a f l u o r i d e i n t o chromium t r i f l u o r i d e i s exothermic ( 4 6 ) . CrF, >CrF + |F 4 3 2 AH = AH f(CrF 3) + AH^.(CrF^) = - 2 6 5 + 287 = +22 The reaction w i l l n a t u r a l l y be f a c i l i t a t e d by heat and the heat of formation of an adduct by chromium t r i f l u o r i d e may also favour the decomposition. REACTION OF CHROMIUM TETRAFLUORIDE WITH SELENIUM FLUORIDE Selenium t e t r a f l u o r i d e i s another covalent f l u o r i d e which has frequently been used to study the reactions of metal f l u o r i d e s . Like the halogen f l u o r i d e s , i t has 3 0 remarkable s o l v e n t p r o p e r t i e s and has a f a i r l y l a r g e l i q u i d range (melting p o i n t : - . . 9 . 3 ° C ; b o i l i n g p o i n t : 1 0 6 ° C ) but i s not as r e a c t i v e nor such a v i g o r o u s f l u o r i n a t i n g agent. I t s s o l v e n t p r o p e r t i e s have r e c e n t l y been d i s c u s s e d (51) and a l k a l i metal complexes o f the type ASeF^ i s o l a t e d , i n d i c a t i n g an i o n i z a t i o n of the type 2 S e F , sSeF + + SeF " 4 3 5 An important d i f f e r e n c e between selenium t e t r a f l u o r i d e and o t h e r f l u o r i d e s o l v e n t s , such as bromine t r i f l u o r i d e , i s t h a t the l a t t e r are o x i d i z i n g agents and tend t o r a i s e an element t o i t s h i g h e s t o x i d a t i o n s t a t e , whereas the former e i t h e r l e a v e s the element i n i t s o r i g i n a l o x i d a t i o n s t a t e or even a c t s as a r e d u c i n g agent. F o r example, vanadium t e t r a f l u o r i d e i s immediately o x i d i z e d by bromine t r i f l u o r i d e t o form vanadium p e n t a f l u o r i d e , w h i l e w i t h selenium 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 forms an adduct ( 3 1 ) . Bromine t r i f l u o r i d e a l s o o x i d i z e s t h a l l o u s compounds to t h a l l i c f l u o r i d e ( 5 2 ) . Selenium t e t r a f l u o r i d e forms PdF-j'SeF^, which on h e a t i n g y i e l d s pure PdF^ ( 5 3 ) . The r e d u c i n g p r o p e r t i e s o f selenium t e t r a f l u o r i d e o r i g i n a t e i n i t s ready o x i d a t i o n t o selenium h e x a f l u o r i d e , which i s a gas a t room temperature. The occurrence o f r e d u c t i o n w i l l o f course a l s o depend on the r e a c t i v i t y o f the metal f l u o r i d e . The s t a b i l i t i e s o f adducts formed by bromine 31 t r i f l u o r i d e and selenium t e t r a f l u o r i d e are w i d e l y d i f f e r e n t . Whereas the " a c i d " adducts o f bromine t r i f l u o r i d e , + — e.g. B r F 2 • AuF/^ , are not very s t a b l e t o heat and u s u a l l y decompose a t 120—150°C, selenium t e t r a f l u o r i d e forms v e r y s t a b l e adducts (7). Adducts c o n t a i n i n g one o r two molecules o f selenium t e t r a f l u o r i d e per molecule of metal f l u o r i d e are known. The r e a c t i o n o f chromium 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 was i n v e s t i g a t e d t o see i f adduct f o r m a t i o n o c c u r r e d . At f i r s t the r e a c t i o n was s t u d i e d a t room temperature but no r e a c t i o n o c c u r r e d . In a second experiment, chromium t e t r a f l u o r i d e and selenium t e t r a -f l u o r i d e were heated t o g e t h e r . R e a c t i o n was observed to occur and appeared t o be complete i n about f i f t e e n minutes. The r e s i d u e a f t e r the removal o f selenium t e t r a f l u o r i d e a t room temperature c o n s i s t e d o f two p h a s e s — one pink and the o t h e r b u f f - c o l o u r e d . When an attempt was made to d i s t i l o f f more selenium t e t r a f l u o r i d e from t h i s heterogeneous r e a c t i o n product a t 100°C, i t was unexpectedly observed t h a t the b u f f - c o l o u r e d p o r t i o n o f the product was c a r r i e d i n t o the c o l d t r a p , presumably w i t h some vapours o f selenium t e t r a f l u o r i d e , l e a v i n g the pure pink product i n the r e a c t i o n v e s s e l . T h i s step f o r t u n a t e l y a f f o r d e d a method o f s e p a r a t i n g the two compounds. The pin k compound i n the r e a c t i o n v e s s e l was found on chemical a n a l y s i s t o correspond t o CrF2»SeF j ! f, c o n t a i n i n g d i p o s i t i v e 32 chromium. The oxidation state was confirmed by a magnetic moment measurement at room temperature, which gave a value o£jum= 5.34 B.M., corresponding to four unpaired electrons, suggesting Cr1"*" in a low f i e l d situation (high spin). The chemical analysis of the buff-coloured compound gave the formula CrF^'SeF^. This compound was very light and not as reactive as the pink compound. It could be stored for longer periods of time without any noticeable change. The magnetic susceptibility measurement on the buff-coloured compound gave a low value for the magnetic moment, which was not satisfactory, since the compound was very light and could not be easily and uniformly packed into the magnetic moment tube. Thus selenium tetrafluoride and chromium tetra-fluoride when heated together yield 1:1 selenium tetra-fluoride adducts of CrF 2 and CrF^. This i s another example of a reaction in which selenium tetrafluoride acts as a reducing agent. 33 V. COMPLEXES DERIVED FROM CHROMIUM TETRAFLUORIDE It was decided to prepare a l k a l i metal complexes containing chromium in the +4 oxidation state. Two types of complex fluoride derivatives seem to be possible for tetrapositive chromium. (i) complexes which contain an al k a l i metal and chromium in a 1:1 atomic ratio, i.e. of the type ACrF c—e.g. KCrF_, RbCrF. etc. 5 5 5 ( i i ) complexes which contain two atoms of al k a l i metal per atom of chromium, i.e. of the type A 2CrF^— e.g. K 2CrF 6, Cs 2CrF 6, etc. During the present investigation, complexes were prepared in a bromine trifluoride medium. After the discovery of bromine trifluoride in 1949, i t s potential use as a non-aqueous solvent was brought to light when the physical properties were studied. Emeleus and Woolf (54) proposed that bromine trifluoride can act as an ionizing solvent according to the equilibrium 2 B r F 3^===^BrF 2 + + BrF^~ and this therefore has been considered the parent substance of a new acid-base solvent system. Bromine trif l u o r i d e has a high value for i t s Trouton fs constant, indicative of association, and i t also possesses a high specific con-ductivity (55). It i s true that useful solvent properties 34 are possessed by other fluorides (e.g. iodine pentafluoride and selenium tetrafluoride) but bromine trifluoride has many advantages, such as i t s large liquid range and high reactivity. For the preparation of the complexes of tetra-positive chromium, bromine trifluoride was selected, since in this medium the possibility of reduction of Cr +4 w i l l be the least. Bromine trifluoride has been used to prepare the fluoro-complexes of the platinum metals (56), gold (57), vanadium (58), and phosphorus, arsenic, antimony, boron, tantalum, and bismuth (59). In the present work, the complexes were prepared by heating together the alk a l i metal chlorides and chromium tetrafluoride in the correct molar ratio in bromine tri f l u o r i d e . A. COMPLEXES OF THE TYPE ACrF, WHERE A - K. Rb. OR Cs.. 1. Potassium pentafluorochromium (IV) Chromium tetrafluoride and potassium chloride in a 1:1 molar ratio were reacted together in bromine t r i f l u o r i d e . The reaction was carried out by heating the reactants for a short time and excess bromine trif l u o r i d e was then removed by d i s t i l l a t i o n in, vacuo. Depending upon the temperature at which the d i s t i l l a t i o n was carried out, two products were obtained. If the excess bromine trif l u o r i d e was removed at 100°C, the residue was a bromine trif l u o r i d e adduct, shown by chemical analysis to be KGrF^-0.5BrF^. The oxidation number of chromium was confirmed by the observed magnetic 35 moment of 3.22 B.M., which showed the presence of two unpaired electrons. I f , however, the removal of excess bromine t r i f l u o r i d e was carried out at 160°C, the product did not contain any bromine t r i f l u o r i d e ; chemical analysis showed the compound to be KCrF^. 2. Rubidium pentafluorochromium (IV) and cesium pentaf luoro chromium (IV) These were s i m i l a r l y prepared by heating chromium t e t r a f l u o r i d e and the respective a l k a l i metal chloride i n bromine t r i f l u o r i d e , and removing excess bromine t r i f l u o r i d e at 160°C to obtain a pure product. The following compounds were thus obtained. complex magnetic moment (B.M.) c r y s t a l St.ruc.^ur.e l a t t i c e Wnskar^s KCrF c.0.5BrF o 5 3 3.22 tetragonal a = 9.168 I c = 13.49 A KCrFfj 3.16 hexagonal a = 8.739 | c = 5.226 A RbCrF r ? 3.17 hexagonal a = 6.985 1 c - 12.12 & CsCrF *0.5BrF o 5 3 3.09 cubic a = 8.107 1 CsCrF 3.09 cubic a = 8.107 1 The complexes obtained contain f i v e atoms of f l u o r i n e per atom of chromium. They do not contain free potassium f l u o r i d e as can be -shown by t h e i r d i f f r a c t i o n patterns which contain no l i n e s c h a r a c t e r i s t i c of potassium f l u o r i d e . At l e a s t from t h e i r formulae these s a l t s contain penta-coordinate chromium. However, coordination number f i v e i n 36 complexes is rare, as i s indicated by the fact that very few complexes of the type A N M X R are known, especially for those containing a tetrapositive central atom. The only such complex containing known at the start of this work was that of manganese (IV), KMriF^, prepared from potassium permanganate and bromine trif l u o r i d e (26). A few other examples that may be cited are T12A1F^, Cs^CoCl^, and (NH.J.ZnCl.. 4 3 5 Many neutral molecules are known which contain five ligands per central atom, e.g. pentahalides of molybdenum and tantalum, etc. In various of these compounds where the stoichiometry might suggest a coordination number of five, the true coordination number i s either higher or lower, apparently depending upon the oxidation number of the central atom. For the pentahalides of niobium and tantalum and for molybdenum pentachloride, i t has been established that in the vapour state these are monomers with a trigonal bipyramidal structure. However, there i s evidence that in the solid state they a l l exist as dimers, M ^ ^ Q , consisting of two octahedral MX^  groups sharing an edge. Similarly, in the case of PF r, PC1_, and PF_C1_, the molecules have 5 5 3 2 been shown to exist as trigonal bipyramidal molecules i n the vapour state (60). However, a study of the phosphorus pentachloride molecule in the solid state by Clark, Powell, and Wells (61) shows that the trigonal bipyramidal arrangement does not persist in the solid state. Thus the various observations of MX^  molecules indicate that a 37 coordination number of five i s unusual in the solid state. The avoidance of odd coordination numbers has been demonstrated by Menzies (62), who suggested that heavy metals might even show an unusual effective atomic number to avoid odd coordination numbers. Sidgwick^s statement (62) that the relation of an odd coordination number to the next higher even value i s that of unsaturation, and Bossett's (62) explanation of the structure of a number of complex lithium compounds i n which the lithium tends to attain a coordination number of two or four, are indications that the tendency towards an even coordination number is displayed through a l l the periodic table. Furthermore, geometric considerations also show that, in general, i f there is space for five ligands around the central atom then six ligands can be equally well accommodated. There i s , therefore, no preference for five coordination as far as spatial considerations are concerned. Crystal structure studies on various complex compounds of the type AnMX^ have revealed that in almost every case an even coordination numbei—six in general— has been attained. A few complexes of this type which can be prepared in aqueous solution form monohydrates, e.g. K^FeCl^'I^O, and i t may be reasonably supposed that these contain ions such as [FeCl^fi^o]"* 2, in which the central atom has attained hexa-coordination. Powell and Wells (63) investigated the structure of anhydrous 3 8 C s 0 C o C l c and found t h a t i t contained t e t r a h e d r a l [ c o C l , | 2 3 ? L 4 J i o n s t o g e t h e r w i t h separate c h l o r i d e i o n s and cesium i o n s . S i m i l a r l y , the compound (NH.) 0ZnCl_ c o n t a i n s t e t r a h e d r a l Hr 3 p ^ZnCl^] groups and f r e e c h l o r i d e and ammonium i o n s . The compound T 1 9AH«V has been shown t o c o n t a i n i n f i n i t e chains - 3 o f A l F ^ octahedra, i n which two corners o f each o c t a -hedron are shared w i t h o t h e r s t o p r e s e r v e t h e s t o i c h i o m e t r y ( 6 4 ) . Thus the above-mentioned s t r u c t u r e d e t e r m i n a t i o n s demonstrate t h a t p e n t a - c o o r d i n a t i o n i n the s o l i d s t a t e v e r y r a r e l y occurs f o r complexes, and i n a l l those cases where the s t o i c h i o m e t r y might suggest i t , a c o o r d i n a t i o n number o f f o u r o r s i x has a c t u a l l y been a t t a i n e d — f o u r i f the o x i d a t i o n number o f the c e n t r a l atom was two, and s i x i f i t was t h r e e . Complexes of the type AMX^ c o n t a i n i n g a t e t r a p o s i t i v e c e n t r a l metal have o n l y r e c e n t l y been prepared, and so f a r no d e t a i l e d s t r u c t u r e d e t e r m i n a t i o n s have been c a r r i e d out. The e x i s t e n c e o f c o o r d i n a t i o n number s i x f o r manganese i n KMnF_ has been suggested ( 6 5 ) based on the observed s i m i l a r i t y o f i t s i n f r a r e d spectrum w i t h t h a t o f K 2MnF^, i n which h e x a - c o o r d i n a t i o n has been e s t a b l i s h e d . Knowing t h a t atoms tend t o a t t a i n t h e i r maximum c o o r d i n a t i o n number a v o i d i n g odd v a l u e s , and t h a t the o x i d a t i o n number o f chromium i n these complexes o f the type ACrF^ i s + 4 , i t can reas o n a b l y be assumed t h a t these complexes c o n t a i n C r F Q ~ 2 o c t a h e d r a l groups, each o f which 39 shares two corners with neighbouring groups to obtain the observed stoichiometry. Detailed structural studies would be very informative. Magnetic moments of these complexes, which w i l l be discussed later, also lead to the view that chromium has attained a coordination number of six in these compounds• B. COMPLEXES OF THE TYPE A 2CrF 6 These complexes of tetrapositive chromium were known previously. The potassium complex ICpCrF^ was prepared by Huss and Klemm (24) by fluorinating a mixture of potassium chloride and chromium chloride. Using the same method, Bode and Voss (25) prepared analogous rubidium and cesium complexes and investigated their crystal structure. Their investigations showed that the potassium and rubidium complexes existed i n two crystalline forms, hexagonal and cubic, while the cesium complex formed only cubic crystals. Their results are given below. la t t i c e constantp K 9CrF 6 cubic a = 8.15 1 «, ^ hexagonal a = 5.69 A; c = 9.34 A Rb0CrF, cubic a = 8.506 A* * b hexagonal a = 5.94 1; c « 9.67 A Cs 2CrF^ cubic a = 9.004 1 In the present investigation, experiments were carried out to prepare these complexes in a bromine trifluoride solvent. Only potassium and cesium complexes 40 were investigated. .1* Potassium hp.y^flunrnnlirnmi nm Chromium tetrafluoride and potassium fluoride in a 1 : 2 molar ratio were refluxed in bromine trifluoride, and excess bromine trifluoride was d i s t i l l e d is vacuo at 1 0 0 °C. Chemical analysis of the product suggested that i t was a bromine trifluoride adduct of the composition K^CrF^'O^BrF^ The tetrapositive state of chromium was confirmed by the observed magnetic measurementAJ?= 3 . 0 3 B.M., which corresponded to two unpaired electrons. An X-ray diffraction photograph of the sample was taken and the pattern was indexed on a tetragonal unit c e l l with a = 4 . 3 3 9 1, c = 5 . 5 0 0 1. The reported structures for pure I^CrF^ were either hexagonal or cubic, which suggested that the presence of bromine trif l u o r i d e in the la t t i c e seriously affected the structure. An attempt was therefore made to see i f this tetragonal compound could be transformed into one of the previously reported structures. Small amounts of .K2GrF^«0.5BrF3 powder were sealed ia vacuo in glass tubes and heated to different temperatures. X-ray pictures of the heated products were found to be different from that of the starting material. A l l the heated samples showed the presence of the same phase, which on indexing was found to be cubic with a = 8 . 1 0 4 1. Thus the tetragonal adduct K^CrF^O^BrFo on heating decomposed to the cubic K^CrF^. 41 Cesium hexaf luoro chromium Similarly Cs^rF^'O.^BrF^ was prepared and had a magnetic moment of 3.34-B.M. An X-ray photograph of the sample was indexed on a cubic l a t t i c e with a = 8.916 1. A sample of this adduct was heated to 135°C, when bromine trifluoride d i s t i l l e d off. The residue on analysis was found to be pure Cs 2CrF^. The results of the present investigation are summarized below. compQun,d, K 2CrF 6»0.5BrF 3 K 2CrF 6 C^ 2CrF^-0.5BrF 3 Cs 2CrF^ magnetic moment (B.M.) 3.66; 3.30 3.06; 3.30 3.14 3.14 crystal structure tetragonal cubic cubic cubic l a t t i c e constants a - 4.339 I c = 5.500 1 a = 8.104 A a = 8.916 1 a = 8.916 1 The values reported by Bode and Voss (24) for the lat t i c e constants of cubic modifications of K 2CrF^ and Cs 2CrF^ are higher than the values obtained in this work. Their,preparations always contained some a l k a l i fluoride in excess which was not discernible in X-ray patterns. The excess a l k a l i fluoride was presumably present as solid solution and may be responsible for the slightly higher values of the l a t t i c e constants. Many complex fluorides of the type A2MF^ are known; in fact almost every element which forms a tetrahalide gives such complexes. The complexes i n which A = lithium or sodium are d i f f i c u l t to form and do not crystallize i n 42 the same types of structures (66) as do the complexes in which A = potassium, rubidium, or cesium. These latt e r complexes usually crystallize in three types of structures— cubic, hexagonal, or trigonal—and contain discrete MX^  octahedra. At the present time, the factors governing the occurrence of these structures are unknown. It has been repeatedly remarked that these three structures possess the same energy and are equally favoured from energetic considerations. However, the calculations of the energies of these structures contain many approximations and therefore may not indicate subtle differences i n energy. The fact that the metals of the f i r s t transition series do not a l l show the same modifications evenly distributed suggests that the structures do depend on both the cations present in the molecule. The observed structures for A 2MF£ complexes of the f i r s t transition series and the ratio of radii of a l k a l i metal to r a d i i of transition metal are given below. structure rM+4 S t r u c t u r e rM+4 rRb + structure M^+4 r C s + T i C H T 0.51 C H T 0.46 C - T 0.40 V - H T 0.465 - H T 0.42 C - T 0.366 Cr C H - 0.42 C H - 0.378 C - - 0.331 Mn C H T 0.405 C H T 0.354 C - - 0.319 C = cubic H = hexagonal T = trigonal It i s seen in the table that potassium salts always 43 exist in either two or three crystalline forms, while cesium salts exist at the most in two forms, indicating that the relative sizes of the cations are important. It i s apparent from the table that for low radius ratios, i.e. equal to or less than 0.331, the complex crystallizes only in the cubic form. As the value of the radius ratio increases, other structures also occur. Thus, at radius ratios between 0.42 and 0.311, complexes exist in two modifications. For radius ratios between 0.46 and 0.51, a l l the three structures may occur. One might expect that with a further increase in the radius ratio cubic structure w i l l disappear, and at s t i l l higher ratios one might expect only one structure. It i s indeed observed for similar compounds of heavier elements, as lead, t i n , platinum, and ruthenium, which w i l l give higher radius ratio values, that the trigonal modification i s the only observed structure (67). The case of vanadium i s intriguing. The modification that has not been isolated is the cubic. This i s surprising and suggests that further research i s necessary. It should be possible to prepare a cubic modification of K^VF^ certainly and of Rb^VF^ quite probably. The other prominent absence is a trigonal variety of K 2CrF^, and one would expect this to exist. If these general correlations are correct, then one would expect only the cubic modification of a potassium complex for which the ratio rM+^ i s less than 0.33. One 44 example of an element which gives a radius ratio less than 0.33 i s tetrapositive s i l i c o n (radius = 0.41 &. rSi +^ = 0.308), and this forms a potassium complex which i s known to exist only in the cubic form,(67). 45 VI. MAGNETIC PROPERTIES The magnetic properties of a substance are characterized by i t s magnetic moment, which for a simple dipole is proportional to the strength of the poles and the distance between them. For a complex system, the observed magnetic moment is the resultant of elementary moments, ultimately of the electrons themselves. These magnetic moments either are int r i n s i c properties of the electron, corresponding to the spin, or result from the orbital motion of the electron. Thus, the magnetic moment in general consists of two parts, namely the "orbital" and "spin 1 1 portions. their spin and orbital moments are not completely cancelled and there remains a permanent magnetic moment. When an external magnetic f i e l d is applied, the permanent moment tends to orient i t s e l f parallel to the f i e l d . This orienting tendency i s opposed by the usual thermal motion: the extent of alignment therefore must decrease with rising temperature. The quantum mechanical treatment of the interaction between the elementary moment and the applied f i e l d yields for the molar s u s c e p t i b i l i t y T the result (68) If an atomic system contains unpaired electrons, 46 where/^" = s t a t i s t i c a l mean square of the moment - the joint contribution of the high frequency-elements of the paramagnetic moment (temperature independent) and of the diamagnetic effect N, k, and T have their usual significance This expression, derived by Van Vleck (68), is identical to one derived by Langevin, except for the addition of the termyV^C Among transition metal compounds, different types of magnetic behaviour have been observed (69) and different expressions for the magnetic moment have been obtained, depending on the spacing of degenerate spin levels in comparison to kT. For example, i f the spacing i s large as compared to kT, the moment/^ i s given by the expression » g/j(J+l) B.M. and i f the spacing i s small as compared to kT, then = 74S(S+1) + L(L+1) B.M. , when S, L, and J are the resultant spin, resultant orbital angular momentum, and total angular momentum respectively, and g i s the Lande spli t t i n g factor, a known explicit function of S, L, and J. S L -2^ e * i + s(g+;u -6 2J(J+1) 47 Clearly, both spin and orbital contribution are included in the expression and must generally be considered. Neighbouring ions in the crystalline or li q u i d state or in solution interact strongly with one another. If the interactions are so strong that the orbital angular momentum cannot be oriented by an applied magnetic f i e l d , the orbital contribution to the magnetic moment does not appear and i s said to be quenched. In the case of ions of short transition series elements i n which the unpaired electrons are near the surface and are almost entirely unscreened from the field s of neighbouring ions, the orbital contributions are almost entirely quenched and indeed i t has long been known (70) that the magnetic moments of the elements of the f i r s t transition series are best given by the spin-only formula y4S(S+l) B.M., the whole of the orbital contribution being neglected. However, i t may be pointed out that the quenching of this orbital contribution i s often incomplete and this deviation can be used to assist i n determining the stereochemistry as discussed later. TEMPERATURE DEPENDENCE OF MAGNETIC SUSCEPTIBILITY Thermal motions retard the alignment of permanent moments and thus decrease the value of the susceptibility with rising temperature. The idealized form of behaviour for the magnetic susceptibility of a paramagnetic substance with temperature T i s given by the Curie Law 4 8 ^ and/^ = 2.84/j^T~, where C = Curie Constant, and X M - molar susceptibility. If the Curie Law i s obeyed, y& i s independent of temperature. The Curie Law i s obeyed with considerable accuracy by a few systems, e.g. [FeF^j"^, but for a majority of cases there are deviations. One cause of deviations i n one unpaired electron systems i s the temperature independent para-magnetism arising from the second order Zeeman effect from higher ligand f i e l d s . This has been allowed for in the Langevin-Debye formula by introducing the term NcC(68). However, the molar susceptibility of many compounds deviates from the requirement of the Curie Law in a way which may be described by a simple modification of this law, the Curie-Weiss Law, = C M T +6 ( # = Weiss Constant) Chromium, li k e other transition metals, can exist in various oxidation states ranging from one to six; of these only two, three, four, five, and six are realized in fluorides and oxyfluorides. Magnetic properties of chromium compounds containing chromium in different oxidation states have been studied extensively at room temperature, but data on their temperature dependence i s not complete. The magnetic behaviour for different oxidation states are 49 described below for a few selected compounds. 1« Or The ground state i s S ^ y 2 5 1 1 1 ( 1 f i v e d electrons are present. From ligand-field theory applications, the expected magnetic moment in an octahedral environment i s 5 . 9 2 B.M. in a weak f i e l d case and 1.73 B.M. in a strong f i e l d case. 2 . Cr** The ground state of this ion i s and i t has four d electrons. In an octahedral environment the expected magnetic moment w i l l correspond to four unpaired electrons in a weak f i e l d case and to two unpaired electrons in a strong f i e l d case. Both types of coordination compound are known (71)« In the present work, the compound CrF^SeF^ has been prepared and the value of i t s magnetic moment was found to be 5.34 B.M. This i s higher than the calculated spin-only value of 4 . 9 0 B.M., indicating a large orbital contribution. This w i l l be discussed in the following section. 3. Cr+3 The ground state of terpositive chromium i s ^ 3 / 2 and i t possesses three electrons in a d shell responsible for magnetic moment. The magnetic moment in an octahedral environment w i l l be due to three unpaired electrons in both weak and strong f i e l d s . The octahedral complexes of terpositive chromium are well known and the observed magnetic moments correspond 50 closely to the spin-only values (71). The compound K^CrF^-H^O (72) has a magnetic moment of 3.79 B.M., while CrF^ has a magnetic moment of 3.9 (73). It is unfortunate that data on the variation of magnetic susceptibility with temperature i s so meagre. The magnetic susceptibilities of a few compounds of chromium at low temperatures have been measured. In C^O^, the observed value of the magnetic moment i s 3.61 B.M., with the Weiss Constant equal to -405° (19). In the present investigation, the following compounds were prepared and their magnetic moments measured. compound, yUZ9k ( C u r i e L a w ) CrF 3 »0 .5BrF 3 3.96 B.M. CrF 3»BrF 3 3.67 B.M. 4. C r + 4 3 The ground state of tetrapositive chromium i s F 2 and the ion has two electrons in the d shell. The expected magnetic moment corresponds to two unpaired electrons in an octahedral environment, whether due to weak f i e l d ligands or to strong f i e l d ligands. Tetrapositive chromium compounds have been mostly studied in the present investigation and therefore w i l l be discussed in detail. Chromium shows an oxidation number of four only in chromium tetra-t-butoxide, chromium dioxide, and chromium tetrafluoride, and in the complexes derived from them. 51 The blue chromium tetra-t-butoxide possesses a magnetic moment of 2.88 B.M. (17). The magnetic measurements on chromium dioxide were made by Bhatnagar (19) and a value of y^= 2.94 and the Weiss Constant equal to 2° were found for this compound. However, the +4 oxidation state of chromium in chromium dioxide has recently been questioned by Russian workers (21). The complex BagCrO^ contains tetrapositive chromium and possesses a magnetic moment of 2.82 B.M. (74). At the start of the present investigation, the magnetic moment of K 2CrF£ only was known among the fluorine compounds containing tetrapositive chromium, and i s 2.8 B.M. (74). In the present investigation, the magnetic moments of many compounds of tetrapositive chromium were measured, and in some cases measurements at low temperatures were also taken. The observed values are given below. Compound CrF 4 KCrF^ KCrF 5'0.5BrF RbCrFr 2.74 B.M. -70° 3.02 B.M. 3.16 B.M. 3.22 B.M. 80° 3.67 B.M. 2.84 B.M. 3.17 B.M. -32° 3.14 B.M. CsCrF^ K 2CrF 6.0.5BrF 3 Cs 2CrF 6.0.5BrF3 3.09 B.M. 3.06 B.M. 3.10 B.M. 105° 3.56 B.M. 52 5t Cr* 5 The ground state of pentapositive chromium ion i s 2 D3^2 a n <* contains only one electron in the d shell, responsible for the magnetic moment. The expected magnetic moment in an octahedral environment—weak f i e l d or strong f i e l d — i s 1.73 B.M. Again, l i k e tetrapositive chromium, this oxidation state i s observed i n oxyanions, in fluorides, and in oxyfluorides. In chromate (V), e.g. Ba-^CrO^^, the magnetic moment has been found to correspond to one unpaired electron ( 7 4 )• No magnetic measurements have been made on chromium pentafluoride. However, the compounds KCrOF^ and AgCrOF^ ( 7 2 ) , RbgCrOGl^ and Cs 2Cr0Cl 5 (71) have been observed to contain one unpaired electron. In the present work, magnetic susceptibilities of oxyfluorides containing pentapositive chromium have been observed to correspond to one unpaired electron, as i s shown in the table below. compound Mz^K ^aW^ CrOF 3 « 0 . 2 5 B r F 3 2 . 0 2 B.M. C r O F 3 - 0 . 2 5 C l F 3 1.83 . B.M. C r 0 F o ' 0 . 2 5 B r F c 1.85 B.M. 3 5 KCrOF4«0.5BrF3 1.73 B.M. The temperature dependence of the magnetic susceptibility of KCrOF 4»0.5BrF 3 was measured, and the Weiss Constant was found to be 53 Magnetic measurements can be discussed under two headings: (i) paramagnetism and i t s temperature dependence ( i i ) paramagnetism and stereochemistry PARAMAGNETISM AND ITS TEMPERATURE DEPENDENCE Experimental observations show that a l l the chromium compounds are spin-free and obey the Curie-Weiss Law. The observed values of the Weiss Constant are f a i r l y large in chromium tetrafluoride and in the complexes derived from i t . It has usually been observed that the Weiss Constant is high In the case of fluorides. The Weiss Constant, according to Van Vleck (68), i s not an atomic property but can arise from interatomic forces and exchange effects. The magnitude of Q shows a marked dependence on the nature of the anion and on the ground state of the atom. As a result of his observations on iron and manganese salts, Van Vleck suggested that & can arise from spin-orbit coupling which contributes to the angular momentum of the system and thus w i l l be smallest for the S states. As i t does not f a l l to zero at in f i n i t e dilution, the Weiss Constant i s not wholly due to exchange effects—hence, exchange effects play only a subordinate role. The statement by Van Vleck that B i s due mainly to the influence of orbital angular momentum, and therefore should be high only for D or F states, does not seem to be always true; both low and high values of & 54 have been observed f o r D or F states, as shown i n the following table. Some examples of the compounds showing a wide range of l v a l u e s i n the D and F states are given. remark arid reference low (75) high (19) high (76) high (76) high present work high present work high present work low present work low present work high (31) high ( 3 D high (31, 77) high (31,77) It i s thus at present not possible to trace the r e a l cause—exchange i n t e r a c t i o n or o r b i t a l i n t e r a c t i o n — o f the high values of the Weiss Constant. G r i f f i t h (78) has shown that i t i s not possible to d i f f e r e n t i a t e between the source of & from the temperature studies of magnetic 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 antiferromagnetic i n t e r -actions are small or i f the spin-orbit coupling occurs between the nearly degenerate leve&s, because both / ion state compound 9 C r + 3 F C r C l 3 32.5 Cr203 405 V + 2 F VC1 ? 565 700 C r + / f F CrF^ 70 KCrF 5'0.5BrF 3 80 K 2CrF 6.0.5BrF 3 105 RbCrF^ 32 C r + 5 D KCr0F 4'0.5BrF 3 +4 V + 4 D VF 4 198 V F 4 # S e F 4 1 3 k K^VFV 118 ^2 v r6 7 8 55 antiferromagnetic interactions as well as spin-orbit inter-actions have similar effects on paramagnetism. For chromium tetrafluoride, a polymerized structure similar to those for titanium tetrafluoride ( 2 9 ) and vanadium tetrafluoride ( 3 1 ) , involving fluorine bridges, has been proposed. In such a structure, an antiferro-magnetic exchange interaction i s very easily possible and might iead to high values of the Weiss Constant. The fluorine bridges can also be considered to distort the regular octahedral symmetry of the CrF^ basic octahedron and result i n two types of fluorine bonds. This w i l l cause a splitting of the low lying t r i p l e t (d^) and a spin~orbit coupling can occur which may give rise to a high value of the Weiss Constant. Thus i t i s apparent that the presence of fluorine bridges can lead to high values ofQ; whether this i s achieved by exchange inter-actions or by spin-orbit interactions cannot be decided by the present theoretical approaches, i n which in addition to other simplifying assumptions, 0 has been treated as independent of the applied f i e l d , which i s especially not true at low temperatures. If the existence of high B values i s associated with bridge atoms, then there i s no reason that they should be observed only in fluorides. Rather, other compounds which contain bridge atoms (halogen or other) should also exhibit high Q values. For example, in the case of vanadium 56 dichloride, the high value of the Weiss constant, & - 565° or 700°, may be associated in part with the presence of chlorine bridges. Similarly, the observed values of the Weiss Constant for the complexes f i t this pattern. The complexes of the type ACrF r possess high 9 values (of the order of 100°) and this may be attributed, at least in part, to the presence of fluorine bridges in the structure. Thus, to conclude, i t can be said that even though i t . i s not possible to separate the contributions of the two factors, v i z . exchange forces and interatomic forces, the magnetic properties of chromium tetrafluoride and i t s complexes can be understood in the light of fluorine bridges. PARAMAGNETIC ATO STEREggimigTRY It has been mentioned earlier i n this chapter that the moments of the f i r s t row transition metal ions are given by the spin-only formula. However, in practice i t i s found that deviations from this spin-only value do occur, and these are mostly towards higher values. The spin angular momentum of an electron i s not affected by forces from other atoms, while on the other hand the interatomic forces are able to quench the orbital angular momentum. The presence of electrica l l y charged particles around a central metal ion gives rise to a ligand f i e l d which has two maini effects: 57 (i) It breaks up the coupling of L and S vectors to some degree, and the ion i s no longer specified by a particular J value. ( i i ) It removes the degeneracy of 2L+1 sublevels associated with the particular L value and splits them. The separations between these sublevels have an important effect on orbital contribution. For an ion with narrow multiplet separation, i.e. hV, T <^kT, the magnetic f i e l d reacts separately with S and L and 2S+1 and 2L+1 sublevels. It i s this distribution over degenerate 2L+1 sublevels that i s responsible for the large orbital contribution in the formula Jl<S{S+l) + L(L+1) If the separation between the s p l i t sublevels i s large as compared to kT, only the lowest (or lower) levels w i l l be occupied. Furthermore, i f this lowest level i s a singlet, then the orbital contribution should be small. Penney and Schlapp (79) have examined the effect of various kinds of crystalline fi e l d s on F and D states for transition metal ions. They found that the single energy level of an ion in the F state i s s p l i t into three new levels when surrounded by a cubic f i e l d of an octahedron of six ligands. The separation between successive levels i s of the order of 1(A cm7"L Usually the octahedron of ligands i s slightly distorted; this may be regarded as 58 equivalent to imposing on the cubic f i e l d a small component of lower symmetry, e.g. tetragonal or rhombic. This w i l l result in a further sp l i t t i n g of two adjacent energy levels into t r i p l e t s , giving in a l l , seven levels for the F state. In this case the t r i p l e t i s lowest and the singlet highest. For an ion in a D state, the cubic f i e l d s p l i t s the orbital level into two levels, which are further s p l i t with a t r i p l e t and a doublet by a small rhombic component. This energy level diagram i s known as a "Stark Pattern". If a tr i p l e t l i e s lowest there i s large orbital contribution but a low lying doublet or singlet is"non-magnetic". Until now only an octahedral arrangement has been considered. It w i l l suffice to say here that in a tetra-hedral environment the "Stark Pattern" w i l l be inverted (69). The "Stark Pattern" for Cr+1+ and Cr +^ in octa-hedral and tetrahedral environments are given in Fig. no. I,(page 59)« The remarkable difference between "Stark Patterns" of F and D states for tetrahedral and octahedral environments i s that in the former, a singlet or doublet l i e s the lowest, and in the latter, a t r i p l e t l i e s lowest. For the tetrahedral case no orbital contri-bution i s expected, as singlet and doublet are'hon-magnetic", and a much closer approximation to the spin-only formula i s to be expected. In the octahedral case a large orbital contribution i s expected as the t r i p l e t l i e s the lowest. In other words, i f the observed values of the 59 Fig. I Stark Patterns for Cr + 2 f and Cr+$ in octahedral and tetrahedral environment (diagrammatic) j free ion no f i e l d free ion no f i e l d cubic | cubic and f i e l d ; rhombic cubic f i e l d f ields fin*. cubic and rhombic fields +5. P STATE OCTAHEDRAL ENVIRONMENT OCTAHEDRAL, ENVIRONMENT TETRAHEDRAL ENVIRONMENT TETRAHEDRAL ENVIRONMENT 6q magnetic moment of ions in the F and D states are appreciably higher than the spin-only values, an octa-hedral arrangement around the ion may be suggested. The observed value o f / * e f f for chromium tetra-fluoride (F state) i s 3.02 B.M. and i s higher than the spin-only value which is 2.83 B.M. for, two unpaired electrons. There is a large orbital contribution suggesting an octahedral environment. Similarly, the values of the magnetic moments observed for complexes of the type ACrF^, containing tetrapositive chromium, are higher than the spin-only value. For example, KCrF^ and CsCrF^ have magnetic moments of 3.16 B.M. and 3.20 B.M. respectively, whereas the calculated spin-only value i s 2.83 B.M. Again, an appreciable orbital contribution i s indicated, suggesting that the central paramagnetic ion has attained hexacoordination. An example of the D state i s the tetrapositive vanadium ion, whose compound vanadium tetrafluoride and i t s complexes are well known. Its magnetic properties have been studied i n detail (31) and the observed value o f > / / i e f f i s B.M., and i s higher than the calculated spin-only value of 1.73 B.M. This suggests an octahedral environment around the ion in vanadium tetrafluoride and also in complexes as K 2vF D and SeF^'VF^, which have values of 2.05 B.M. and 2.32 B.M. respectively for their magnetic moments. However, the observed value ofy^-Qff 61 for vanadium trif l u o r i d e (F state) i s only 2.55 B.M. (72) and i s noticeably lower than the spin-only value, but in the absence of data over a temperature range, no remarks can be made. Hexacoordination of vanadium has already been suggested (31) in analogy with the structure of titanium tetrafluoride proposed at f i r s t by Htlckel (29). Recent work on the heat capacity of titanium tetrafluoride (32) also supports this view. Thus, the observed values of the magnetic moments of the fluorine compounds of tetrapositive chromium and vanadium show large orbital contribution which i s in accord with hexacoordination of the central metal ion. 62 VII. REACTION OF CHROMIUM TRIOXIDE AND POTASSIUM DlCHROMATE WITH HALOGEN FLUORIDES Oxides react very differently with bromine trif l u o r i d e (80) and investigations of these reactions have resulted in the preparation and isolation of many new fluorides and oxyfluorides. Oxides can be divided into two classes, depending on whether or not they react with the fluoride. (i) oxides which do not react or which react very slowly: in this category are included BeO, MgO, ZnO, CdO, HgO, CaO, Mn02, F e ^ , etc. ( i i ) oxides which react with bromine trifluoride by exchanging oxygen with fluorine: these can be further divided into two groups— (a) replacement of oxygen i s complete, yielding a f u l l y fluorinated product. Since bromine trifluoride is known to oxidize and fluorinate at the same time, the products contain the metal in the highest oxidation state. This category includes Ti 0 2 , As 20 3, Sb20^, and Se02, which give TiF^, As 2F^, SbF^BrF^, and SeE^ respectively. (b) replacement of oxygen is partial, and oxy-fluorides result. For example, V 20^ reacts with bromine trifluoride to form VOF^. The reactions of the oxides of the f i r s t series of transition metals show a gradual decrease in the tendency of 63 oxygen in the oxide to be replaced by fluorine. Thus, while titanium dioxide replaces a l l of i t s oxygen, vanadium pentoxide and chromium trioxide replace only two-thirds, and the oxides of the next two elements do not react. Why the replacement of oxygen by fluorine becomes more d i f f i c u l t as we proceed from titanium to manganese and iron cannot be explained at the present stage, as l i t t l e i s known about the structures and thermochemistry of oxyfluorides, and about the mechanism of reaction. However, the important point to note is that the reaction of GrO^ and BrF^ i s the only one of i t s kind, as the product contains the metal atom in a lower oxidation state (+5) than in the starting material (+6). A. PACTIONS, WITH flHRQMWfl T R I P C T B The reaction of bromine trif l u o r i d e with chromium trioxide was investigated for the f i r s t time by Sharpe and Woolf (26). They observed that two-thirds of the oxygen was liberated and the product contained chromium in the +5 oxidation state. The removal of excess bromine trif l u o r i d e at a high temperature could not be achieved as i t resulted in volatilization of the product, and thus pure CrOF^ could not be isolated. During this investigation, reactions of chromium trioxide with bromine trif l u o r i d e , bromine penta-fluoride, and chlorine trifluoride were carried out in an attempt to prepare CrOF^ free of any solvate. Chromium trioxide reacted with a l l of these vigorously at room temperature and the product in each case contained 64 pentapositive chromium, as indicated by the magnetic measurements and supported by chemical analyses. The results of the reactions are given below. magnetic product analysis measurements react iar tg . #F %Cr ZM. A>9L formula x l O - 0 (B.M.) CrO, + obs. 4 4 . 5 3 3 . 4 BrFo (liq.) calc. 44.81 32.71 1 7 1 5 2 . 0 2 C r 0 F o - 0 . 2 5 B r F o -> 3 3 C r O o + obs. 4 7 . 7 3 0 . 2 BrF^ (liq.) calc. 4 7 . 9 3 0 . 8 1 4 4 3 1 . 8 5 CrOF^'O^BrF^ CrO, + obs. 48.OO 3 4 . 2 CIF^ (liq.) calc. 4 8 . 1 1 3 5 . 1 3 1 4 1 8 I . 8 3 C r O F 3 ' 0 . 2 5 0 ^ Reactions of chromium trioxide with bromine penta-fluoride and chlorine tri f l u o r i d e were carried out to see i f . i t . i s possible either to replace a l l of the oxygen in the molecule, or to obtain an oxyfluoride containing chromium i n the +6 oxidation state, in view of the fact that both of these agents are considered to be stronger fluorinating agents than bromine tr i f l u o r i d e . Neither of these two reactions, carried out below a temperature of 50°C, gave the desired product. Therefore, i t was decided to perform a reaction using a higher temperature, and with the fluorinating agent in the gaseous phase; chlorine trifluoride, being the more reactive of the two, was selected. k stream of chlorine trifluoride was passed over the heated oxide. The reaction product sublimed at the reaction temperature and a dull-red solid collected in the trap following the s i l i c a reaction tube. This appeared 6 5 completely different from the product obtained as a result of the reaction carried out using liquid chlorine t r i f l u o r i d e . This red product was paramagnetic, the magnetic moment corresponding to one unpaired electron, indicating that chromium was present in the +5 oxidation state. Chemical analysis showed the product to be identical with the liquid phase reaction product, CrOF 3«0.25ClFj. This indicated that the two compounds were identical chemically and differed only in state of aggregation or other physical properties. The one obtained in the vapour phase reaction was more compact, solid, and appeared crystalline, while the one obtained i n the liquid phase reaction was less dense and appeared amorphous. In order to see i f the dull-red compound obtained in the vapour phase reaction could be transformed into the liquid phase reaction product, a sample of the former was carefully heated in a s i l i c a tube. At a temperature of about 75°C the compound expanded considerably and occupied almost the whole of the tube; this f i n a l product was identical i n appearance to the liquid-phase reaction product. The composition of the compound was unchanged, as shown by analysis. Thus, the dull-red vapour phase product was converted, on heating, into the yellowish substance obtained with the liquid phase reaction, apparently a change only in the state of aggregation. These oxyfluoride adducts are very reactive and 66 fume i n air giving yellow fumes of chromyl fluoride. They are hydrolysed by water, yielding Cr J and CrO^ ions in solution. The products are orange in colour and react with glass when stored for long periods, even under vacuum. However, they can be handled for short periods in glass vessels, and manipulated under dry-box conditions. The reaction with glass presumably i s due to the halogen fluoride produced as a result of dissociation. Attempts were made to take X-ray photographs of these adducts but the capillaries containing the speciments exploded during exposure. Both s i l i c a and pyrex capillaries were used. The X-ray pattern of the dull-red product obtained from the vapour phase reaction was taken, but decomposition was seen to have occurred inside the capillary. It was therefore not possible to say whether the pattern was due to the original substance or to the decomposition product. The fluorination of chromium trioxide with many different fluorinating agents can therefore be summarized as follows. fluorinating % of displaced aeent product oxveren reference S F 4 Cr0 2F 2 33.3 (81) I F 5 Cr0 2F 2 33.3 (82) SeF 4 Cr0 2F 2 33.3 (83) BrF-j CrOF 3 « 0 . 2 5 B r F 3 66.6 present work BrF 5 CrOF 3 «0.25BrF 5 66.6 present work G1F3 Cr0F Q . 0.25ClF o J 3 66.6 present work 67 The percentage of displaced oxygen in these reactions f a l l s in line with the general order of reactivity of these fluorinating agents. The weaker fluorinating agents displace only a third of the oxygen, while the stronger ones displace two-thirds. Oxyfluorides of chromium The possibility of the formation of oxyfluorides is apparent from the fact that the sizes of fluorine and oxygen are not very different and hence they can mutually replace each other in compounds without causing much structural change. Thus, some oxyfluorides of transition metals show a remarkable similarity in physical properties to binary fluorides of similar empirical composition (e.g. MoOF^ has melting and boiling points similar to those of MoF/j), while others resemble oxides. Replacement of a fluorine atom by oxygen, however, w i l l require a change in the oxidation number of the metal or a change in the coordination number i f two fluorine atoms are replaced. Knowing that the maximum oxidation number of chromium is +6 and that i t shows lower, oxidation states of +4 and +5, the following oxyfluorides, at least in theory, are possible. oxidation state QMYfluflr 5,435 • C r + 6 Cr0 2F 2, CrOF^ J-5 Cr T? CrOF3, Cr0 2F 2 Cr"1"4 CrOF C r + 3 CrOF 68 The oxyfluoride C r 0 2F 2, chromyl f l u o r i d e , i s well known and evidence f o r the existence of CrQF^ was given by Sharpe and Woolf (26). In the present i n v e s t i g a t i o n , unsuccessful attempts were made to prepare pure CrOF^; however, adducts of CrOF^ were i s o l a t e d . The compound CrOF^ was not formed i n any reaction, and t h i s i s not e a s i l y understandable since the analogous compounds of molybdenum and tungsten (84,85) are well known. However, t h i s p a r a l l e l s the f a c t that chromium hexafluoride i s not known, although MoF^ and WF^ are w e l l known compounds. Si m i l a r l y , the compound CrOF 2 should be capable of i s o l a t i o n since the analogous compounds TiOF 2 (86), VOF 2 (87), and W0F2 (88) are known. With further work, i t i s possible that the missing oxyfluorides w i l l be prepared. B. REACTION OF DICHROMATES WITH HALOGEN FLUORIDES The reaction of potassium dichromate with bromine t r i f l u o r i d e has been previously studied by Sharpe and Woolf (26). They reported the: formation of KCrOF^•0.5BrF^, which on heating to 130—150°C l o s t the combined bromine t r i f l u o r i d e , y i e l d i n g pure KCrOF^, containing pentapositive chromium. The oxidation state of chromium was confirmed by a magnetic moment measurement (72) which gave a value of 1.76 B.M. i n the present work, the reaction of bromine t r i f l u o r i d e with potassium dichromate was repeated, and furthermore the reactions of bromine pentafluoride and chlorine t r i f l u o r i d e with potassium dichromate were 69 investigated. These latter reactions were studied in order to see i f bromine pentafluoride and chlorine t r i f l u o r i d e , well known as fluorinating agents stronger than bromine trifl u o r i d e , could either replace a l l of the oxygen in the molecule or yield a product of type KCrOF^, containing chromium in the +6 oxidation state. However, i t was found that i n a l l cases the product of reaction was KCrOF^, containing chromium in the +5 oxidation state. With bromine trif l u o r i d e , the product KCrOF •0.5BrF_ 4 3 was obtained as reported by previous workers: i t could be prepared free of bromine trifluoride when heated to 160°C in vacuo. X-ray photographs of both products, the adduct and the pure compound KCrOF^, were taken and were found to be identical, indicating that inclusion of bromine trifluoride in the l a t t i c e does not alter the crystal structure. The reaction of bromine pentafluoride and potassium dichromate resulted i n a product which gave an X-ray pattern different from that of KCrOF^'O.5BrF^. Chemical analysis revealed that the product was a bromine pentafluoride adduct, and that chromium was in the +5 oxidation state since the compound was paramagnetic, the magnetic moment corresponding to one unpaired electron. The bromine pentafluoride contained in the complex could be very easily removed by heating the compound in vacuo at 160°C to yield pure KCrOF^, as shown both by chemical analysis and by the X-ray pattern, identical with that obtained from the product of reaction of 70 potassium dichromate with bromine t r i f l u o r i d e . The reactions of bromine f l u o r i d e s with potassium dichromate can be formulated as the following: l l 6 3. t cl"fc K 2 C r 2 ° 7 + B r F 3 »KCrOF, •0.5BrF« 1,60°P. ^KCrOF K 2Gr 20 + BrF >KCrQF, »0.5BrF 5 h S § S ° 6 t > KCrQF A reaction of chlorine t r i f l u o r i d e with potassium dichromate at room temperature also gave a paramagnetic product whose X-ray pattern was l i k e that of KCrOF^. The compound KCrOF. i s highly reactive. I t 4 decomposes i n a i r and therefore needs to be handled i n dry-box conditions. I t dissolves i n water giving chromic and chromate ions i n solution, according to the equation 3KCr0F^ + 5H 20 >3KF + CrF^ + 2H 2CrO^ + 6HF . Magnetic s u s c e p t i b i l i t y measurements were carried out at d i f f e r e n t temperatures down to 85°K. A plot of r e c i p r o c a l molar s u s c e p t i b i l i t y versus temperature gave a value of the Weiss Constant equal to +4°. Compared to those f o r binary f l u o r i d e s and complex f l u o r i d e s , the value of the Weiss Constant i s remarkably low. Unfortunately, no binary f l u o r i d e of Cr*-> i s known, and therefore no comparison can be made. However, the f l u o r i d e and complex f l u o r i d e s of V +^, i s o e l e c t r o n i c with Cr +^, are known to possess high values of the order of 150° (31). Whether or not the low 71 value of 9 in KCrOF^ is associated with the introduction of oxygen in the molecule cannot be decided at present. It i s furthermore remarkable to note that while the presence of bromine trif l u o r i d e i n the l a t t i c e of KCrOF^ does not affect the crystal structure of KCrOF^, this i s not so with bromine pentafluoride, whose adduct possesses a structure different from that of KCrOF^. This difference produced by bromine pentafluoride i s most probably due simply to i t s larger size as compared with bromine tri f l u o r i d e . 72 VIII. THE ANALYTICAL DETERMINATION OF FLUORINE The accurate determination of fluorine i s s t i l l a major problem of analytical chemistry, and during recent years when fluorine has been put to manifold uses (from atomic research to fluoridation of water) a suitable, simple, and accurate method has become highly desirable. Indeed, during the past several years numerous modified or new methods have been described in the literature. In spite of this, few of the methods are as yet completely satis-factory. Excellent and detailed reviews on the analytical chemistry of fluorine have also appeared during the last few years (89,90). A l l the methods for the determination of fluoride depend on the formation of either insoluble metal fluorides and • fluorosilicates, or of stable, soluble, metal fluo-, borofluo-, or s i l i c o fluoride complexes. Since many other anions can show similar behaviour, most of the methods require separation of fluoride. The su i t a b i l i t y of a certain method i s determined by the amount of fluorine and by the nature of the cations present in the compound. If the fluorine in the sample i s so contained that i t i s not easily amenable to precipitation (e.g. i f i t exists as complex ions) the fluoride i s separated and then determined. The methods for determining fluoride can be divided into two classes: 73 (i) methods involving separation of fluoride. In this the procedure involves two s t e p s — f i r s t l y , a separation of fluoride from cations, and secondly, determination of the amount of fluoride. In this class are included the pyrolytic method, the ion-exchange method, and the Winter-Willard method. ( i i ) methods applied directly to the fluorine-containing compound or to a solution of the compound. In this class are discussed the equivalent weight method and the Null-point potentiometric method. A. METHODS INVOLVING SEPARATION OF FLUORIDE 1. Pvrolvtic method In this method the sample containing fluorine i s decomposed to liberate hydrofluoric acid which i s condensed in water and then titrated with a standard a l k a l i solution. Pyrolytic methods were f i r s t developed in work for the Atomic Energy Commission (91,92) and further studied at Oak Ridge National Laboratory (93). Two techniques using pyrolysis have been used. The f i r s t method (94,95) uses superheated steam while the second method involves using a stream of moist oxygen instead of steam (94,96). In the present work the samples were decomposed by steam, and details of the method are described in the experimental section. Steam was passed over the sample of chromium 74 tetrafluoride contained in a heated platinum boat and the vapours of hydrofluoric acid and steam were condensed in water. It was observed that the condensate was colourless at f i r s t but that later the condensate was yellow. As the pyrphydrolysis was continued, the intensity of the yellow colour in the condensate increased. The yellow colour was due to chromate ions, as a precipitate of lead chromate was obtained when the solution was tested for chromate. This was presumably formed by the pyrohydrolysis of chromyl fluoride, produced by the reaction of chromium tetrafluoride and steam: Cr0 2F 2 + 2H20 > H 2 C r 0 4 + 2 H F This indicated that i t i s not possible to separate fluorine from chromium by this method and that the condensate could not be used for t i t r a t i o n with sodium hydroxide since i t contained H 2Cr0 4. Pyrohydrolytic decomposition of chromium tetrafluoride was thus not suitable for analytical purposes. 2. Ion exchange method The use of ion-exchange resins to separate ions from one another i s well known and this has been used for the isolation of fluoride from other anions and cations. Two types of methods are possible, one using cation-exchange resins and the other using anion-exchange resins. The use of an anion-exchange resin would involve adsorption of fluoride ions on the resin (97) followed by elution with a suitable solution such as sodium acetate (98) 75 or sodium hydroxide (99). In theory this method seems very suitable but in practice various complications are associated with i t . For example, a solution of a fluoride of tetrapositive or pentapositive chromium w i l l invariably contain terpositive and hexapositive chromium, the former -3 as the anionic complex CrF^ , in addition to simple fluoride ions. Naturally, the adsorption of chromate ions and the anionic complex is to be considered. Also, ions of the -3 type CrF£_ x(OH) x w i l l be present and their elution rates w i l l cause complications. Taking into account the possible interference by chromate ions in the solutions containing chromate and fluoride ions, and by anionic complexes in +3 solutions containing Cr , no attempt was made to use anionic resins. The use of cation-exchange resins would involve passing the solution through a column of cation-exchange resin in the hydrogen ion form. Cations w i l l be adsorbed on the resin and the anions, (including fluoride), w i l l pass into the effluent. The use of cation-exchange resins also suffers from the defect that anionic complexes of the type CrF^""^ and CrF^_>x(OH)x"*^ w i l l pass through i f they are not decomposed by the resin. Separations with cation-exchange resins have been effected nicely for solutions which do not contain complex ions (100). In the present work, an aqueous solution of chromium tetrafluoride was reduced with sulphur dioxide, excess of 76 which was removed by heating slowly. The solution was then passed through a column of cation-exchange resin (Dowex-50). The effluent appeared colourless and a dark-green chromium band was observed in the column. However, when the effluent was made alkaline and heated, a precipitate of chromic hydroxide formed. This indicates that chromium forms stable anionic complexes with fluoride ions and hence a complete separation of chromium'-from fluoride ions was not possible under the conditions employed. However, i t i s possible that by a systematic study of the complexes, and by changing the experimental conditions, i t might be possible to effect a separation of fluoride ions from chromium in this way. Is Winter-Willard method In 1933, Winter and Willard (101) discovered that fluoride could be separated from other elements by volatilization as hexafluosilicic acid, from perchloric or sulphuric acid. The method i s applicable for a l l substances soluble in or decomposed by perchloric or sulphuric acid. The solution is maintained at 135°C in order to avoid excessive acid in the d i s t i l l a t e . Distillations using perchloric and sulphuric acids were performed during this investigation. When perchloric acid was used to decompose the fluorides of chromium, i t was observed that the d i s t i l l a t e always was yellowish in colour, due to chromium present in the hexapositive 77 oxidation state. A completely colourless d i s t i l l a t e , free of hexapositive chromium, could not be obtained. When sulphuric acid was used, i t was observed that by d i s t i l l i n g 25 ml. of solution with 35 ml. of concentrated sulphuric acid at 135°C, the d i s t i l l a t e always possessed a yellow colour. It was, however, noticed that the intensity of the yellow colour in the d i s t i l l a t e depended on the amount of acid in the flask, and that the d i s t i l l a t e was colourless when 25 ml. of concentrated sulphuric acid were heated with 25 ml. of just acidic solution of unknown. If the amount of sulphuric acid in the d i s t i l l a t i o n flask was more than the "optimum" amount, 25 ml., the d i s t i l l a t e was yellow. In a few determinations when the solution containing fluoride was acidic, a yellow colour in the d i s t i l l a t e was again observed, even with 25 ml. of concentrated sulphuric acid. If the amount of the acid was less than 25 ml., the results were low, presumably because of incomplete decomposition of complex anions of chromium (CrF^~^). If the chromium i n solution was present as chromate, the d i s t i l l a t i o n with sulphuric acid again gave a d i s t i l l a t e containing colour. This suggests that chromium should be present in the terpositive oxidation state i f possible and that too much sulphuric acid should be avoided as an excess causes oxidation of chromic to chromate, causing the appearance of chromium in the d i s t i l l a t e . 78 After separating fluorine as hexafluosilicic acid by volatilization, the determination of fluoride can be carried out by many methods. Those which have been largely used are li s t e d below. (i) gravimetric methods (94): these include precipitation as calcium fluoride, as lithium fluoride, as triphenyl t i n fluoride, and as lead chlorofluoride. ( i i ) volumetric methods (94): these involve the use of many different salt solutions as titrants, e.g. zirconium, aluminum, iron (III), cerium, and yttrium salt solutions. The most frequently used titrant i s thorium nitrate. ( i i i ) Null-point potentiometric method (102)—discussed later. The d i f f i c u l t y in the detection of the end point in the volumetric method using thorium nitrate i s well known and careful buffering of the solution i s required. -2 If some yellow colour due to CrO^ ions i s present, this w i l l interfere with the detection of the end point. Precipitation as calcium fluoride gives a precipitate which i s very gelatinous, f i l t e r s with great d i f f i c u l t y , adsorbs other ions, and may become colloidal. It has also been shown that at 105°G calcium fluoride loses 0.5$ of i t s weight through partial conversion to Ga(0H)F, and at 400°C, 2.5% conversion to the oxide may occur and both of 79 these compounds retain about 1% HF adsorbed even at 1000°C (103). The conversion factor i s not favourable for lithium fluoride. Precipitation as triphenyl t i n fluoride i s a very good method but requires carefully controlled conditions— the precipitation should be performed in 60—70% alcoholic solution. Moreover, the precipitant i t s e l f is not very soluble. Precipitation as lead chlorofluoride i s the most gen-erally used method even though a r i g i d adherence to experi-mental procedure i s required. The precipitate i s granular, settles readily, and i s very easy to f i l t e r . The conversion factor i s favourable. Unfortunately, the composition of the precipitate varies from the stoichiometric ratio and the results can never be better than + 0.5% (104). However, the advantages of this method outweigh those of a l l others and therefore in this study the fluoride in the Winter-Willard d i s t i l l a t e was always precipitated as lead chlorofluoride, which could be weighed as such or dissolved in dilute n i t r i c acid and i t s chloride content determined by Volhard's method and from i t , the fluoride calculated. Since there was a danger of the d i s t i l l a t e being contaminated with sulphate and chromate ions, determination was completed volumetrically. pt M E T H O D S imoium NO sEPmnpN, 1. Equivalent weight determination method If an alkaline solution containing chromate and fluoride ions i s passed through a cation exchanger, the 80 cations w i l l be removed and the anions generated as free acids, while excess al k a l i w i l l be changed into water. NaoCr0, ^ HoCrO,. H + + HCrO " n 2 u x u 4 ^ " 4 H + + CrO^"2 NaF ^ N HF v ^ H + + F" NaOH V VHQH ~-H+ + 0H~ The eluent w i l l contain free acid, which can be titrated with a standard alk a l i solution. The normality of the eluent can thus be determined and the equivalent weight calculated. If a weighed amount of a soluble compound of chromium or a compound that can be hydrolysed by water (e.g. chromium tetrafluoride) i s taken into water and oxidized to chromate by means of hydrogen peroxide in an alkaline medium, the solution obtained contains chromate and fluoride ions with an excess of a l k a l i . This, on passing through a cation exchanger, w i l l liberate free acids, as shown below. CrF, H2°2 > NaoCr0, + 4NaF 4 NaOH 7 2 4 Na2CrOif^= ^ H 2CrO^ s H + + H C r O / f % = ^ 2H+ + Cr0~ 2 4NaF s ^ 4HF s ^ 4H + + 4F~ Equivalent weight of CrF^ = 7 x molecular weight 6 = 21.33 81 The solution containing free acids can be titrated and the normality of solution determined. Knowing the weight of substance taken, the equivalent weight can be calculated. By knowing the percentage of chromium in the compound, i t s equivalent weight, and i t s magnetic moment, an unambig-uous molecular formula can be assigned to the compound. As an example, three compounds, CrF^, CrF,., and CrOF^ are considered below. magnetic moment 2.83 two unpaired electrons 1.73 one unpaired electron 1.73 one unpaired electron By determining the equivalent weight, i t would therefore be easy to distinguish between CrF^ and CrOF-j, while the percentage of chromium determination and magnetic moment w i l l distinguish between CrF^ and CrF^, whose equivalent weights are very similar. It i s thus possible to avoid the direct determin-ation of fluorine, which for fluorides of chromium can be extremely d i f f i c u l t for the reasons already described. 2. Null-point potentiometric method This method, developed by O'Donnell and Stewart (102), i s based on the complexing of Ce (IV) by fluoride ions. In equiv. Compound. we^ gh.t, & chromium CrF. 21.33 40.63 CrF c 21.00 35.38 5 CrOF^ 25.00 41.60 82 a solution containing both Ce (III) and Ce (IV) ions, addition of fluoride ions results in the lowering of the Ce (IV):Ce (III) ratio and thus in the lowering of the redox potential. In this method an unknown fluoride solution i s added to one Ce (IV):Ce (III) half c e l l and standard fluoride solution i s added to a similar half c e l l u n t i l the potential difference i s zero. The fluoride ion concentration in both half cells i s then the same and i s calculated from the amount of standard fluoride solution added. The authors showed there i s no interference by-chloride, sulphate, and nitrate ions, but acetate, oxalate, phosphate, and molybdate ions caused interference. They did not investigate possible interference caused by cations, apart from observing that a l k a l i or ammonium ions did not interfere; usually cations were removed before fluoride was determined. In order to avoid the tedious and time-consuming step of separation of fluoride ions, i t was decided to examine the effect of chromate and chromic ions on the applicability of this method. The following observations were made: (i) Using standard sodium fluoride solutions, the results were accurate and reproducible. ( i i ) Using sodium fluoride solution containing chromic ions, the results were low and the sensitivity of the potential changes towards the addition of 83 fluoride decreased considerably at the end point, and sensitivity to the addition of small amounts of water increased considerably. The results were not reproducible, ( i i i ) Addition of potassium dichromate solution to one half c e l l and an equal volume of water to another half c e l l did not result in the development of potential difference. Furthermore, ti t r a t i n g with sodium fluoride and potassium dichromate solution, the results were not reproducible— perhaps some oxidation-reduction phenomena were taking place. This method i s based on the complexing of Ge (IV) species present in the solution and presupposes the availability of a l l of the fluoride ions for this purpose. Now, i n solutions of a l k a l i metal fluorides, i t is quite reasonable to expect that a l l of the fluoride ions are available, but i n solutions containing cations capable of forming complex ions with fluoride, the situation w i l l be different. The availability of the fluoride ions w i l l depend on the s t a b i l i t y constant of various complex ions and other factors affecting the ionic equilibria. Another important factor which w i l l affect the method considerably i s the possibility of the occurrence of oxidation-reduction reactions in the half c e l l , especially when large concen-trations of chromate ion are present and when the acidity 84 i s very high. Fluoride ions, the "universal addend" ( 1 0 5 ) , are well known to form complex ions in aqueous solution with metallic ions. Indeed, the formation of stable fluoride complexes forms the basis of a l l colorimetric, fluorometric, potentiometric, and other methods for determination of fluoride. Naturally, the presence of any cation which can form complex ions with fluoride w i l l interfere with the ava i l a b i l i t y of fluoride ions for complexing Ce (IV) and the extent of interference w i l l depend on the degree of dissociation of the complex. If i t i s completely or readily ionized, then there w i l l be l i t t l e or no interference, but in other cases, the s t a b i l i t y constant of the complex w i l l indicate the extent of interference. The interference by uranium ions in fluoride determination by the Null-point potentiometric method has been reported by the authors, who found that the results were very low, indicating that a l l fluoride ions were not available to complex Ce (IV). These authors also believe that the method is not suitable for solutions containing f e r r i c ion. In short, two equilibria need to be considered: the ionization of complex ions formed by cations other than cerium, and the complexing of Ce (IV) to lower the redox potential. The f i r s t step i s very important as on this depends the amount of fluoride available to the Ce (IV) ions. If 85 the s t a b i l i t y constant of the complex i s high, the equil-ibrium concentration of complex ion w i l l be considerable, keeping some fluoride ions bound. The amount of fluoride ions to bind Ce (IV) w i l l be less than in the absence of other complex cations, and consequently, the results w i l l be low. In order to assess the value of this reasoning, i t i s appropriate to compare the st a b i l i t y constants of various complex ions. Appropriate Logarithms of Formation Constants for F i r s t Fluoride Complexes (Qfc) ion log of formation constant ion log Ki +2 1.5 T i + 4 6.5 Ca + 2 1.5 9.8 Zn + 2 1.0 U 0 2 + 2 4.5 Ag + 0 Fe +3 5.0 Cd + 2 0.5 C r + 3 4.5 Ce+3 4.0 A l + 3 6.0 v o 2 + 3.0 A higher value of log K-j^  means a higher equilibrium concentration of complex ions, i.e. low ionization. It i s quite reasonable to expect that cations with a value of logarithm of formation constant higher than that of Ce (III) w i l l interfere, e.g. A l + 3 , F e + 3 , etc. The cations that w i l l not interfere w i l l be those having a value of log K-j_ less than 3 or 2.5 . The s t a b i l i t y of CrF/- J ions has been mentioned by 86 Kleram and Huss (24) who observed that even concentrated sulphuric acid does not decompose the complex very easily. The value of log of formation constant i s 4.5, which i s higher than that for Ce (III) complex. It i s therefore not surprising that the ionic equilibrium w i l l be affected and low results w i l l be obtained for the fluoride determin-ation. Another important point to consider is the role of pH and of concentration changes. In the original method, addition of unknown solution and of titrant i s balanced by additions of equal amounts of d i s t i l l e d water. When the pH of the unknown solution i s near seven, i t s addition w i l l scarcely affect the pH, and this w i l l be completely balanced by the addition of water to the other half c e l l . On the other hand, i f the unknown solution is acidic or alkaline, then i t s addition to one half c e l l and addition of d i s t i l l e d water to another w i l l not give comparable pH values. This imbalance of pH w i l l disturb the various ionic equilibria which are sure to exist where complex-forming cations are present, and this w i l l affect the potential changes. For example, using a l k a l i fluoride solutions, i t i s observed that the determination i s not very sensitive to the addition of small amounts of water— one m i l l i l i t r e or so—but when i t contains cations (such as Cr (III) ), the potential becomes very sensitive to water addition. 8 7 The other possibility i s to oxidize chromic ions to chromate and then use this solution. If an alkaline solution is used, there i s danger of precipitation of basic cerium salts ( 1 0 6 ) . If an acidic solution is used oxidation-reduction reactions can occur. The oxidation potentials of Ce (IV) and Cr (VI) are very similar ( 1 0 7 ) . Ce (IV), Ce (III)/Pt + 1 . 4 5 C r 2 0 7 ~ 2 , Cr (III)/Pt + 1 . 3 6 In small concentrations, dichromate ions should not cause any d i f f i c u l t y . In large concentrations, however, especially when fluoride ions are present in the solution (which w i l l complex Ce (IV) and reduce i t s concentration and hence the potential) dichromate ions w i l l interfere seriously and render the method unsuitable. The foregoing review of various methods for the determination of fluorine indicates that to date the most widely applicable method i s that of precipitating as lead chlorofluoride following the separation of combined fluorine by a Winter-Willard d i s t i l l a t i o n . The present investigation demonstrates that the use of perchloric acid for Winter-Willard d i s t i l l a t i o n should be avoided when the solution contains cations which can be oxidized or which form very stable fluoro-complexes. Sulphuric acid i s more suitable than perchloric acid but there i s danger of interference by the sulphate ion. The best procedure i s to use sulphuric acid for d i s t i l l a t i o n and precipitate SB fluorine as lead chlorofluoride, and then complete the determination volumetrically. The Null-point potentiometric method was not found directly applicable for the compounds encountered in the present research. Using this method after separat-ing the combined fluorine by Winter-Willard d i s t i l l a t i o n i s rather tedious (and i s of no advantage). Its application to the solution directly i s determined by the a b i l i t y of the cations to form fluoro-complexes. If the cations present in the solution do not form stable complexes, this method is applicable. On the other hand, i f the cations present do form stable complexes, the Null-point potentiometric method w i l l give very low results. The equivalent weight determination method is very simple and requires no special equipment. This method is applicable to those cations which form stable complexes but which can be oxidized to oxyanions. The present investigation was concerned with fluorides of chromium, and because chromium could be oxidized to chromate, this method was particularly suitable. It gave good and reproducible results, and was therefore often used. 89 DC. TETRAFLUORIDES OF THE FIRST TRANSITION SERIES The f i r s t transition series includes the metals from scandium to copper. The maximum oxidation states attained by these are (108): Sc Ti V Cr Mn Fe Co Ni Cu 3 4 5 6 7 6 4 2 2 The possibility of the formation of a tetrafluoride exists for the elements after scandium, and tetrafluorides of titanium, vanadium, chromium, and manganese are known. Iron does not show a +4 oxidation state, most probably because of the extra s t a b i l i t y of f e r r i c ion containing five electrons i n the d orbital. The elements cobalt and nickel show +4 oxidation states in fluoro-complexes only. Some guidance concerning the relative s t a b i l i t i e s of oxidation states can be obtained by considering the values of the ionization potentials (109), which are given below. Ti V Cr Mn Fe Co Ni 4th I.P. 24 48 49.6 It i s observed that the value of the fourth ionization potential increases in the series, indicating the increasing d i f f i c u l t y of obtaining the elements in a +4 oxidation state. S t r i c t l y speaking, this problem of 90 the r e l a t i v e s t a b i l i t i e s o f o x i d a t i o n s t a t e s i s very-complex and i s not amenable t o a simple g e n e r a l i z a t i o n . R e c e n t l y Maksimova (110) has evaluated f o r the elements the f u n c t i o n p, d e f i n e d as p = w T h i s f u n c t i o n r e p r e s e n t s t h e f r a c t i o n o f the energy expended i n removal o f the l a s t e l e c t r o n to o b t a i n the o x i d a t i o n number w. A c c o r d i n g t o the author, the q u e s t i o n o f the e x i s t e n c e o f v a r i o u s o x i d a t i o n s t a t e s can be s o l v e d on the b a s i s o f the v a l u e s of p. T h i s appears to be t r u e i n a q u a l i t a t i v e way but the e x i s t e n c e of a p a r t i c u l a r o x i d a t i o n s t a t e i s dependent v e r y l a r g e l y on the l i g a n d s . The v a l u e s o f p f o r the +4 o x i d a t i o n s t a t e f o r the elements of the f i r s t t r a n s i t i o n s e r i e s are g i v e n below. T i V Cr Mn Fe Co Ni Cu p: .482 .474 .479 .488 .493 .474 .476 .473 The g e n e r a l r u l e g i v e n by Maksimova i s t h a t t h e l e s s the d e v i a t i o n from the v a l u e o f p c h a r a c t e r i s t i c f o r the group, the g r e a t e r the p r o b a b i l i t y of the i o n having the charge corresponding t o t h a t group. I f f o r o x i d a t i o n number +4, the v a l u e o f p f o r t i t a n i u m i s taken to optimum, then i t i s seen t h a t the d e v i a t i o n i s maximum i n the case of i r o n and t 91 copper, and t h i s i s i n accord w i t h the f a c t t h a t no t e t r a f l u o r i d e o f these or complexes t h e r e o f are known. However, t h i s would a l s o p r e d i c t the e x i s t e n c e of n i c k e l t e t r a f l u o r i d e and c o b a l t t e t r a f l u o r i d e , f o r which the d e v i a t i o n i s l e s s than or equal to t h a t of vanadium t e t r a -f l u o r i d e , but the spontaneous decomposition of vanadium t e t r a f l u o r i d e at room temperature i s c o n s i s t e n t w i t h t h i s , and might a l s o p r e d i c t i n s t a b i l i t y f o r n i c k e l t e t r a f l u o r i d e and c o b a l t t e t r a f l u o r i d e . Of the known t e t r a f l u o r i d e s o f the f i r s t t r a n s i t i o n s e r i e s , manganese t e t r a f l u o r i d e has been i s o l a t e d o n l y r e c e n t l y , and as y e t not much i s known about i t s chemical p r o p e r t i e s ( 3 0 ) . PREPARATION AND PHYSICAL PROPERTIES OF THE TETRAFLUORIDES A l l the t e t r a f l u o r i d e s are formed when the metal i s f l u o r i n a t e d . T i tanium t e t r a f l u o r i d e i s a white s o l i d and b o i l s at 284°C ( 7 ) . Vanadium t e t r a f l u o r i d e i s a s o l i d and sublimes at 1 0 0 — 1 2 0 ° C ( 3 D w i t h d i s p r o p o r t i o n a t i o n , w h i l e chromium t e t r a f l u o r i d e i s a l s o a s o l i d and sublimes at 158°C in. vacuo. Vanadium, chromium, and manganese i n the +4 o x i d a t i o n s t a t e possess one, two, and t h r e e unpaired e l e c t r o n s i n the d o r b i t a l s and are t h e r e f o r e paramagnetic. The observed magnetic moments agree w i t h t h i s , and are g i v e n i n the f o l l o w i n g . 92 A>bst /^alc^on^y) Const, ^obs. -/calc. reference VF 4: 2.17 1.73 198° . 4 4 ( 3 D CrF 4: 3 .02 2.83 70° .19 present work MnF^: 3 .83 3 . 8 7 1 0 ° . 0 4 (30) It i s remarkable to note that the deviation from the spin-only values decreases from vanadium to manganese, indicating decreased orbital contribution to magnetic moment. The value of the Weiss Constant also f a l l s in the series, and this may be due to the increased covalent character of the tetrafluoride. To conclude, a comparison of the chemical properties of vanadium tetrafluoride and chromium tetrafluoride i s given in Table 1 (page 9 3 ) . Titanium tetrafluoride i s omitted mainly because i t contains no d electrons and the possibility of oxidation to a higher oxidation state does not exist. Manganese tetrafluoride i s omitted because l i t t l e has yet been reported about i t s chemical properties. 93 Table 1 A comparison of the chemical properties of vanadium tetrafluoride and chromium tetrafluoride. (a) with water hydrolysed to hydrolysed to V0 + < i + 4F~ C r + 3 + CrOT + 4F~ 4 (b) with SC>2 and SO^ no reaction no reaction (c) with NH-3 (liq.) formed 1:1 adduct no reaction ' ' • ' •> VF 4«NH 3 (d) with BrF 3 oxidation to VFr no reaction (room temp.) (e) with BrFo oxidation to VFc yields green (boiling? p CrF3»0.5BrF3 (f) with SeF, formed 1:1 adduct no reaction (room temp.) SeF^-VF^ (gj with SeF, formed 1:1 adduct yields C^-SeF^ (boiling) SeF 4-VF 4 CrF 2.SeF 4 (h) with IF^ no reaction no reaction (i) AC1 + BrF 3 AVF6 ACrF^ (j) 2AC1 + BrF 3 A 2CrF 6 A = K, Rb, or Cs 94 E X P E R I M E N T A L 95 X. GENERAL TECHNIQUES Progress in fluorine chemistry has been retarded because of the high reactivity of fluorine and many fluorides. Fluorine i t s e l f i s the most powerful oxidizing agent known, while chlorine and bromine fluorides are almost as reactive as fluorine. Thus, extreme precautions must be taken in the storage of these compounds. For example, fluorine reacts very readily with organic materials such as vacuum grease and o i l , and i t i s necessary to select materials for laboratory apparatus very carefully. We know now that a wide range of suitable materials i s avail-able, including most metals, ceramics, and siliceous bodies. The development of fluorocarbons and of fluorocarbon polymers has further extended the range of fluorine resistant materials. The usual material for apparatus, glass, reacts with fluorine and many fluorides, producing hydrofluoric acid which not only contaminates the products but also in many cases masks the true chemical properties of the compounds being studied. It i s true that metallic apparatus could be used, but the advantages of glass are well known and previous workers have shown that borosilicate or quartz glass can be used provided the apparatus has been baked dry on the exposed surface, preferably under vacuum. Also, i n the use of reactive fluorides such as halogen fluorides, which have been widely used in this research, 96 care should be taken to use as l i t t l e vacuum grease as possible, since t h i s i s r e a d i l y attacked by these f l u o r i d e s . In t h i s study, therefore, most experiments were performed i n a l l - g l a s s vacuum systems using techniques described previously (111). The s u s c e p t i b i l i t y of f l u o r i d e s to hydrolysis also requires, i n addition to rigorously dried apparatus, the use of reagents which are completely free of any traces of moisture. Before being reacted with f l u o r i d e s , a l l the reagents used i n t h i s study were d i s -t i l l e d from drying agents i n a l l - g l a s s apparatus. At PJffiPMATXVE T j j C H T O V B j g Preparative techniques i n t h i s work involved the use of f l u o r i n e , bromine t r i f l u o r i d e , bromine pentafluoride, and chlorine t r i f l u o r i d e as f l u o r i n a t i n g agents and as solvents i n complex-forming reactions. 1. Fluorine Fluorine was obtained from a 6 l b . cylinder which was clamped i n a v e r t i c a l p o s i t i o n and well shielded with bricks on two sides and with fume-hood walls on the other two sides. The cylinder valve was opened by a key having a handle 4 f t . long. Two Hoke st a i n l e s s s t e e l needle valves i n series were used to reduce the pressure of the f l u o r i n e gas, which was 400 l b s . / i n . i n the cylinder. The high pressure side was made from h a l f - i n c h s t a i n l e s s s t e e l tubing and was connected to the cylinder by a compression f i t t i n g , a Teflon gasket being employed. A st a i n l e s s s t e e l 97 Bourdon type pressure gauge was screwed and silver-soldered into this part of the system. The low pressure side of the system was constructed from quarter-inch copper tubing, with the exception of the sodium fluoride chamber (to remove hydrogen fluoride), which was of half-inch diameter. A l l valves on the low pressure side of the system were brass bellows-sealed needle valves. Also on the low pressure side, a safety blowout valve was provided in which excess pressure of fluorine was reduced by bubbling through a fluorocarbon o i l (Hooker Chemicals F S-5). This could also be used, before carrying out any reaction, to give a rough estimate of the fluorine flow-rate. 2. Bromine trifluoride Bromine trifluoride was taken from a cylinder provided by Matheson Co., Inc. A train of four traps was connected to the valve on the cylinder of bromine trifluoride by Teflon tubing. The traps were provided with break-seals and with constrictions in their side arms, by means of which they could be sealed off. The traps were dried under vacuum by flaming and the three traps following the cylinder were cooled by liquid nitrogen baths and the valve of the cylinder was opened; bromine trifluoride condensed in the trap closest to the cylinder. When a sufficient quantity of bromine trifluoride had d i s t i l l e d , the system was sealed off from the cylinder. The bromine trifluoride in the f i r s t trap was separated into three portions by d i s t i l l i n g into other traps. A l l the traps 98 were sealed off and kept at -78°G un t i l required. The bromine trifluoride obtained from the cylinder contained bromine pentafluoride as an impurity. Because in some reactions i t was desirable to use bromine trifluoride free of pentafluoride, the trifl u o r i d e was prepared i n the laboratory by direct fluorination of bromine. Bromine dried oyer phosphorus pentoxide was taken in a glass reaction vessel and nitrogen-diluted fluorine was bubbled through i t at a slow rate. The reaction vessel was connected to a train of traps provided with break-seals and was surrounded by a beaker containing carbon tetrachloride, which was kept cool by dropping some liquid nitrogen onto i t s surface from time to time. The fluorination of bromine proceeded very smoothly, as could be seen by the gradual disappearance of the bromine colour. After two to three hours, the bromine in the vessel had changed to a clear yellow l i q u i d . The reaction vessel was then sealed off from the fluorine supply and flushed with nitrogen. The bromine trifluoride was d i s t i l l e d into a storage trap provided with a break-seal. Ii Bromine pentafluoride Bromine pentafluoride was taken from a cylinder supplied by Matheson Co., Inc., and was condensed into traps in the same manner as bromine tr i f l u o r i d e . L. Chlorine trifluoride Chlorine trifluoride was supplied by Matheson Co., Inc., and was similarly taken into traps. 99 B. APPARATUS FOR REACTIONS WITH BROMINE TRIFLUORIDE, CHLORINE TRIFLUORIDE, BROMINE PENTAFLUORIDE, AND SELENIUM TETRAFLUORIDE.  The reaction vessel was made of s i l i c a tube of 12 mm. diameter closed at one end and connected to a 6 cm. length of s i l i c a tube of 10 mm. diameter. The 10 mm. tube was joined to an extended B-10 ground s i l i c a cone. The extended cone was used to prevent any reaction of the fluorinating agent with grease. Before each use the reaction vessel was cleaned with chromic acid and dried. In summary then, to minimize as far as possible the contact of grease with reactive fluorides, the follow-ing modifications were found useful. (i) use of extended cones ( i i ) use of sockets with inlet tubes C. REACTION WITH HALOGEN FLUORIDES This involved condensation of halogen fluoride onto reactants in the reaction vessel and then heating the contents of the vessel to the desired temperature under atmospheric pressure. After the completion of the reaction, excess halogen fluoride was d i s t i l l e d away. Removal of the excess of halogen fluoride was accomplished by d i s t i l l a t i o n in vacuo f at f i r s t at room temperature and then at higher temperatures. Bromine pentafluoride could be d i s t i l l e d off much more easily than bromine trifluoride, which required heating to a temperature, of l60°C for complete removal. Chlorine tri f l u o r i d e was very easily d i s t i l l e d . 100 A typical example of a preparation involving a halogen fluoride i s the preparation of potassium penta-fluoro-chromium (IV), KCrF^, and i s described below. Apparatus was set up as shown in F i g . no. II (page 101) and was thoroughly dried and evacuated. It was disconnected from the vacuum line and then a i r was allowed to enter very slowly while trap J was surrounded by a C0 2—alcohol mixture to condense any moisture. The s i l i c a vessel containing weighed quantities of chromium tetrafluoride and potassium chloride (1:1 molar ratio) was attached to the system by means of a B-10 joint and the system was evacuated again. The break-seal of trap B containing bromine trifluoride, which was surrounded by the C0 2—alcohol bath, was broken magnetically and any more volatile impurities were pumped off. Stopcock T.^as closed, the s i l i c a reaction vessel was surrounded by a liquid nitrogen dewar, and the C0 2—alcohol bath surrounding trap B was removed. As trap B warmed up, bromine trif l u o r i d e was condensed into the reaction vessel. Bromine tri f l u o r i d e in trap B melted, and then the trap was turned around through an angle of 180° very cautiously, causing the liquid bromine trifluoride to run down into the reaction vessel. After the transfer of bromine trif l u o r i d e , trap B was sealed off and removed. The system was disconnected from the vacuum line and traps L, K, and J were surrounded by C0 2—alcohol baths. The tap T was opened very slowly to l e t in a i r . The 102 cooling of traps L, K, and J prevented the entry of moisture. The liquid nitrogen bath surrounding the reaction vessel was removed and the s i l i c a vessel was allowed to warm to room temperature. At this stage the s i l i c a vessel was trans-ferred from F to G, and F was closed by a cone. The s i l i c a reaction vessel was heated by means of a small flame for ten to fifteen minutes and when the reaction was complete, the vessel was surrounded by a C 0 2—alcohol bath and tap T was closed. The tube V was connected to the vacuum l i n e . The trap L was surrounded by a liquid nitrogen dewar and vacuum was applied very slowly by opening carefully the tap T. The C 0 2—alcohol bath surrounding the reaction vessel was removed and excess bromine trifluoride was d i s t i l l e d off very slowly. Particular care was needed while d i s t i l l i n g off excess bromine trif l u o r i d e , as a quick d i s t i l l a t i o n rate results i n spurting, and the recording of weights becomes meaningless. When no more bromine trifluoride was d i s t i l l i n g over at room temperature, the reaction vessel was surrounded by boiling water. An o i l bath maintained at a temperature of 160°C was used to obtain a product free of bromine tr i f l u o r i d e . In order to ensure complete removal of bromine tri f l u o r i d e , the substance had sometimes to be heated for twenty-four hours. After complete removal of bromine trif l u o r i d e , the reaction vessel was sealed off and kept in the dry-box. 103 XI. ANALYTICAL TECHNIQUES The required analytical techniques involved quantitative determinations of chromium, fluorine, bromine, and chlorine. During a l l analytical work, an automatic Sartorius analytical balance was used. The chemicals used were of AnalaR or CP. grades and standard analytical techniques were followed throughout (107). Chromium was determined volumetrically by oxidation of the salt to chromate with hydrogen peroxide in alkaline solution. The excess peroxide was decomposed by boiling the alkaline solution. The solution was acidified and i t s dichromate content determined by the addition of an excess of standard ferrous ammonium sulphate solution and titr a t i o n of the excess of the latter with standard deci-normal potassium dichromate. Procedure.—A ground glass stoppered conical flask containing 25 ml. d i s t i l l e d water and 5 gm. sodium hydroxide was taken. Powdered sample (0.3—0.5 gm.) was accurately weighed into a small glass tube and the tube was dropped into the conical flask which was very quickly stoppered. Vapours of chromyl fluoride were formed in many cases, which took some time to dissolve. Hydrogen peroxide (10—15 ml.) was added and the solution turned clear. When a l l of the Cr (III) had been oxidized, the solution was heated for an hour to decompose the excess 104 peroxide. The solution, when cool, was acidified with 9 N sulphuric acid and transferred to a 100 ml. volumetric flask. For each determination 10 ml. of this solution were used. To 10 ml. of this solution in a 250 ml. conical flask were added 20 ml. of standard ferrous ammonium sulphate solution, 100 ml. of 2 N sulphuric acid, and 0.5 ml. of N-phenylanthranilic acid. This was titrated with standard potassium dichromate. The volume of ferrous salt solution consumed by dichromate i n the original solution was calculated, and from this the percentage of chromium in the sample was found. B, BROMINE, W CHimCMB Halide ion was determined by precipitation as silver halide by adding an excess of standard silver nitrate solution, and then ti t r a t i n g the excess silver nitrate with standard potassium thiocyanate (107). C, FLUQFJNE, The determination of fluorine was greatly compli-cated by the formation of very stable chromium complex ions CrF^ . Various analytical methods were tried and their values assessed. These methods have been reviewed in an earlier chapter. 1. Methods involving separation Pvrohvdrolvsis.—In this method the fluorine i s li b e r -ated as hydrogen fluoride which is determined titrimetric-1 0 5 a l l y with sodium hydroxide, using phenolphthalein as an indicator. Two techniques using pyrolysis have been described in the li t e r a t u r e — t h e f i r s t uses steam to decompose fluoride at an elevated temperature, and the second method involves using a stream of moist oxygen. In the present work steam was used to decompose the sample. The apparatus consisted essentially of a s i l i c a tube of 2 cm. diameter and 4 0 cm. length, having a 3 5 / 2 0 B.S. s i l i c a socket at one end and a B - 1 0 ground s i l i c a cone at the other. The pyrex steam condenser was attached to the s i l i c a tube by way of a B - 1 0 ground s i l i c a cone. The steam preheater and steam trap made of quarter-inch internal diameter copper tubing were attached to the other end of the s i l i c a tube by means of a brass b a l l machined to f i t the 3 5 / 2 0 B.S. s i l i c a socket. The brass b a l l and the B - 1 0 s i l i c a cone were provided with inset tubes to prevent seepage at the joints. The sample was heated by means of an electric furnace. Steam was generated in a 1 1 . flask with three necks—one taking the air inlet bubler, another the moist air/steam outlet, and the third a thermometer. To carry out a fluorine determination a platinum boat was weighed to a constant weight after heating i t to 3 0 0 °C in a stream of steam and then in a stream of hydrogen. The sample was transferred to the platinum boat in the dry-box, and the boat was re-weighed and transferred quickly into the s i l i c a tube. The hydrolysis was carried out by 106 f i r s t passing moist air over the sample, then slowly raising the temperature of the water in the bubbler together with the temperature of the furnace, u n t i l eventually the steam was passing over the sample at 300°C. At f i r s t a colourless d i s t i l l a t e collected, but later a yellow "chromium11 colour was seen in the d i s t i l l a t e , and as the pyrohydrolysis was continued, the intensity of the yellow colour also increased. The d i s t i l l a t e naturally could not be used for estimations of hydrofluoric acid owing to the impurity of chromic acid. Decomposition of the sample by pyrohydrolysis was thus found to be inapplicable to fluorides of chromium. Ion exchange.—The use of ion-exchange resins to separate cations from anions is well known. It was desired to see i f this method could be used to separate chromium from fluorine by running the solution down a column containing cation-exchange resin. The resin (Dowex-50) was stirred with portions of water u n t i l a clear liquid was obtained on decanting from the resin. The resin was then transferred in a water slurry to an ion-exchange column, before backwashing and re-settling repeatedly with water to form a resin bed through which the solution flowed in an even non-channelling stream. The column was then eluted with concentrated hydrochloric acid to convert the resin into the hydrogen form and to remove other cations, particularly f e r r i c ions, originally sorbed onto the resin. Elution was then continued with dilute 107 hydrochloric acid and f i n a l l y with water u n t i l the effluent was free from chloride ions. In order to carry out a separation, a solution was prepared by dissolving 1.05 gm of chromium tetrafluoride, oxidizing i t to chromate with hydrogen peroxide, and then diluting the solution to 250 ml. An aliquot portion ( 2 5 ml.) of this solution was heated with concentrated hydrochloric acid and alcohol to reduce chromate to the chromic state. The solution was then diluted and passed through the ion-exchange column. The eluant appeared colourless and a dark green band was seen in the column. Concentration of eluant, after making i t alkaline, gave a precipitate of chromic hydroxide, indicating that some chromium had passed as an anion complex. Thus i t was not possible under these experimental conditions to separate chromic from fluoride ions by passing the solution through the column. Winter-Willard method (101) .—Winter and Willard found that fluoride may be volatilized as f l u o s i l i c i c acid from a perchloric or sulphuric acid solution in the presence of quartz, glass, or porous plate with steam vapour as the carrier gas. In this work sulphuric acid was used to decompose fluoride, as perchloric acid was not found suitable. A sample of the compound was accurately weighed and dissolved in a sodium hydroxide solution contained in a 108 beaker. The solution was a c i d i f i e d u n t i l clear and then transferred to a 250 ml. f l a s k . Of t h i s solution 50 ml. were taken into a 250 ml. d i s t i l l i n g f l a s k equipped with a thermometer, a dropping funnel, and a condenser. Concen-trated sulphuric acid (25 ml.) was added through the dropping funnel and mixed with the solution, the solution was brought to a b o i l , and the l i q u i d i n the f l a s k was maintained at 135—140°C by adding water slowly from the dropping funnel. The d i s t i l l a t e was collected i n a beaker containing 25 ml. of d i l u t e sodium hydroxide sol u t i o n . Approximately 300 ml. of d i s t i l l a t e was collected over a period of two hours. The estimation of f l u o r i n e i n the d i s t i l l a t e could be completed i n two ways, gravime t r i c a l l y and volumetrically. (i) g r a v i m e t r i c a l l y The Winter-Willard d i s t i l l a t e was a c i d i f i e d with n i t r i c acid, using bromophenol blue as an in d i c a t o r . The solution was then made al k a l i n e by careful addition of sodium hydroxide, a c i d i f i e d with hydrochloric acid, and 1 ml. hydrochloric acid added i n excess. The solution was heated to 80°C and 5 gm. of lead n i t r a t e were added with constant s t i r r i n g . The solution was heated to b o i l i n g , and when the lead n i t r a t e had dissolved, 5 gm. of sodium acetate were added, r e s u l t i n g i n the p r e c i p i t a t i o n of lead chlorofluoride. The white c r y s t a l l i n e p r e c i p i t a t e was digested on a water bath f o r an hour and l e f t overnight. The p r e c i p i t a t e was 109 f i l t e r e d off on a sintered-glass crucible (weighed to a constant weight), washed once with cold water, five times with a saturated solution of lead chlorofluoride, and f i n a l l y with water once more. The crucible was dried to a constant weight at 1 3 4 — 1 4 0 °C and the percentage of fluorine calculated. ( i i ) volumetrically The fluoride in the d i s t i l l a t e was precipitated as lead chlorofluoride as described above i n the procedure for gravimetric determination, and instead of f i l t e r i n g through a sintered glass crucible, the precipitate was f i l t e r e d on a No. 42 Whatman f i l t e r paper and washed as above. The f i l t e r paper with the precipitate was transferred to the beaker i n which the precipitation had been carried out, and stirred to a pulp i n 100 ml. 5% n i t r i c acid. The beaker was heated on a steam bath and an excess of standard silver nitrate solution was added and the determination of chlorine was carried out by the Yolhard method ( 1 0 7 ) . The percent-age of fluorine was thus calculated. In the present work, i t was observed that the d i s t i l l a t e always contained sulphate ions and the precipitate of lead chlorofluoride contained the impurity of lead sulphate. The determinations of fluorine, therefore, were always completed by the volumetric method. 2. Methods involving no separation Equivalent weight determination.—A neutral or alkaline 110 solution containing chromate and fluoride anions was passed through a cation-exchange resin column. This resulted in the generation of free acids—H oCr0. and HF. <• 4 The eluant was titrated with standard alka l i and the equivalent weight of the compound was determined. From a knowledge of the chromium percentage, equivalent weight, and the oxidation state of chromium i n the compound, i t was possible to decide the molecular formula of the compound. The ion-exchange column used has been described earlier i n this chapter. Before carrying out the determinations with unknowns, i t was deemed necessary to check the validity of this method,-to see whether chromate ions and fluoride ions could be determined both separately and when present together. For this purpose, a standard sodium fluoride solution and a standard potassium dichromate solution and then a mixture of the two were passed in turn through the resin column, and the resulting acid solutions were titrated with standard a l k a l i . The end points were determined graphically and the results of a typical series are given below. 10.00 ml. of NaF solution after elution required 7.50 ml. NaOH 10.00 ml. of I^C^Oy solution after elution required 3.40 ml. NaOH 10.00 ml. NaF + 10.00 ml. K 9Cr 90 7 after elution required 10.85 ml. NaOH c ' It i s observed that the t i t r e value for the mixture i s equal I l l to the sum of the values for individual solutions, suggesting that this method i s applicable and that the two ions do not interfere. A solution of chromium tetrafluoride was prepared as follows: chromium tetrafluoride (0.4536 gm.) was weighed accurately into a beaker containing 5 gm. of sodium hydroxide dissolved in 25 ml. of d i s t i l l e d water. Hydrogen peroxide was added to oxidize the chromic ion to chromate. After the oxidation was complete, the excess hydrogen peroxide was removed by boiling the alkaline solution care-f u l l y . The peroxide-free yellow solution was transferred to a 100 ml. volumetric flask and 10 ml. of this solution were taken for each determination. The solution (10 ml.) was diluted with 20 ml. of water and passed through the ion-exchange column and the eluant was titrated with standard sodium hydroxide. A value of 21.84 was obtained for the equivalent weight (calc. for CrF^: 21.33). Null-point potentiometric method (102).—Two 250 ml. beakers were used as half cells and were connected by an agar-potassium chloride salt bridge. Two clean platinum wire electrodes were dipping into the half cells and were connected to a potentiometer. The beakers were provided with s t i r r i n g rods. A solution 0.005 M with respect to Ce (IV) and Ce (III) was prepared, and 50 ml. of this solution were pipetted out in each half c e l l . The e.m.f. of the c e l l was checked to be zero. Two standard solutions— 112 one of sodium f l u o r i d e (11.120 gm/l.) and one o f potassium dichromate (10.810 gm/l.)—were prepared t o study the e f f e c t of chromate and chromic i o n s on the accuracy o f the method. ( i ) Standard sodium f l u o r i d e s o l u t i o n (5 ml.) was added to one o f the h a l f c e l l s and 5.00 ml. of d i s t i l l e d water t o the o t h e r to account f o r the volume changes. The p o t e n t i a l d i f f e r e n c e was read and brought t o zero by c a r e f u l a d d i t i o n of standard sodium f l u o r i d e s o l u t i o n t o the o t h e r h a l f c e l l . T h i s r e q u i r e d 5.00 ml. of standard sodium f l u o r i d e s o l u t i o n . ( i i ) Two h a l f c e l l s were s e t up and the p o t e n t i a l d i f f e r e n c e checked t o be zero. To one h a l f c e l l 5 ml. of chromic s u l p h a t e s o l u t i o n (0.5%) were added and the p o t e n t i a l d i f f e r e n c e was a g a i n found to be zero. ( i i i ) Another s e t o f h a l f c e l l s was s e t up and the p o t e n t i a l d i f f e r e n c e was zero. Standard potassium dichromate s o l u t i o n (5 ml., 10 ml., and 15 ml.) was added s u c c e s s i v e l y t o one h a l f c e l l and an equal amount of water was added t o the o t h e r . There was no change i n the p o t e n t i a l d i f f e r e n c e between the two h a l f c e l l s . ( i v ) A s o l u t i o n c o n t a i n i n g both f l u o r i d e i o n s and chromic i o n s was prepared. Standard potassium dichromate (10.810 gm/l; 50 ml.) was taken i n a beaker, reduced w i t h sulphur d i o x i d e , and b o i l e d t o remove excess s u l p h u r d i o x i d e and to reduce the volume. To t h i s , 50 ml. of 113 standard sodium fluoride solution (11.28 gm/l.) was added and the mixture was transferred to a volumetric flask. This was solution A. In another volumetric flask 50 ml. of sodium fluoride solution (11.28 gm/l.) were diluted to 100 ml. and used as a t i t r e . The fluoride concentration in both solution A and the t i t r e was the same. Solution A (10 ml.) was added to one half c e l l and titrated. This required 9.00 ml. of t i t r e to bring the potential back to zero. In another run 8.95 ml. of t i t r e were required. This was approximately 10% less than the amount theoretically required and the results were not reproducible. It was also noticed that near the end- point the sensitivity decreased considerably, and the potential was very sensitive to changes in the water concentration. A solution of chromium tetrafluoride was prepared by dissolving 0.4218 gm. of chromium tetrafluoride in water (25 ml.) and sulphuric acid (1 ml.) i n a beaker. The solution was transferred to a 100 ml. volumetric flask and 10.00 ml of this solution were used for each determin-ation. Two determinations were carried out and gave the results 37.75% fluorine, and 71.27% fluorine (CrF^ requires 59.37% fluorine). Due to serious errors in the results and a lack of reproducibility, this method was not useful for determination of fluorine in chromium tetrafluoride. 114 XII. MAGNETIC MEASUREMENTS AND X-RAY INVESTIGATIONS A. MAGNETIC MEASUREMENTS Magnetic s u s c e p t i b i l i t i e s were measured at room temperature and at lower temperatures u t i l i z i n g a Gouy balance. The apparatus has e a r l i e r been described (112) i n d e t a i l . The magnetic balance consisted of a semi-micro balance standing on a slate-topped table, the s l a t e top being mounted on an a n t i - v i b r a t i o n mounting. The magnet used was a Varian 4 i n . electromagnet (Model V 4084) with 2 i n . tapered pole faces. The pole gap was set at one inch and with a current of 2 amp. per winding section; a f i e l d of approximately 15 kilogauss was obtained. The current and hence the f i e l d was maintained constant by a current regulator. 1. Room temperature measurement The compound, contained i n a tube sealed under vacuum, was taken into the dry-box. The container was opened and the compound powdered i n a dry agate mortar and then transferred to a trap having a side tube of a size suitable f o r the sample holder, a glass top, and a B-14 cone and socket. This trap had been baked dry before use. The trap containing the sample was evacuated and then disconnected from the vacuum l i n e by closing the glass top. The compound was tipped into the side tube, an appropriate length of which was then sealed o f f 1 1 5 carefully. The specimen was f i t t e d into the brass holder and suspended from the balance into the pole gap. The weight of the sample was recorded at f i r s t without any magnetic f i e l d and then in the f i e l d . The specimen tube was removed and taken into the dry-box and emptied into another container. The specimen tube was washed with water, dried, and suspended in the magnetic f i e l d . The weights in the f i e l d and without any f i e l d were recorded. The tube was then f i l l e d to the same length as the unknown sample with standard HgCo(SCN)^ ( 1 1 3 ) . A great deal of care was taken to ensure similar packing in both cases. The specimen tube, now containing the standard, was suspend-ed in the pole gap and the change in weight on application of the f i e l d recorded. From a knowledge of the weights and changes in weights of unknown and standard, the magnetic moment of the sample could be calculated by the formula /A = 2 . 8 3 9 w( s) xXg( s) x M ( x ) x T where Aw^  = change in the weight of unknown A w ( s ) ~ change in the weight of standard w(x) = weight of the unknown w ( s ) = w e i g h t of the standard ^g(s) = Gram Susceptibility of the standard = 1 6 . 4 4 x 10 c.g.s. units M(x) = molecular weight of the unknown 116 /L. Low temperature measurement The equipment used was a modified form of that used by Fig g i s and Nyholm (114). Low temperatures were produced by using l i q u i d nitrogen, and the apparatus could be set at any desired temperature i n the range of 90—298°K. Measurements were made as already described, and at each ' temperature f i f t e e n to twenty minutes were allowed to s t a b i l i z e the c i r c u i t and the temperature. From the measurement with the standard, HgCo(SCN) 4, at room temper-ature, a tube constant C was calculated and t h i s constant was used to calculate the s u s c e p t i b i l i t y at various temper-atures. The molar s u s c e p t i b i l i t y (?^) could be obtained and then the r e c i p r o c a l of molar s u s c e p t i b i l i t y was plotted against absolute temperature to obtain the Weiss Constant. As an example, measurements on chromium t e t r a f l u o r i d e and the r e s u l t i n g calculations are shown i n Table 2, page 117. The plot of the value of - J — versus T, which gave a AM value of Q - -70°K, i s shown i n F i g . n o . I l l (page 118). B. X-RAY INVESTIGATIONS X-ray powder photographs were taken on a General E l e c t r i c unit with a camera of 14.4 cm. diameter, using Cu-K^ - r a d i a t i o n . The samples were sealed i n 0.5 mm., thin-walled X-ray c a p i l l a r i e s (Pantak Ltd.) i n a dry-box to protect them from atmospheric moisture. In general, the d i f f r a c t i o n patterns obtained possessed dark background 117 Table 2 Magnetic susceptibility measurements of chromium tetrafluoride. weight of chromium tetrafluoride I.6425 gm. weight of standard 1.248O gm. room temperature 21°C (294°K) change in weight (Aw) for standard 0.0200 gm. change in weight (aw) for empty tube 0.0007 gm. Y for standard 16.4 x 10~6 o C = 16.44 x 1.2A.fi x IQ" 6 = x 1 Q -6 C t = 991 x IO" 6 = 603.6 x 10"6 1.642$ , % -6 * M » - 6 -4- A c u r i e / ^ f f ( C u r i e -m^p,» iU AW x, 1,0 0 , x ; o u L,aw) W e i s s Law) x 102 (B.M.) (B.M.) 294° .0405 24.46 3127 3.20 2.74 3.02 245 .0484 29.20 3746 2.68 2.72 3.06 220 .0517 31.20 3994 2.51 2.66 3.04 198 .0546 32.92 4217 2.37 2.60 3.02 170 .0645 38.93 4983 2.10 2.61 3.12 148 .0687 41.47 5319 1.89 2.52 3.04 123 .0757 45.69 5849 1.70 2.41 3.02 119 and showed no lines at high angles. Consequently, no intensity measurements were possible and therefore no detailed structure determinations were made. 120 X I I l . PREPARATION AND REACTIONS OF CHROMIUM TETRAFLUORIDE A. PREPARATION OF CHROMIUM TETRAFLUORTITE Chromium tetrafluoride was prepared by the fluorin-ation of powdered chromium metal at 300—350°C (22). Fluorination was carried out in a nickel reactor tube 12 i n . long and 1 i n . in diameter. One end of the nickel tube was flanged so that the nickel boat containing the powdered metal to be fluorinated could be introduced. This end could be sealed by bolting a nickel plate, with a 1/4 i n . exit tube silver-soldered to i t , to the flange compressing a lead washer to make a gas-tight seal. The 1/4 i n . exit tube in the faceplate was connected to the train of traps by a brass compression f i t t i n g . The other end of the nickel tube was silver-soldered to a 4 i n . length of 1/4 i n . nickel tubing which could be connected to the fluorine supply line with a 1/4 in. compression f i t t i n g . The f i r s t two of the train of four traps following the reactor were each equipped with capillary constrictions and break-seals. The other two traps were used to prevent any back diffusion of moisture into the apparatus. To prepare chromium tetrafluoride, about 8—10 gm. of powdered chromium metal were weighed out into the nickel boat (6 in. x 3/4 in.) which was then placed into the nickel reactor tube. The flange was bolted into place and then the nickel reactor tube was placed in a tube furnace. The reactor tube was connected to the fluorine-nitrogen 121 supply on one end and to the train of traps on the other. Before fluorinating, i t was absolutely necessary to dry the apparatus as well as possible. This was achieved by pumping on i t while the furnace was adjusted to a temperature of 200°C. The glass traps were occasionally heated by means of a Bunsen flame, with the vacuum s t i l l applied. Then nitrogen gas was let into the reactor tube very slowly and the system was disconnected from the vacuum lin e . Nitrogen gas was then passed through the system for two hours while the chromium was maintained at a temperature of 350°C. When the apparatus was dry and f i l l e d with nitrogen, the glass traps were cooled by dry ice—al c o h o l baths. Fluorine gas diluted with nitrogen was then passed over the heated metal and the reaction commenced. Dense yellow vapours condensed in the f i r s t cold trap and as the fluorination was continued, a thin red layer was observed to deposit on the inside of the glass tube connecting the traps with the reactor tube and also the amount of yellow condensate in the trap increased. After twenty minutes the fluorine supply was stopped and the system was flushed with nitrogen gas, while the furnace was switched off. When fluorine had been completely displaced by nitrogen and the reactor tube was at room temperature, the system was connected to a vacuum line and the traps were sealed off from the nickel tube. Nitrogen gas was l e t in the nickel tube and the faceplate 122 was removed. A dark glassy solid was noticed to have been deposited in the exit tube of the faceplate and also on the inside of the tube adjacent to the flange, i.e. the region of the tube just outside the furnace. This was chromium tetrafluoride, and was scraped off from the tube under a fast flow of nitrogen, and was taken into a weighing tube which was then placed in a desiccator. In every preparation 2—4 gm. of product were obtained. The boat inside the reactor was found to contain some unreacted metal covered by a green substance which was later shown to be chromium tr i f l u o r i d e . In one fluorination reaction a s i l i c a reaction tube was used instead of a nickel reactor tube, in order to observe the fluorination more closely. The s i l i c a tube was 10 i n . long and 3/4 i n . in diameter, and was connected to a train of traps on one side and to the fluorine supply on the :other by means of.a B-14 standard ground glass joint. A small nickel boat containing chromium metal was placed in the tube. This tube was surrounded by a mantle heater. After the apparatus had been dried, the tube was heated to a temperature of 350°C and fluorine gas diluted with nitrogen was passed through. Chromium metal in the nickel tube began to glow and bright blue vapours were seen in the hot zone of the tube. These vapours were carried with the stream of fluorine-nitrogen gas mixture and condensed in the cooler part to a dark green solid, later shown by 1 2 3 analysis to be chromium tetrafluoride. The amount of heat produced during the reaction was so considerable that the s i l i c a tube melted at the hottest point and a hole was blown in the tube, indicating that s i l i c a could not be used for carrying out the fluorination. Analysis.—Chemical analyses for chromium and fluorine were carried out as described in an earlier chapter. Found: Cr, 40.$2, 40.00; F, 59.63. Calc. for CrF. : 4 Cr, 40.63; F, 59.37%. B. REACTIONS OF CHROMIUM TETRAFLUORIDE 1. Reaction with water Chromium tetrafluoride was weighed into a beaker containing 50 ml. d i s t i l l e d water. It reacted vigorously with water and a solution containing chromate and chromic ions was produced. It was acidified to dissolve the small amount of chromic hydroxide, and made up to 100 ml. Of this solution 10.00 ml. were used for the determination of hexapositive chromium. In another 10.00 ml., total chromium was determined after oxidation of Cr ; to Cr . The ratio of Cr :Cr was 1:2, indicating a hydrolysis of the type 3 Cr + 4 + 4H20 Cr0 4~ 2 + 2Cr + 3 + £H + 2. Fluorination of chromium tetrafluoride Fluorination was carried out in a s i l i c a tube similar to the one used for the fluorination of chromium. A small nickel boat was dried and a small amount of 124 powdered chromium tetrafluoride was transferred into i t in the dry-box. The boat was placed in the s i l i c a tube and fluorination was carried out at a temperature of 100—150°C. Higher temperatures resulted in reaction with the s i l i c a tube and decomposition of chromium tetrafluoride. Signs of any extensive reaction were not observed. However, because of some traces of oxygen in the fluorine and some derived from the reaction with the s i l i c a tube, small amounts of chromyl fluoride were formed and collected in the cold trap. The yellow chromyl fluoride formed a layer on the inside of the connecting glass tubes. Because of the messy nature of the reaction, i t was stopped and the product in the boat, on analysis, was found to be chromium tetrafluoride. (Found: Cr, 39-95, 40.26. Calc. for CrF^: Cr, 40.63%.) 3. Reaction with liquid ammonia Gaseous ammonia (Matheson Co., Inc.) was carefully dried by condensing i t on to metallic sodium in an all-glass d i s t i l l a t i o n system which had been baked dry. An excess was then d i s t i l l e d from sodium on to chromium tetrafluoride and allowed to melt. There were no signs of reaction and the residue after the removal of ammonia was found to be chromium tetrafluoride. (Found: Cr, 39.8. Calc. for CrF^: Cr, 40.6%.) 4 . Reaction with sulphur dioxide Sulphur dioxide from a cylinder was passed through two tubes containing phosphorus pentoxide and was 1 2 5 condensed at -184°C i n a glass trap f i t t e d with a break-seal and a c a p i l l a r y c o n s t r i c t i o n . The trap was evacuated and sealed at the con s t r i c t i o n when s u f f i c i e n t sulphur dioxide had colle c t e d . The reaction of sulphur dioxide with chromium t e t r a f l u o r i d e was performed i n the usual a l l - g l a s s system which had previously been baked dry. The trap containing sulphur dioxide was attached to the apparatus. Then the s i l i c a reaction vessel containing chromium t e t r a -f l u o r i d e was attached to the reaction assembly by means of a B-10 j o i n t . The system was evacuated and sulphur dioxide d i s t i l l e d i n vacuo on to chromium t e t r a f l u o r i d e contained i n the reaction vessel, and the trap o r i g i n a l l y containing sulphur dioxide was sealed o f f . The reaction vessel was then allowed to warm to room temperature. After two hours sulphur dioxide was d i s t i l l e d o f f and the residue was found to be chromium t e t r a f l u o r i d e . (Found: Cr, 39.89. Calc. f o r CrF^: Cr, 40.63%.) 5. Reaction with sulphur t r i o x i d e Pure dry«/-sulphur t r i o x i d e was obtained by d i s t i l l i n g a mixture of fuming sulphuric acid and phosphorus pentoxide at 100°C. The sulphur t r i o x i d e was d i s t i l l e d i n vacuo into a second trap provided with a break-seal, which was then sealed o f f and attached to an a l l - g l a s s reaction assembly, and the reaction was carried out as with sulphur dioxide. There were no v i s i b l e signs of reaction; the product a f t e r removal of sulphur t r i o x i d e had the same 126 weight as the starting material, and was shown by chemical analysis to be chromium tetrafluoride. (Found: Cr, 39.96. Calc. for CrF^: Cr, 40.63%.) &« Reaction with selenium tetrafluoride Selenium tetrafluoride was prepared by fluorination of selenium (115) and was stored in a trap provided with a break-seal. In all-glass apparatus was set up as for reaction with bromine tr i f l u o r i d e . Reaction with selenium tetrafluoride was carried out at room temperature and also at the boiling point of selenium tetrafluoride. (i) at room temperature Powdered chromium tetrafluoride (0.2230 gm.) was taken in a s i l i c a reaction vessel and an excess of selenium tetrafluoride d i s t i l l e d on to i t from a trap which was sealed off after the d i s t i l l a t i o n was complete. Air was let into the system through traps cooled by a C0 2—alcohol mixture to ensure removal of water from the a i r . The s i l i c a reaction vessel was allowed to warm to room temper-ature. There were no visible signs of reaction. The powdered chromium tetrafluoride was allowed to remain in contact with the liquid selenium tetrafluoride for fifteen minutes, after which a l l the traps were cooled to -78°C and connected to the vacuum l i n e . The dewar surrounding the s i l i c a reaction vessel was removed and selenium tetra-fluoride was allowed to d i s t i l over into the adjacent cooled trap. The reaction vessel was heated to 90°C to 127 remove a l l of the selenium tetrafluoride. D i s t i l l a t i o n in. vacuo was continued u n t i l there was no selenium tetra-fluoride d i s t i l l i n g and the reaction vessel was then sealed off. There were no visible signs of reaction and the weight of the starting material had not changed. On analysis the residue was found to be chromium tetrafluoride. (Found: Cr, 40.55. Calc. for CrF^: Cr, 40.63%.) ( i i ) at the boling point of selenium tetrafluoride, 106°C An apparatus similar to that used in the preceeding experiment was set up and powdered chromium tetrafluoride was taken into the s i l i c a reaction vessel. Selenium tetra-fluoride from a trap was d i s t i l l e d on to i t and the reaction was carried out in exactly the same way as in the previous experiment, except that the reaction vessel was heated carefully so that the selenium tetrafluoride refluxed. Reaction between chromium tetrafluoride and selenium tetra-fluoride occurred on heating, and the reaction vessel was heated for fifteen minutes, after which i t was allowed to, cool to room temperature. Excess selenium tetrafluoride was removed by d i s t i l l i n g i t under vacuum at room temper-ature and when no more was d i s t i l l i n g the reaction vessel was sealed off. The s i l i c a reaction vessel was seen to contain a non-homogeneous product consisting of at least two phases. The vessel was again connected to a train of cooled traps with break-seals by means of a B-10 joint and vacuum was applied. The reaction vessel was surrounded by 128 a beaker containing boiling water in order to remove selenium tetrafluoride completely. It was unexpectedly observed that a buff-coloured portion of the solid product was swept through into the cold trap. The reaction vessel was maintained at the temperature of boiling water u n t i l the transfer of the buff-coloured product was complete. It was also noticed that the reaction vessel then contained only one type of product, needle-like crystals of a pink colour. At this stage the reaction vessel and also the trap contain-ing the light buff-coloured compound were both sealed off and the contents of each examined separately. Pink product.—The substance gave a positive qualitative test for selenium, suggesting that i t was a selenium tetra-fluoride adduct. Chromium and fluorine analyses gave the formula CrF 2«SeF 4. (Found: Cr, 22.00; F, 47.50. Calc. for CrF 2-SeF 4: Cr, 21.24; F, 46.75%.) The magnetic susceptibility was measured at room temperature and gave a value of 5.34 B.M. for the magnetic moment. Magnetic susceptibility = 11900 x 10~^ c.g.s. units The X-ray powder pattern of the compound gave a complex diagram and no attempt was made to index i t . Buff-coloured product.—The trap containing the buff-coloured product was opened in the dry-box and the contents were examined by X-ray, magnetic, and analytical methods. It was a very light compound. Chemical analysis showed 1 2 9 that i t contained selenium and was therefore probably a selenium tetrafluoride adduct. Chromium and fluorine determinations gave the formula CrF^'SeF^. (Found: Cr, 19.7; F, 50.00. Calc. for CrFySeF^: Cr, 19.7; F, 50.40%.) Magnetic measurements were d i f f i c u l t , presumably because of the very low density of the solid, and also because i t could not be packed very tightly. Therefore, no importance can be attached to this measurement. lz Reaction with iodine pentafluoride In an all-glass well dried reaction assembly similar to the one used for reaction with bromine t r i -fluoride, iodine pentafluoride was condensed on to a weighed quantity of chromium tetrafluoride taken in a s i l i c a reaction vessel. The reaction vessel was then allowed to warm to room temperature and chromium tetrafluoride was kept in contact with liquid iodine pentafluoride for fifteen minutes. There were no signs of reaction. Iodine pentafluoride was removed at 100°C. There was no significant change i n the weight and the residue, on analysis, was found to be chromium tetrafluoride. (Found: Cr, 39.98, 40.34. Calc. for CrF,: Cr, 40.63%.) 4 A similar experiment was carried out; iodine penta-fluoride was condensed on to chromium tetrafluoride contained in the s i l i c a reaction vessel. The reaction was carried out at the boiling point of iodine pentafluoride, 130 100.94°C. The reactants were heated together for fifteen minutes but no signs of reaction were observed. Excess iodine pentafluoride was then removed. A temperature of 100°G was used to obtain a product free of iodine penta-fluoride. When d i s t i l l a t i o n of iodine pentafluoride was complete, the reaction vessel was sealed off. There was no change in the weight of chromium tetrafluoride, and chemical analysis showed that i t was unchanged chromium tetrafluoride. (Found: Cr, 3$.5- Calc. for CrF^: Cr, 40.63%.) The percentage of chromium was low because of some adsorbed iodine pentafluoride. 8. Reaction with bromine trifluoride (i) at room temperature A weighed sample of powdered chromium tetrafluoride was taken i n a s i l i c a reaction vessel and attached to an all-glass reaction assembly to which a trap containing bromine trifluoride had also been attached. Bromine trifluoride was condensed on to the reactant in the reaction vessel. The trap originally containing bromine trifluoride was sealed off and the reaction was carried out at atmos-pheric pressure and at room temperature, as described earlier. Excess bromine trifluoride was removed, a temp-erature of 100°C being used to remove the f i n a l traces. The residue in the reaction vessel appeared identical to the starting material and by chemical analysis was found to be unchanged chromium tetrafluoride. (Found: Cr, 40.55. 131 Calc. for CrF, : Cr, 40.63%.) 4 ( i i ) at the boiling point of bromine trifluoride, 127°C The usual all-glass reaction assembly was assembled and baked dry. To i t , a trap containing bromine trif l u o r i d e was attached by means of a B-14 joint. The s i l i c a reaction vessel containing a weighed amount of chromium tetrafluoride was attached by means of a B-10 joint. The bromine t r i -fluoride was condensed in the reaction vessel and the trap originally containing bromine trifluoride was sealed off. The reaction assembly was opened to the a i r , care being taken to avoid entry of moisture. The reaction vessel was warmed to room temperature and heated slowly with a low Bunsen flame un t i l the bromine trifluoride was boiling; the reaction vessel was maintained at this temperature for twenty minutes. A greenish powder was seen to form. Excess bromine trifluoride was removed by d i s t i l l a t i o n under vacuum at 120°C, and when d i s t i l l a t i o n was complete, the reaction vessel was sealed off and taken to the dry-box. Samples were taken for chemical analysis and for X-ray and magnetic measurements. Qualitative chemical analysis showed the presence of bromine and hence probably the product was a bromine trifluoride adduct. Quantitative analysis showed i t to be CrFy0.5BrF 3. (Found: Cr, 30.00; equiv. wt. 25.5. Calc. for CrF^-O.SBrF^: Cr, 29.4%; equiv. wt. 25.4.) Magnetic susceptibility measurements were carried 132 out at room temperature (294°K) and the following values were obtained. XM = 6583 x 10 6 c.g.s. units, ^ 2 9 4 = 3 , 9 6 B # M > The value of the magnetic moment corresponds to three unpaired electrons, indicating that chromium is in an oxidation state of +3. 1* Reaction with bromine pentafluoride A weighed sample of chromium tetrafluoride was taken in the s i l i c a reaction vessel and attached by means of a B-10 joint to the usual type of reaction assembly with a trap containing bromine pentafluoride. After evacuating the apparatus, bromine pentafluoride was condensed into the reaction vessel and the reaction was carried out at room temperature in the usual way. After the tetrafluoride had stood in contact with liquid bromine pentafluoride for fifteen minutes, the reaction assembly was connected to the vacuum line and bromine pentafluoride was removed by d i s t i l l a t i o n into a trap cooled by liquid nitrogen. When a l l the bromine pentafluoride had been removed, f i r s t at room temperature and then at 100°C, the reaction vessel was sealed off and the contents analysed. Chromium determination showed the product to be unchanged chromium tetrafluoride. (Found: Cr, 39.52. Calc. for CrF^: Cr, 40.63%.) In another experiment the reaction was performed similarly, with the exception that the reaction vessel was heated very slowly with a low Bunsen flame. On heating the 133 reaction vessel, bromine pentafluoride boiled off in a short time. The residue was found to be chromium tetrafluoride. (Found: Cr, 39.80. Calc. for CrF^: Cr, 40.63%.) 10. Reaction with a mixture of bromine trif l u o r i d e and bromine peataXluorj,4e _ A weighed sample of chromium tetrafluoride was refluxed with a mixture of bromine trifluoride and bromine pentafluoride. The reaction mixture was observed to be deep-red in colour. After the removal of the volatile bromine fluorides at 100°C, a reddish residue was l e f t in the reaction vessel, which was then sealed off from the reaction assembly and taken into the dry-box. Samples of the residue were taken for chemical analysis and magnetic measurements. Chemical analysis indicated the presence of bromine, and a quantitative determination of chromium showed the compound to be CrOF3'0.25 BrF^. (Found: Cr, 33.8, 32.9. Calc. for CrOF3'0.25 BrF^: Cr, 32.71%.) Magnetic susceptibility measurements were carried out at room temperature (294°K) and the following values were obtained. 134 XIV. COMPLEXES DERIVED FROM CHROMIUM TETRAFLUORIDE JL* POTASSIUM FETOFLUQRQQHRQfflyM (IV) Preparation. 1—Potassium pentafluorochromium (IV) was prepared by heating together a mixture of chromium tetra-fluoride and potassium chloride in a 1:1 molar ratio. Potassium chloride rather than potassium bromide was used because when the latter i s employed bromine collects in the glass connecting tubes. Potassium chloride used was of AnalaR grade and was well dried before use. The preparation was carried out as described in the general experimental technique. Two preparations were carried out, one in which bromine trif l u o r i d e was d i s t i l l e d off at 100°C, and another in which i t was removed at 160°C. (i) preparation I After heating together the mixture of chromium tetrafluoride, potassium chloride, and bromine tri f l u o r i d e , the reaction vessel was allowed to cool to room temperature and then excess bromine trifluoride was removed by d i s t i l l i n g in. vacuof at f i r s t at room temperature, and then at 100°C. When d i s t i l l a t i o n was complete, the reaction vessel was sealed off and taken to the dry-box. Samples were taken for chemical, magnetic, and X-ray analyses. Analysis.—Qualitative tests showed the presence of bromine, indicating that even at 100°C a l l bromine t r i -fluoride had not been removed, and that possibly a bromine trifl u o r i d e adduct had been formed. Quantitative analysis 135 gave the formula KCrF^0.5BrF 3 f o r the compound. (Found: Cr, 20.00; F, 49.10. Calc. f o r KCrF 5«0.5BrF 3: Cr, 20.43, F, 48.50%.) Magnetic measurement.—The magnetic susceptibility-was measured at room temperature and gave a value of 3.22 B.M. f o r i t s magnetic moment, ind i c a t i n g the presence of two unpaired electrons, i . e . chromium (IV). With another sample the measurement of magnetic s u s c e p t i b i l i t y was carried out over a temperature range, down to -175°C. The values obtained are given below, i n Table 3. Table 3 Magnetic s u s c e p t i b i l i t y measurements of KCrF^0.5BrF 3 X , v TO-6 ACurie-Weiss Temp (K) AM, x ^ / ( C u r i e Law) 7 Law) (B.M.) (B.M.) 98° 8830 2.65 3.56 123 8063 2.82 3.63 148 7182 2.92 3.63 171 6485 2.99 3.62 198 5847 3.05 3.62 226 5474 3.18 3.67 253 5040 3.19 3.67 294 4485 3.22 3.67 The plot o f - r * 1 — versus T gave a value of *M 0 = -80° 136 Crystal structure.—The X-ray powder pattern was indexed on a tetragonal l a t t i c e with a = 9.168 I c = 13.49 A The values of the observed and calculated s i n 2 $ values are given in Table 4 (page 137). ( i i ) preparation II Another reaction using chromium tetrafluoride, potassium chloride, and bromine trifluoride was carried out in exactly the same way, except that the temperature at which bromine trif l u o r i d e was d i s t i l l e d off was 160°C. This was done in order to get a product free of bromine tri f l u o r i d e . In this experiment, chromium tetrafluoride (0.6112 gm.) and potassium chloride (0.3582 gm.) were heated together in bromine t r i f l u o r i d e . The weight of the. product was 0.9750 gm. D i s t i l l a t i o n of bromine trif l u o r i d e was carried on for twenty-four hours, after which the reaction vessel was sealed and taken to the dry-box. Analysis.—Qualitative analysis showed no precipitate of silver bromide on addition of silver nitrate to a solution of the compound. Quantitative analysis showed that the compound was KCrF^. (Found: Cr, 26.00; F, 49.50. Calc. for KCrF^: Cr, 27.94; F, 51.07%.) Magnetic measurement.—The magnetic susceptibility 137 Table 4 Calculated and observed sinffi values for KCrFr*0.5BrF sin 2t9 s i n ^ hkl obs. calc. intensity obs. 112 :..o6o3 .0599 w. 212 .1079 .1067 v.w. 004 .1144 .1152 m. 220 .1234 .1246 v.v.w. 300 .1405 .1402 m. 310 .1598 .1558 v.v.w. 302 .1684 .1690 w. 204 .1750 .1775 v.w. 313 .2194 .2206 v.v.w. 224 .2395 .2398 v.w. 314 .2725 .2710 w. 225 .3004 .3046 m. 332 .3101 .3092 m. 324 .3174 .3177 w. 206 .3264 .3216 v.v.w. 334 .3970 .3956 v.v.w. 522 .4811 .4806 v.v.w. 514 .5209 .5202 v.v.w. 128 .5384 .5387 v.w. 426 .5719 .5702 v.v.w. 540 .6361 .6387 v.v.w. 604 .6743 .6760 v.v.w. 630 .7000 .7011 v.v.w. 00; 10 .7202 .7202 v.v.w. 517 .7555 .7578 v.v.w. 550 .7773 .7770 v.v.w. 702 .7933 .7922 v.v.w. 447 .8514 .8513 v.v.w. 643 .8717 .8747 v.v.w. la t t i c e : tetragonal a = 9.168 A c = 13.49 A 138 measurement gave a value of ^ = 4193 x 10' "6; /294 = 3.16 B.M. Crystal structure.—The X-ray powder pattern was indexed on a hexagonal unit c e l l with a = 8.739 A c = 5.226 1 The calculated and observed sin^L7 values are given i n Table 5 (page 139). B. RUBIDIUM PENTAFLU0R0CHR0MIUM (IV) Preparation.—Chromium t e t r a f l u o r i d e (0.7324 gm.) was taken i n the s i l i c a reaction vessel, and to i t well dried rubidium chloride (0.69 gm.) was added. The s i l i c a reaction vessel was then attached to a reaction assembly and bromine t r i f l u o r i d e condensed on i t . The reaction was carried out as f o r preparation II of the potassium analogue; excess bromine t r i f l u o r i d e was removed at a temperature of 160°C. The product weighed 1.3841 gm. (Calc. f o r RbCrF^: 1.32 gm.) A n a l y s i s . — Q u a l i t a t i v e tests indicated that no bromine was present, suggesting that the compound was not a bromine t r i f l u o r i d e adduct, and quantitative analysis gave the formula RbCrF^. (Found: Cr, 21.5; F, 40.7. Calc. f o r RbCrF^: Cr, 22.4; F, 40.7%.) Magnetic measurement.—The magnetic s u s c e p t i b i l i t y measurement at room temperature (21°C) gave a value of 139 Table 5 Calculated and observed s±r?Q values for KCrF hkl sin%> obs. sin20 calc. intensity obs. 110 .0314 .0310 w. 111 .0518 .0527 w. 210 .0726 .0724 v.w. 220 .1244 .1242 m.w. 212 .1584 .1592 w. 003 • I960 .1953 v.w. 320 .I960 .1966 v.w. 203 .2322 .2367 v.v.w. 303 .2887 .2884 v.v.w. la t t i c e : hexagonal a = 8.739 i c = 5.226 1 140 magnetic moment/^y= 3.17 B.M., corresponding to two unpaired electrons. The magnetic susceptibility measurements were then taken on another sample over the temperature range -194°C—23°C. The values computed are given below in Table 6. Table 6 MagnPt . - i r sugnppt.i h i 1 i t.y m M R D r e m p n t s n f •RhflrTSV V _£ /(Curie-Weiss Temp (K) AM> x 1 0 . /(Curie Law) x Law) (B.M.) (B.M.) 73° 9550 2.36 2.84 118 7245 2.62 2.96 157 5935 2.76 3.01 188 5234 2.80 3.04 197 4841 2.82 2.99 202 4754 2.78 2.99 246 4228 2.88 3.08 265 3987 2.84 3.09 294 3918 2.84 3.14 The plot of ^ f— versus T gave a value of A M 0 = -32° Crystal s^ru^-bure.--The X-ray powder photograph of RbCrF^ was indexed on the basis of an hexagonal lat t i c e with 0 a = 6.985 A c = 12.119 A 141 The calculated and observed sin 9 values are given in Table 7 (page 142). C, CESIUM. PEN^AFLUOR QCHROMXUM, (IJ) Preparation.—Chromium tetrafluoride (0.4597 gm.) was transferred to the s i l i c a reaction vessel in the dry-box and to this, O.7604 gm. of previously dried cesium bromide were added. The s i l i c a reaction vessel was attached to the reaction assembly and bromine trifluoride condensed in i t . The reactants were heated together and the reaction was completed as in the case of KCrF_ (preparation II); 5 d i s t i l l a t i o n of excess bromine trif l u o r i d e was carried out at 160°C. When d i s t i l l a t i o n was complete, the reaction vessel was weighed and taken to the dry-box for analysis and characterization. The weight of the product was 1.073 gm. (calc. for CsCrF : 1.004 gm.). Analysis.—A qualitative test showed the absence of bromine. Quantitative analysis gave the formula CsCrF^. (Found: Cr, 18.00; F, 32.8. Calc. for CsCrF^: Cr, 18.55; F, 33.95%.) Magnetic measurement.—The magnetic susceptibility was measured at room temperature and the results are as follows: Crystal structure.—The X-ray pattern was indexed on a cubic l a t t i c e with a = 8.107 1 . The indices of a l l but one 142 Table 7 Calculated and observed sinffi values for RbCrF hkl s±n 29 obs. s i n2 9 calc. intensity obs. 110 .0485 .0486 m. 111 .0680 .0688 m. 203 .1013 .1007 w. 202 .0806 .0807 w. 105 .1157 .1157 m. 213 .1484 .1491 w. 303 .1833 .1817 w. 313 .2159 .2147 w. 108 .2696 .2709 w. 315 .3136 .3121 v.v.w. 405 .3580 .3587 v.v.w. 502 .4290 .4229 v.v.w. lat t i c e : hexagonal a = 6.985 I c = 12.119 1 143 arc on the pattern were either odd or even as for a face-centred l a t t i c e . The values of the observed and calculated sin Q are given in Table 8, below. Table 8 Calculated and observed sin 2Q values for CsCrF,. hkl s in20 obs. sin 2§ calc. intensity obs. 200 .0366 .0361 m. 220 .0722 .0722 m. 311 .0987 .0993 m. 222 .1079 .IO83 v.w. 320 .1169 .1173 v.w. 420 .1782 .1806 v.w. 422 .2136 .2167 v.v.w. 511) 333) .2406 .2438 v.v.w. la t t i c e : cubic a = 8.107 A D. REACTION OF CHROMIUM TETRAFLUORIDE AND SODIUM FLUORIDE (1:1) IJ BROMINE TRIFLUORIDE, Chromium tetrafluoride and re-crystallized sodium fluoride in a 1:1 molar ratio were heated together in bromine trifluoride in a s i l i c a reaction vessel. Upon d i s t i l l i n g off excess bromine trifluoride, two solid phases were clearly discernible in the reaction vessel. There were no signs of the formation of the complex NaCrF^. 144 E. REACTION OF CHROMIUM TETRAFLUORIDE AND SILVER CARBONATE [111) JJ BRgfflE TRJFLUORIDE ;  Chromium tetrafluoride and well dried silver carbonate in a 1:1 molar ratio were taken into the s i l i c a reaction vessel, which was then attached to a reaction assembly and bromine trifluoride was condensed on the reactants. The reaction was carried out in the usual way and excess bromine trif l u o r i d e was removed at l60°C. A blackish residue was l e f t in the reaction vessel, and was presumably impure. The X-ray powder pattern of the compound was different from those of silver carbonate and silver fluoride, and was complex. No attempt was made to index i t . F. REACTION OF CHROMIUM TETRAFLUORIDE AND BARIUM FLUORIDE m B M W TjamoRipE Chromium tetrafluoride and barium fluoride in a 1:1 molar ratio were taken into the reaction vessel, which was attached to the reaction assembly, and bromine t r i -fluoride was condensed in i t . The reaction was carried out by heating the s i l i c a reaction vessel for fifteen to twenty minutes, after which excess bromine tr i f l u o r i d e was d i s t i l l e d off. Two phases were discernible in the reaction vessel, indicating that no complex formation had occurred. G. PREPARATION AND CRYSTAL STRUCTURE OF POTASSIUM PENTA-FLUQRQMANQANITE (III Potassium pentafluoromanganite (IV) was prepared 145 by the method of Sharpe and Woolf (26). Potassium permanganate and bromine t r i f l u o r i d e were heated together i n a s i l i c a reaction vessel and excess bromine t r i f l u o r i d e was removed at 160°C over a forty-eight hour period. An X-ray picture of the product was taken i n order to compare i t with those of the complexes of chromium of the type A C r F y The X-ray powder pattern was indexed on a hexagonal l a t t i c e with a = 11.445 I c = $.208 1 The calculated and observed s i n 2 Q v a l u e s are given i n Table 9 (page 146). H, PQTASS1UM, Hj^LUQROCrffiOMIUM, (IV) Preparation.—Chromium t e t r a f l u o r i d e (0.6100 gm.) was weighed into the s i l i c a reaction vessel and to t h i s , 0.7022 gm. of potassium chloride, previously dried, were added. The vessel was attached to a reaction assembly of the type used f o r preparation of KCrF^, by means of a B-10 j o i n t . Bromine t r i f l u o r i d e was then condensed on to the reaction mixture and the reaction was completed by heating the reactants f o r ten minutes. Excess bromine t r i f l u o r i d e was removed f i r s t at room temperature and then at 100°C. When the d i s t i l l a t i o n of bromine t r i f l u o r i d e at t h i s temperature was complete, the reaction vessel was 146 Table 9 Calculated and observed sin 2ff values for KMnF hkl sin 2£ obs. s i n2 0 calc. intensity 110 .0184 .1809 m. 111 .0271 .0268 s. 102 .0414 .0408 m. 202 .0591 .0589 V . S . 220 .0719 .0723 m. 221 .0816 .0811 m.w. 302 .0894 .0891 v.w. 222 .1073 .1072 w. 321 .1227 .1233 w. 004 .1408 .1392 w. 500 .1509 .1507 m.w. 333 .1621 .1628 m. 214 .1813 .1814 w. 323 .1904 .1928 v.v.w. 413 .2010 .2049 m. 512 .2202 .2217 m.w. 520 .2352 .2352 v.w. 440 .2840 .2894 v.v.w. 442 .3258 .3242 v.v.w. 540 .3660 .3678 v.v.w. la t t i c e : hexagonal a = 11.445 A" c = 8.255 I 147 sealed off and taken into the dry-box. Samples were taken for chemical, magnetic, and X-ray analyses. Analysis.—Qualitative analysis showed the presence of bromine, indicating that the product was probably a bromine trifl u o r i d e adduct. Chromium and fluorine analyses showed that the compound was K2CrF^»0.5BrF3. (Found: Cr, 16.30; F, 46.93. Calc. for K2CrF6.0.5BrF 3: Cr, 16.62; F, 45.56%.) Magnetic measurement.—Magnetic susceptibility measure-ments at room temperature gave a value of magnetic moment A" 3.30 B.M. Another sample was prepared and magnetic suscepti-b i l i t y measurements at different temperatures were carried out. The results are given in Table 10, below. Table 10 Magnetic susceptibility measurements of K^CrF^/O.5BrFn y ir>-6 M~ /(Curie-Weiss Temp (K), x 1 0 /(Curie Law) y Law) (B.M.) (B.M.) 123° 6699 2.58 3.51 143 6209 2.67 3.52 173 5542 2.78 3.52 199 5045 2.84 3.51 223 4789 2.93 3.55 248 4498 2.99 3.57 294 3954 3.06 3.56 The plot of — i - versus T gave a value of ^M Q = -105° 148 Crystal structure.—The X-ray pattern of the compound was indexed on a tetragonal unit c e l l , with the following l a t t i c e constants: a = 4 . 3 3 9 I c = 5 . 5 0 0 A The values of the calculated and observed sin 2 6? are given Table 1 1 , below. Table 11 Calculated and observed sin 2# values for K o C r F ^ . Q . m - r V * sin 2 t9 sin2<9 hkl obs. cqlp, intensity obs. 100 .0315 .0315 m. 101 . 0 5 0 9 .0511 s. 110 . 0 6 2 9 . 0 6 3 3 m. 111 . 0 8 3 3 . 0 8 2 6 w. 200 .1255 . 1260 w. 112 . 1 4 0 4 . 1414 m. 002 .1760 . 1 7 6 4 m. 202 . 2 0 4 7 . 2 0 4 4 v.w. 212 . 2 3 4 7 . 2 3 5 9 w. 203 . 3 0 0 7 . 3 0 2 4 v.w. 310 . 3 1 5 1 .3150 v.w. 222 . 3 2 9 2 . 3 3 0 4 v.w. Effect of heat on the structure of K o C r F ^ . Q . 5 B r F 3 . — A few samples of the tetragonal compound were sealed in 149 pyrex glass tubes in, vacuo. The glass tubes were heated in an oven at different temperatures for varying lengths of time in order to remove the bromine trifluoride present. Vapours o f bromine—bromine trif l u o r i d e were observed in every tube in the oven. The tubes were cooled slowly after the samples had been heated for the desired length of time, taken into the dry-box, and samples for X-ray examination were prepared. The results are given below. Time X-ray Tfimp. (C) (hrs t) Remarks pattern 120° 240 slight reaction with glass sharp 170 70 reaction with glass not sharp 250 24 reaction with glass not good 360 5 slight reaction with glass good A l l these pictures showed common lines, indicating the presence of the same phase in a l l samples. These lines were due to a phase different from the compound K^CrF^-O^BrF^. The X-ray pictures on measurement gave a set of sin^fp values which were indexed on a cubic l a t t i c e with a = 8.104 A. The calculated and observed values of s i n 2 0 are given in Table 12 (page 150). The photographs also showed very faint lines due to some other phase, probably an impurity. T. CESIUM HEXAFLU0R0CHR0MIUM (IV) Preparation.—Chromium tetrafluoride (0.4918 gm.) and well dried cesium chloride (I.264 gm.) in a 1:2 molar ratio 150 Table 12 Calculated and observed sin B values for K^CrF^ hkl sin 2 0 obs. s i n2 0 calc. intensity 111 .0272 .0270 s. 220 .0724 .0722 m.s. 222 .1083 .1082 s. 400 .1442 .1443 s. 311 .1724 .1714 v.w. 422 .2159 .2164 w. 511) 333) .2431 .2435 m.w. 440 .2877 .2886 m. 531 .3152 .3157 v.w. 620 .3590 .3608 w. 622 .3965 .3968 v.w. 444 .4318 .4329 v.w. 551) 711) .4562 .4600 v.v.w. 642 .5007 .5051 v.v.w. 733 .6097 .6043 v.v.w. la t t i c e : cubic a = 8.104 A 1 5 1 were weighed into the s i l i c a reaction vessel which had been previously baked dry to a constant weight. The reaction vessel was attached to the reaction assembly and evacuated. The break-seal of the trap containing bromine trifluoride was broken magnetically and bromine trifluoride condensed on to the reaction mixture in the s i l i c a vessel. When a l l bromine trifluoride had d i s t i l l e d off, the trap originally containing i t was sealed off. The reaction was performed by opening the reaction assembly to the a i r and heating the reaction vessel with a small flame. When the reaction was complete, excess bromine trifluoride was d i s t i l l e d under vacuum f i r s t at room temperature and then at 1 0 0 °C. When d i s t i l l a t i o n was complete, the reaction vessel was sealed off and taken into the dry-box. Analysis.—Qualitative analysis showed the presence of bromine; hence, the product was probably a bromine trifluoride adduct. Chemical determinations of chromium and fluorine gave the formula Cs^rF^O^BrF-j for the compound. (Found: Cr, 1 0 . 6 5 ; F, 2 8 . 1 0 . Calc. for Cs 2CrF6* 0.5BrF 3: Cr, 1 0 . 4 0 ; F, 2 8 . 5 0 % . ) Magnetic measurement.—The magnetic susceptibility was measured at room temperature and the results are as follows: Crystal structure.—The X-ray pattern was indexed on a cubic l a t t i c e with a = 8 . 9 1 5 6 1 . In Table 1 3 (page 1 5 2 ) the 152 Table 13 Calculated and observed s i n 2 ^ values f o r C s o C r F ^ - O . 5 B r P s i n 2 ^ obs. s i n 2 ^ c a l c . i n t e n s i t y obs. I l l .0234 .0224 v.w. 200 .0310 .0298 v.w. 211 .0486 .0448 m. 220 .0606 .0597 m.s. 310 .0756 .0747 v.w. 222 .0903 .0896 m.s. 400 .1205 .1195 m. 420 .1506 .1494 m. 422 .1793 .1793 m. 440 .2392 .2390 m.w. 600) 442) .2688 .2689 v.w. 620 .2974 .2988 v.v.w. 622 .3270 .3287 v.v.w. 444 .3564 .3585 v.v.w. 711 .3827 .3809 v.v.w. 642 .4154 .4183 v.v.w. l a t t i c e : cubic a = 8.9156 1 153 calculated and observed sin 2t9 values are given. The indices of a l l the reflections observed are either even or odd, indicating that the l a t t i c e i s face-centred. Effect of heat on Cs^rF^O^BrFo.—The complex ( 0 . 6 2 0 4 gm.) was weighed into a small s i l i c a tube joined to a B-10 cone. This was fixed on to an assembly of three traps by means of a B-10 socket. The system was evacuated and the two traps next to the s i l i c a tube were surrounded with a C O 2—alcohol bath. The s i l i c a tube was surrounded by an o i l bath which was then heated to 110°C. The temperature was slowly raised to 135°C and the s i l i c a tube kept at this temperature un t i l bromine trifluoride no longer d i s t i l l e d off (seventy-two hours). The s i l i c a tube was cooled to room temperature and the weight recorded. The tube was then taken into the dry-box and i t s contents were analysed. Qualitative tests showed an absence of bromine. Chromium determination showed that the compound was Cs 2CrF 6. (Found: Cr, 1 2 . 2 . Calc. for Cs 2CrF 6: Cr, 12.04%.) The X-ray powder pattern of the specimen was similar to that for Cs^rF^.O^BrFo. 154 XV. REACTIONS OF CHROMIUM TRIOXIDE AND POTASSIUM DICHROMATE At • REACTIONS OF CHROMUM TRIPHDE 1. Reaction with bromine trifluoride at room temperature The reaction of chromium trioxide with bromine trifluoride was carried out in the usual apparatus, u t i l i z i n g the s i l i c a reaction vessel. A weighed quantity of chromium trioxide, previously dried at 120°C, was taken in the reaction vessel and the vessel attached to the system, which was then evacuated. Chromium trioxide was kept at a temperature of 60—70°C while evacuation was being carried out and the system baked. Bromine trifluoride from a trap was condensed on to; the chromium trioxide in the s i l i c a reaction vessel. When a l l -the bromine trifluoride had been transferred, the trap originally containing bromine t r i -fluoride was sealed off. The system was then disconnected from the vacuum line and opened to the air , care being taken to avoid the entry of moisture. The reactants were allowed to warm to room temperature. A liquid phase and a garnet-red solid phase were observed. There was effer-vescence which decreased with time, and the garnet-red phase dissolved. After twenty to twenty-five minutes, the effervescence had nearly ceased and a deep-red coloured precipitate was observed. Excess bromine trifluoride was removed at f i r s t at room temperature and then at 100°C by d i s t i l l a t i o n i n vacuof and when d i s t i l l a t i o n was complete the reaction vessel was sealed and i t s contents examined. 155 Analysis.—Chemical analysis indicated the presence of bromine, and quantitative determination of chromium and fluorine showed the product to be CrOF^•0.25BrF 3. (Found: Cr, 3 3 . 4 ; F, 44.5. Calc. for CrOFy 0 . 2 5 6 ^ : Cr, 32 .71; F, 44.81%.) Magnetic measurements.—Magnetic susceptibility measurements at room temperature confirmed that the oxidation state of chromium in this compound was +5. The measurements gave the following values, corresponding to one unpaired electron. Crystal structure.—An X-ray diffraction photograph of the powder showed no lines. 2* Reaction with bromine trifluoride at 126°C In a second experiment, a sample of dry chromium trioxide was taken in the s i l i c a reaction vessel and bromine trif l u o r i d e was condensed on to i t . The reaction was carried out at atmospheric pressure. When the reaction was complete, as indicated by the cessation of effervescence, excess bromine trifluoride was d i s t i l l e d off in. vacuo. at f i r s t at room temperature and later at 120°C. The reaction vessel was surrounded by an o i l bath at 120°C and l e f t overnight to obtain a product free of bromine tri f l u o r i d e . After a period of twenty-four hours, the reaction vessel was sealed off and taken into the dry-box. = 1715 x 10 ,-6 c.g.s. units; 156 The product of the reaction was a light-green substance, resembling a chromic compound. It was examined by chemical analysis, and magnetic measurements and an X-ray photograph were taken. Analysis.—Quantitative analysis showed that the compound was CrF^•BrF^. (Found: Cr, 2 1 . 6 ; F, 4 6 . 5 , 4 8 . 5 . Calc. for CrF 3«BrF 3: Cr, 2 1 . 1 4 ; F, 47 .17%. ) Magnetic measurement.-—Magnetic susceptibility measure-ments were carried out at room temperature and the following values obtained: yCM = 5682 x l C f 6 c.g.s. u n i t s ; / / 6 2 9 4 = 3 , 6 7 B- M-The value of the magnetic moment indicates that chromium is present in the + 3 oxidation state. Crystal structure.—An X-ray diffraction photograph of the product showed only one broad diffraction line. 1* Reaction with bromine pentafluoride Dry chromium trioxide was transferred to the s i l i c a reaction vessel which was then attached to the usual type of reaction assembly by means of a B-10 joint. The reaction assembly, to which a trap containing bromine pentafluoride had already been attached, was being evacuated, during which time i t was also heated. Meanwhile the sealed trap of bromine pentafluoride was kept surrounded by a dewar con-taining liquid nitrogen. When the apparatus was dry, the break-seal of the bromine pentafluoride trap was broken magnetically, the dewar surrounding the trap was removed, 157 and bromine pentafluoride condensed into the reaction vessel, which was cooled i n l i q u i d nitrogen. A f t e r d i s t i l l a t i o n of bromine pentafluoride was complete, the reaction assembly-was disconnected from the vacuum l i n e and dry a i r was l e t i n slowly. The reaction was carried out by l e t t i n g the reaction vessel warm to room temperature at atmospheric pressure. Although the reaction occurred at room temper-ature, the vessel was heated s l i g h t l y (to 40.5°C) to ensure completion of the reaction. A f t e r f i f t e e n minutes, the reaction assembly was connected to the vacuum l i n e and excess bromine pentafluoride was removed by d i s t i l l a t i o n i n vacuo. After ten hours of d i s t i l l a t i o n at 80°C, a reddish powdery mass was l e f t i n the s i l i c a reaction vessel. The reaction vessel was then sealed o f f from the system and taken into the dry-box, where on opening the reaction vessel, i t was observed that the product of t h i s reaction was more reactive than was the product obtained from the reaction of chromium t r i o x i d e with bromine t r i -f l u o r i d e ; i t fumed i n the dry-box giving yellow vapours of chromyl f l u o r i d e , and reacted much more ra p i d l y with glass. A sample of the product was taken i n an X-ray c a p i l l a r y and an X-ray picture was taken. A n a l y s i s . — A sample (0.2909 gm.) was taken i n a small weighed tube f o r chemical analysis. The tube was dropped into an alkaline (sodium hydroxide) solution contained i n a 250 ml. conical f l a s k with a ground glass stopper. There 158 was vigorous reaction, with the evolution of yellow fumes, and the flask was agitated u n t i l these fumes dissolved. Qualitative tests indicated the presence of bromine, and quantitative analysis showed that the compound was CrOF 3 » 0 . 2 5 B r F 5 . (Found: Cr, 3 0 . 2 ; F, 4 7 . 7 , 4 8 . 5 ; Br, 1 3 . 3 . Calc. for CrOF 3 . 0 . 2 5 B r F 5 : Cr, 3 0 . 6 ; F, 4 7 - 9 ; Br, 1 1 . 9 % . ) Magnetic measurement.1—The magnetic susceptibility measurement was carried out at room temperature (21°C) and the following values obtained: X n = 1443 x 1 0 " 6 c.g.s. units; = 1.^5 B.M. Crystal structure.—An X-ray photograph was taken but showed no diffraction lines; the sample decomposed during exposure. 4. Reactions with chlorine trifluoride (i) liquid phase reaction Chlorine trifluoride was condensed in a s i l i c a reaction vessel containing a sample (0 . 4 3 5 8 gm.) of well dried chromium trioxide. The reaction, carried out at room temperature, occurred smoothly; during i t the chlorine trifluoride was boiling. When the reaction was complete, as indicated by the cessation of effervescence, unreacted chlorine trifluoride was d i s t i l l e d i n vacuo at room temperature. During the last stages of d i s t i l l a t i o n , the s i l i c a reaction vessel was heated to 50°C, and was sealed off when d i s t i l l a t i o n was complete. A light, orange-red powder was observed in the s i l i c a vessel, which was taken 1 5 9 into the dry-box and opened; this product also fumed in the dry-box. Analysis.—Qualitative analysis showed the presence of chlorine, indicating that the product was probably a chlorine trifluoride adduct. Quantitative determinations of chromium and fluorine showed the product to be CrOF 3 . 0.25Cll? y (Found: Cr, 3 4 . 2 0 ; F, 4 8 . 0 , 4 7 . 5 . Calc. for CrOF 3 «0.25ClF 3 : Cr, 3 5 . 1 ; F, 48.1$.) Magnetic measurement.—The magnetic susceptibility was measured at room temperature and the following values obtained: % M = 1 4 1 8 x 1 0 " 6 c.g.s. units; J^g^ = 1 » 8 3 B.M. Crystal structure.—No X-ray diffraction lines were observed in the photograph; the sample decomposed during exposure. ( i i ) vapour phase reaction Since the previous reaction did not permit the use of high temperatures—the boiling point of chlorine trifluoride i s 12.1°C—it was decided to carry out a similar reaction, this time by passing the vapours of chlorine trifluoride over heated chromium trioxide. This, reaction was performed in a s i l i c a reaction tube. A s i l i c a reaction tube having a B-14 socket at one end and a pyrex-silica graded seal at the other, was connected to a train of traps by means of the graded seal. 1 6 0 Each trap was provided with a break-seal. The last trap was connected to a vacuum line with a stop-cock in the li n e . On the other side, the s i l i c a tube was attached to a trap by means of a B-14 joint, which in turn was connected to a cylinder of chlorine trifluoride by means of Teflon tubing. The s i l i c a reaction tube was surrounded by a mantle heater. A small nickel boat containing chromium trioxide was placed inside the s i l i c a reaction tube and the assembly was evacuated. During evacuation, carried out for twelve hours, the s i l i c a tube was heated to 120°C. The assembly was occasionally heated by a Bunsen flame in order to bake i t dry. After the apparatus was dry, the trap following the chlorine tri f l u o r i d e cylinder was cooled with liquid nitrogen, and a suitable amount of chlorine trifluoride condensed into i t from the cylinder. While this trap was cooled, vacuum was applied to remove the more volatile impurities. The stop-cock between the traps and the vacuum line was closed, and the traps following the s i l i c a reaction vessel were cooled, while the dewar of liquid nitrogen surrounding the trap containing chlorine t r i -fluoride was removed. Chlorine trifluoride was d i s t i l l e d through the s i l i c a tube and condensed in the cooled trap following i t . During i t s passage through the tube, chlorine trifluoride reacted with the hot chromium trioxide. A dull-red solid, the reaction product, condensed in the cold 161 trap. During this reaction, chlorine trifluoride was passed back and forth unti l a l l the chromium trioxide had disappeared, and then the train of traps was sealed off from the s i l i c a tube. Vacuum was applied and a more volatile gas was observed to escape, indicating the liber-ation of oxygen during the reaction. The chlorine trifluoride which had condensed i n the trap along with the reaction product was separated from i t by letting the trap warm to room temperature and keeping the other traps cooled in liquid nitrogen. This caused condensation of chlorine trifluoride in the cold traps, leaving behind the dull-red solid in the f i r s t trap. When no more chlorine trif l u o r i d e was d i s t i l l i n g into the cold traps, the trap containing the product of reaction was sealed off and transferred into the dry-box. Analysis.—Qualitative analysis indicated the presence of chlorine, and chromium and fluorine determinations gave the composition CrOFyO^ClF^. (Found: Cr, 34.24, 34.5; F, 49.5, 50.0. Calc. for CrOF3«0.25ClF3: Cr, 35.1; F, 48.1%.) Magnetic measurement.—Magnetic susceptibility was measured at room temperature and the following values obtained: X M = 1308 x 10~ 6 c.g.s. units; LL = 1.76 B.M. 162 Crystal structure.—An X-ray picture of the sample was taken, but the capillary exploded during exposure. Effect of heat on the vapour phase reaction product.— The dull-red solid obtained from the vapour phase reaction was taken in a s i l i c a tube and attached to a train of traps by means of a B-10 joint. The system was connected to the vacuum line by means of a glass stop-cock. The two traps following the s i l i c a tube were cooled in liquid nitrogen, and the s i l i c a tube containing the dull-red product was immersed in an o i l bath. The stop-cock was closed and the o i l bath heated gradually. At a temperature of 75°C, the compound in the s i l i c a tube began to expand, and when the s i l i c a tube was maintained at this temperature for a few minutes, a considerable increase in volume occurred; the f i n a l product was identical in appearance to that obtained in the liquid phase reaction. However, the composition was unchanged, as shown by chromium analysis. R. REACTIONS OF POTASSIUM DICHROMATE 1. Reaction with bromine trifluoride This reaction was carried out in a s i l i c a reaction vessel attached to the usual reaction assembly. A sample of previously dried potassium dichromate was taken into the s i l i c a reaction vessel and bromine trifluoride condensed on i t from a trap, which was sealed off after the bromine trifluoride had d i s t i l l e d off. The apparatus was opened to the air with due precaution, and a reaction occurred at 163 room temperature as indicated by the effervescence at the surface. When the reaction was complete, excess bromine trifluoride was d i s t i l l e d off .in vacuo. A temperature of 100°C was used to remove bromine tri f l u o r i d e . After d i s t i l l a t i o n was complete, the s i l i c a vessel was sealed off and a sample of the product was taken out in the dry-box and analysed. Analysis.—Qualitative tests indicated the presence of bromine, and chromium analysis gave the formula KCrOF4*0.5BrF 3 . (Found: Cr, 1 9 . 8 8 . Calc. for K C r 0 F 4 . 0 . 5 B r F 3 : Cr, 20 .66%.) Magnetic measurement.—The magnetic susceptibility, measured at room temperature, confirmed that chromium was in the +5 oxidation state. Susceptibility measurements were then carried out over a temperature range of 85—294°K and the results are given in Table 14» below. Table 14 Magnetic susceptibility measurements of KCrQF^.Q.5BrF3 Temp (K) % M » * IO" 6 /(Curie Law) (B.M.) 85° 3617 1.58 116 2776 1.61 191 1773 1.65 217 1575 1 .66 239 1426 1 .66 294 1265 1.73 A plot of - i — versus T gave a value of 0 = + 4 ° . A M 164 Crystal structure.—A sample of the compound as prepared was examined by X-rays. The powder pattern was sharp and was indexed on the basis of an orthorhombic la t t i c e , with a = 13 .24 1 b = 10.31 I c = 8.317 1 Another sample of the complex KCrOF 4 »0.5BrF 3 was taken in a s i l i c a tube and heated in, vacuo to a temperature of 160°C. This removed the bromine trifluoride and pure KCrOF^ was obtained, as indicated by a negative response to a test for bromide ions and by chromium analysis. An X-ray picture of this sample was taken and was found to be identical to the one obtained from the bromine t r i -fluoride adduct. 2. Reaction with bromine pentafluoride . Dry potassium dichromate (I .6764 gm.) was weighed into the s i l i c a reaction vessel which was then joined to a reaction assembly by means of a B-10 joint. The reaction assembly had a sealed trap containing bromine pentafluoride cooled in liquid nitrogen. The apparatus was evacuated and dried, and then the break-seal of the bromine pentafluoride trap was broken and bromine pentafluoride was d i s t i l l e d into the s i l i c a vessel which was cooled in liquid nitrogen. The reaction assembly was opened to the air and the reaction 165 vessel allowed to warm to room temperature when the reaction occurred. When the reaction had taken place, the assembly-was connected to the vacuum line and excess bromine penta-fluoride was d i s t i l l e d off from the reaction vessel into a trap cooled in liquid nitrogen. The reaction vessel was heated to a temperature of 100°C to drive off a l l bromine pentafluoride, and when d i s t i l l a t i o n at this temperature was complete, the reaction vessel was sealed off and i t s contents were examined. Analysis.—Qualitative analysis showed the presence of bromine, indicating the compound was an adduct, and quantitative analysis gave the formula KCrOF 4.0 .5BrF^. (Found: Cr, 19 .00 ; F, 4 4 . 5 . Calc. for KCrOF.«0.5BrFc: 4 > Cr, 19 .20 ; F, 45.64%.) Magnetic measurement.—Magnetic susceptibility measurements were carried out at room temperature with the following results: X M = 1237 x 1 0 " ° c.g.s. units; y^o^ = 1-75 B.M. C r y s t a l stru^turs.—A sample of the compound as prepared was taken into a capillary tube and an X-ray picture was taken. The powder pattern was different from the one obtained for KCrOF^'O^BrF-j. It was, however, complex, and no attempt was made to index i t . Another sample of the compound KCr0F4«0.5BrF<j was taken in a small s i l i c a tube with a B-10 cone, by means of which i t was attached to a train of traps. The traps were 1 6 6 cooled by liquid nitrogen and connected to the vacuum li n e . The s i l i c a tube was then sealed off and taken into the dry-box where i t was opened and i t s contents examined with X-rays. The powder pattern of this substance was identical with that of KCrOF^, indicating the decomposition of the solvate KCrOF4*0.5BrF5 into KCrOF^. 3 . Reaction with chlorine tri f l u o r i d e The reaction of potassium dichromate with chlorine trifluoride occurred smoothly and the product obtained gave a diffraction pattern characteristic of KCrQF^. C. REACTION OF CALCIUM CHROMATE WITH BROMINE PENTAFLUORIDE A sample of calcium chromate which had been thoroughly dried was taken into a dry s i l i c a reaction tube, which was then joined to a reaction assembly by means of a B-10 joint. Bromine pentafluoride from a trap was condensed in the s i l i c a reaction vessel containing calcium chromate. The reaction was carried out at room temperature and at atmospheric pressure. Bromine pentafluoride reacted with i t smoothly with an evolution of gases. When the effervescence ceased, indicating the completion of the reaction, the excess of pentafluoride was d i s t i l l e d off in vacuo. The product was heated to 160°C by means of an o i l bath in order to drive off a l l bromine pentafluoride. When bromine pentafluoride no longer d i s t i l l e d off, the s i l i c a vessel was sealed off and taken to the dry-box. 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