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Studies in 57Fe and 121Sb Mossbauer spectorscopy Scott, James Charles Stewart 1973

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STUDIES IN F e AND 5 7  1 2 1  S b MOSSBAUER SPECTROSCOPY  by JAMES CHARLES STEWART SCOTT B.Sc.(Hons.), University of B r i t i s h Columbia, 1965  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of Chemistry  We accept t h i s thesis as conforming to the required standard  THE UNIVERSITY OF BRITISH COLUMBIA JULY, 1973  In p r e s e n t i n g  t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the requirements f o r  an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e  and  study.  I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e copying of t h i s t h e s i s f o r s c h o l a r l y purposes may by h i s r e p r e s e n t a t i v e s .  be granted by  the Head of my  I t i s understood t h a t copying or p u b l i c a t i o n  of t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be written  Department or  permission.  Department The U n i v e r s i t y o f B r i t i s h Columbia Vancouver 8, Canada  allowed without  my  i  ABSTRACT  A number of iron carbonyl complexes of the general formula LFe (CO)g (L = f luoroalicyclic-bridged di (tertiary arsine or phosphine)) 2  have been investigated by Mossbauer spectroscopy and the results are consistent with the known structure of f^farsFe2(CO)g. of derivatives of the general formulae L, LFe ( C O ) ( L  m  L LFe (CO)  b  m  = monodentate ligand) ,  2  (L° = bidentate ligand, chelating), and L LFe (CO)^ (L =  C  2  As well, a number  4  b  2  bidentate ligand, bridging) have been examined.  The Mossbauer parameters  are consistent with substitution trans to the iron-iron bond in L LFe (CO) ,j. m  2  Mossbauer and infra-red data show complexes of the type  L LFe (CO), have structures similar to f.AsP f,AsPFe„(CO).. Mossbauer 2 4 4 4 2 4 spectroscopy shows that in complexes of the type ^ U ^ C C O ) ^ the ligands bridge the two iron atoms and are coordinated cis to the iron-iron bond. The usefulness of the magnetic perturbation technique for removal of ambiguities in the assignment of Mossbauer spectral parameters i n low-spin iron compounds having two iron sites has been demonstrated. 121 Sb Mossbauer spectra were obtained for the following compounds: (C H ) SbX (X = C l , OH, NCS), 6  5  4  (C H ) SbCl 6  5  2  3  (C H ) SbX  and (CgH^SbtOOH.  5  5  3  2  (X = OCOCH , NCS, N0 , ^(OCrO^), 3  3  The data for the (CgH^SbX and  (CgH^) SbX complexes are consistent with trigonal bipyramidal structures 3  2  for these compounds (except for the acetate) with the X groups in the axial positions.  The additive model for quadrupole splittings has been success-  fully applied to these and some related compounds.  The linear relationship  ii  2 between the isomer shift and e qQ for many of the compounds of the type (CgH,.) ,jSbX2 suggests a-bonding plays a dominant role i n determining their Mossbauer parameters.  Possible structures are examined for both  (C,H ).SbCl_ and (C,H )„Sb(0)0H. c  C  For (C.H )„Sb(OCOCH„)„, octahedral C  structures with one ester-like and one bidentate acetate group are compatible with the observed parameters.  + A number of cations of the general formula X Sb (Fe (CO)Tr-C H ) . ° n 2 5 5 4-n 0  (X = C l , Br, I, CF^, CgH , n - C ^ ; 5  n  c  c  = 1, 2, 3, but not a l l combinations)  have been studied by ^ Fe and ^"''Sb Mossbauer spectroscopy. 7  These  compounds are nominally isoelectronic with the extensively studied neutral t i n derivatives X Sn(Fe(CO)„(1T-C.-H,.)) . and a number of correlations n 2 5 5 4-n between their respective spectral parameters have been investigated.  The  121 Sb isomer shifts were found to overlap with the ranges of isomer shifts characteristic of Sb(III) and Sb(V), hence the assignment of a formal oxidation state to antimony i n these compounds has l i t t l e justification. + 121 For the complexes R SbFe(CO) ( T T - C ^ ) (R = » n ^) 2 quadrupole coupling constants e qQ are positive while the coupling constant in the corresponding t i n derivative (n-C^Hg) ^SnFe (CO) (FT-C^-H,.) has been C  3  2  H  6  - 0  t  h  e  s b  5  2  found to be negative.  Isomer shift data for  57 119 121 Fe, Sn and Sb as well  as the carbonyl stretching frequencies for the Fe(CO) (iT-C^H^) group indicate 2  Fe-Sb ir-bonding i s more important than Fe-Sn TT-bonding, although a-bonding effects are the dominant factor i n determining the Mossbauer spectral parameters.  iii  TABLE OF CONTENTS Page ABSTRACT  i  ACKNOWLEDGEMENTS  x  INTRODUCTION  1  THE MOSSBAUER E F F E C T  2  EXPERIMENTAL  31  RESULTS AND D I S C U S S I O N PART 1  LFe (CO) 2  COMPLEXES AND T H E I R D E R I V A T I V E S  6  39  (A)  LFe (CO)  (B)  L LFe (CO)  5  (C)  L LFe (CO)  4  65  (D)  L LFe (CO)  4  76  (E)  THE I R O N - O L E F I N BOND I N L F e „ ( C O ) , AND Z o THEIR DERIVATIVES  2  39  6  m  2  .  .  .  .  .  .  C  2  b  2  PART 2  Ph  PART 3  R Sb(Fe(C0) Cp). X COMPOUNDS n 2 4-n 121  5-n c  SbX  n  DERIVATIVES  .  .  .  (A)  (C)  Sb MOSSBAUER PARAMETERS 5 7  F e MOSSBAUER PARAMETERS  THE CORRELATION OF  5 7  90  .  95  .  o  (B)  57  .  121  .  .  .  123  .  .  .  139  F e MOSSBAUER P A R A -  METERS WITH THE CARBONYL STRETCHING FREQUENCIES I N THE I . R  145  BIBLIOGRAPHY  153  APPENDIX I  163  APPENDIX II  173  iv  LIST OF TABLES Table I  Page(s) Mossbauer Parameters at,80°K f o r L F e ( C 0 ) 2  Compounds II  .  42  Possible Assignments of the Mossbauer Parameters for f f a r s F e ( C 0 ) 6  III  &  2  44  6  Mossbauer Parameters at 80°K f o r L L F e ( C 0 > m  2  5  Compounds IV  59  Mossbauer Parameters at 80°K f o r L ° L F e ( C 0 ) 2  Compounds V  4  •  68-69  Mossbauer Parameters at 80°K f o r L L F e ( C O ) b  2  4  Compounds VI  77-78  Magnetic Perturbation Results  .  .  .  .  VII  1 2 1  S b Mossbauer Parameters at 9°K (This Work)  VIII  1 2 1  S b Mossbauer Spectra at 4.2°K (Previous Studies)  IX  97 98  Application of the Additive Model to Predict the 2 e qQ Values f o r R,. SbX^ Compounds n  X  .  93  NQR Data - At Room Temperature  .  .  .  .  .  . .  102 103  121 XI  Sb Mossbauer Parameters f o r Compounds of the Type R Sb(Fe(C0)„Cp) . X n 2 4-n  124-125  J r  XII  Isomer S h i f t s of Nominally Isoelectronic Antimony and T i n Complexes  136  V  LIST OF TABLES (Continued) Table XIII  Page(s) ~*^Fe Mossbauer Parameters for.Compounds of the Type R Sb(Fe(CO) Cp) _ X n  XIV  2  4  n  .  .  .  .  .  .  140  ~^Fe Mossbauer Parameters for Fe(C0) Cp Groups 2  Bonded to Tin and Antimony XV  "*^Fe Mossbauer and  141  Parameters for Some  X MFe(C0) Cp Derivatives 3  XVI  .  2  "^Fe Mossbauer Parameters and Some X M(Fe(C0) Cp) Derivatives 2  2  2  146  Parameters of .  .  .  .  147  vi  LIST OF FIGURES Figure  1  Page  Schematic of a T y p i c a l Mossbauer Spectrometer Using Transmission Geometry and Absorber Cooling  2  .  .  .  5  An Approximate Energy Level Diagram f o r a "*^Fe Nucleus Subjected to an e.f.g. (n = 0) and Then to an Applied Magnetic F i e l d at an Angle 8 to the P r i n c i p a l Component of the e.f.g.  12  121 3  Energy diagram f o r an  Sb Nucleus Subjected to  Non-Zero A x i a l l y Symmetric e.f.g. 4  14  Some Regular Structures and the Point Charge Expressions  f o r the Components of Their EFG Tensors.  Where the P r i n c i p a l Axes are Determined by Symmetry, the Components are Designated by Upper Case Subscripts. Otherwise, the P r i n c i p a l Axes can Only be Found by Diagonalization of the Tensor f o r Each Case Under Consideration 5  22  Schematic Diagram of the Apparatus Employed f o r 121 Obtaining  Sb Mossbauer Spectra  33  6  Schematic of Magnetic Perturbation Apparatus  7  The Structures of Some T y p i c a l Ligands Used i n this Work  40  The Structure of f.farsFe„(CO),  41  8  .  .  36  vii LIST OF FIGURES (Continued) Figure 9  Page "* Fe Mossbauer Spectra of f a r s F e ^ ( C O ) ^ and 7  (PhO) Pf AsPFe (CO) 3  10  4  2  .  5  .  .  .  .  T y p i c a l Spectra Produced i f Assignment  .  .  45  (a) i s  Correct with Site I = 0.34 mm/sec, n = 0 and S i t e I I = 0.30 mm/sec, n = 0 11  .  T y p i c a l Spectra Produced i f Assignment  .  .  .  49  .  50  .  51  (b) i s  Correct with S i t e I = 1.00 mm/sec, n <= 0 and Site II = 1.06 mm/sec, n = 0 12  .  .  .  .  "* Fe Mossbauer Spectrum of f ,f arsFe_ (CO) , i n an o z o Applied Longitudinal Magnetic F i e l d of 50kG 7  Showing Experimental Points and T h e o r e t i c a l F i t 13  ^ F e Mossbauer Spectrum of f^AsPFe (CO)^ i n an 7  2  Applied Longitudinal Magnetic F i e l d of 50kG Showing Experimental Points and T h e o r e t i c a l F i t  .  .  .  52  57 14  Fe Mossbauer Spectrum of f o s F e ( C 0 ) ^ 2  i n an  Applied Longitudinal Magnetic F i e l d of 50kG Showing Experimental Points and T h e o r e t i c a l F i t 15  5 7  .  .  .  53  F e Mossbauer Spectrum of (PhO) Pf AsPFe (C0) i n 3  4  2  5  an Applied Longitudinal Magnetic F i e l d of 50kG Showing  16  Experimental Points and T h e o r e t i c a l F i t  .  .  .  58  The Structure of f.AsP f.AsPFe_(CO). 4 4 2 4  .  .  .  66  C  viii  L I S T OF FIGURES  (Continued)  Figure 17  Page " ^ F e Mossbauer S p e c t r a of f AsP°f farsFe (CO) 4  4  2  f^fos^f^fosFe,,(CO)^  67  4  57 18  and  b Fe Mossbauer Spectrum o f  Applied Longitudinal Experimental Points  f^fos  f o s F e ^ ( C O ) ^ in  Magnetic F i e l d of and T h e o r e t i c a l  an  50kG S h o w i n g  Fit  .  .  .  83  121 19  The  Sb M o s s b a u e r S p e c t r u m o f P h S b C l 121 2  w i t h n = 0 and the  Sb M o s s b a u e r S p e c t r u m o f  T y p i c a l Compound o f Ph Sb(N0 ) 3  3  t h e Type R^SbX,^  Correlation of  Fit  a  Namely 99  2  121 20  Showing  3  of  Derivatives  the of  2 Sb I . S .  a n d e qQ f o r  t h e Type P h S b X 3  .  2  a Number .  .  .  104  121 21  Sb M o s s b a u e r S p e c t r u m o f P h S b ( 0 A C ) 3  Improvement o f  n = 22  1 2 1  0.0  Fit  for  n. = 0 . 4 6  (b)  Showing  2  Over That  (a)  115  S b Mossbauer Spectrum of C l S b ( C p ( C O ) F e ) 2  Illustrating That  for  Improvement o f  n = 0.0  + P F  2  Fit  for  2  f| = 0 . 4 6  6  (b)  Over 126  + Sb M o s s b a u e r S p e c t r u m f o r  —  CISb(Cp(CO) Fe) PF . + — Sb M o s s b a u e r S p e c t r u m o f P h S b F e ( C O ) C p P F Showing A l t e r n a t e F i t s to the D a t a . I n (a) t h e Fitting 2 P a r a m e t e r s w e r e 5 = - 6 . 7 m m / s e c , e qQ = + 9 . 4 m m / s e c , 2  3  f i  121  24  ~  (a)  121 23  for  3  2  6  127  ix  LIST OF FIGURES (Continued) Figure  Page r = 2.9 mm/sec, n = 0.0. In (b) , 6 = - 6.5 mm/sec, 2 e qQ = - 2.9 mm/sec, T = 4.0 mm/sec, n = 0.0. The 2 Fit with e qQ>0 i s Clearly Preferable  25  128  The Correlation of ^^Sn and "^^Sb Isomer Shifts. The Straight Line, Based on the Assumption of Equivalent Electron Density at the Two Nuclei, i s after Ruby. The Points for the Isoelectronic Pairs are Labelled Using the Notation M = Sn or Sb and Fe = +  Fe(C0) Cp 2  26  .  .  .  .  .  137  57 + — Fe Mossbauer Spectrum of PhSb(Cp(CO)_Fe). PF, i n A  J  O  an Applied Longitudinal Magnetic Field of 30kG. The 2 Sign of e qQ i s Clearly Positive  144  X  ACKNOWLEDGEMENTS  I wish to express my thanks to Dr. J.R. Sams for his patience and guidance of this work.  I am particularly grateful for his invaluable  assistance in the past few months. I should like to extend my sincere appreciation to Dr. J.N.R. Ruddick for his advice and encouragement during the early course of the antimony work and for his synthesis of the five-coordinate antimony derivatives. As well, our discussions of some of the experimental problems were particularly stimulating. To Mr. L.S. Chia, a particular vote of thanks for his many hours of hard labour i n the separation and purification of the iron carbonyl complexes and for free access to his unpublished experimental data. I am also indebted to Dr. D.J. Patmore for the contribution of his synthetic s k i l l s to the preparation of the antimony-iron compounds. Finally, I appreciate the contributions of the rest of the Mossbauer group, including Lia Sallos, Troy Lassau and Tsin Bik.  INTRODUCTION  One of the most important problems confronting the chemist i s the e l u c i d a t i o n of the structure and bonding i n new compounds.  or unusual  In order to solve this problem there i s recourse to many  physical methods, but i n p a r t i c u l a r the various forms of spectroscopy play a dominant r o l e .  The applications of nuclear magnetic  resonance,  infrared and e l e c t r o n i c absorption spectroscopies to the elucidation of structure and bonding are w e l l known''".  Among the spectroscopic tech-  niques which have been employed i s Mossbauer spectroscopy or nuclear 2 gamma resonance spectroscopy , with which t h i s thesis i s concerned. Spectroscopic techniques rely on the i n t e r a c t i o n between atoms or molecules and electromagnetic r a d i a t i o n . form of the emission  This usually takes the  or absorption of one or more quanta of radiation  (in Raman spectroscopy,  this process i s somewhat more complex since the  energy corresponding to the t r a n s i t i o n i s added to or subtracted from a 3 scattered photon ).  4 Owing to c e r t a i n r e s t r i c t i o n s , the energies (and  thus the frequencies) of these quanta are normally confined to a c e r t a i n d e f i n i t e range of values.  Thus, a l l forms of spectroscopy may  be  characterized by the p a r t i c u l a r type of t r a n s i t i o n s which may be studied (e.g. r o t a t i o n a l , v i b r a t i o n a l , e l e c t r o n i c ) and by the range of the e l e c t r o magnetic spectrum  to which they are confined.  For t h i s reason the i n s i g h t s  into chemical behaviour which can be gained by any given form of spectroscopy are somewhat limited by the p a r t i c u l a r spectroscopic "window" which i s employed.  For example, one could not expect to use exactly the same spectro-  2  scopic tools to study both Raman and infrared t r a n s i t i o n s .  Although  the type of information to be gained i s very s i m i l a r , the techniques are very d i f f e r e n t ^ .  Mossbauer spectroscopy, For example, only n u c l e i  too, has i t s c h a r a c t e r i s t i c l i m i t a t i o n s .  i n s o l i d materials or i n very viscous  liquids  may be studied^', the number of isotopes to which the technique i s applicable i s l i m i t e d since the e f f e c t has never been observed i n isotopes l i g h t e r than ^K.  As w e l l , there are p h y s i c a l l i m i t a t i o n s such as very  short h a l f - l i v e s of radioactive precursors, or extremely broad or extremely narrow linewidths.  A l l these factors mean that there are only  about a dozen isotopes which are currently i n use f o r routine  chemical  applications and n a t u r a l l y an i n d i v i d u a l spectroscopist i s somewhat limited i n h i s access to those isotopes which  are a v a i l a b l e .  Nevertheless, f o r those isotopes to which the e f f e c t i s applicable, Mossbauer spectroscopy  provides some valuable information  about such diverse phenomena as the chemical e f f e c t s of nuclear transformations, magnetic phase t r a n s i t i o n s , electron d e n s i t i e s , and molecular symmetry^.  So although a Mossbauer spectroscopist's "window" on chemical  behaviour i s a very small one, f o r those elenents to which i t can be applied, some unique s t r u c t u r a l and bonding information may be derived, p a r t i c u l a r l y from measurements of the isoner s h i f t and quadrupole s p l i t t i n g .  The purpose of this thesis i s to i l l u s t r a t e how Mossbauer spectroscopy  can be employed to study structure and bonding.. In p a r t i c u l a r  the techniques and theories which have been so successfully applied i n  3  elucidating the structures of iron and t i n compounds  have been  121 applied to the interpretation of the  Sb Mossbauer spectra i n two  series of antimony compounds. The f i r s t series of compounds of the type Ph^_ SbX n  n  (Ph =  phenyl, X = electronegative group, NCS, NO.j> etc., n = 1, 2, 3) i s an  g extension of the earlier work of Long e_t a l . and represents the f i r s t explicit^ application of partial quadrupole splitting (p.q.s.) theory to the interpretation of the electric f i e l d gradients  (e.f.g.) i n these  antimony compounds. In the second set of compounds of the type R _ Sb{Cp(CO) Fe} 4  X  n  2  (R = Cl, Br, Ph, etc., Cp = 7T-cyclopentadienyl, X = large anion,  PFg , etc., n = 1, 2, 3), the theory of p.q.s. has also been applied. 57 As well, the  Fe resonance has also been studied i n order to elucidate  the nature of the antimony-iron bond. The nature of the iron-iron bond as well as the structures of a number of substituted iron carbonyls bridged by di(tertiary arsines and phosphines) has also been investigated.  Magnetic perturbation  techniques have been used as a check on the assignment of the Mossbauer parameters for the two iron sites i n these compounds as well as to investigate the signs of the e.f.g. at both sites.  4  THE MOSSBAUER EFFECT  The Mossbauer effect depends on the recoil-free emission and absorption of y-rays by suitable nuclei.  Although there are many isotopes  for which the effect has been demonstrated, most of the experimental results to date have dealt with either ^ Fe or ^"^Sn. 7  This i s not  surprising in view of the ease of working with these isotopes. In a conventional Mossbauer experiment in transmission geometry, the y-rays from an appropriate single-line source are Doppler modulated, and, after passing through a suitable absorber, are detected and counted.  The range of source velocities i s normally such that the  Y-rays from the source go in and out of resonance with the absorber. The spectra consist of a plot of transmitted counts versus Doppler velocity  9  (Figure 1). In most cases, except for certain compounds of iron and t i n , i t  is necessary to cool both the source and absorber at least to liquid nitrogen temperature.  This i s because f , the resonance fraction or the probability Si  of recoil-free absorption of a Y-ray from the source, i s of the form^ 2  (1)  f  2  -E <X > = exp(-^ 5-) (*cr  a  2  where E^ is the Y-ray energy, <X > the mean square vibrational amplitude of the nucleus in the direction of the y-ray,  sometimes referred to as the  Debye-Waller factor, -fi is Planck's constant over 2TT, and c the velocity of light in vacuo.  A similar equation holds for f , the probability of recoil-  FIGURE 1.  Schematic of a T y p i c a l Mossbauer Spectrometer Using Transmission Geometry and Absorber Cooling.  c  TYPICAL SPECTRUM  ©  0 u  N T  S  ~\A/~  OSCILLOSCOPE  DATA OUTPUTS:  VELOCITY  PAPER TAPE  DISPLAY  L_ 400 WORD  HIGH VOLTAGE  MEMORY  POWER SUPPLY  ANALYZER  PULSE HEIGHT ANALYZER  AMPLIFIER  PICKUP SIGNAL »  *  -<  MOSSBAUER  r  UNIT  y-SOURCE DETECTOR  DEWAR — »  1  DRIVE  TIME BASE GENERATOR  TRANSDUCER '-DRIVE SIGNAL H  OPTICAL BENCH  SYNCHRONIZATION  SIGNAL  M  O  %  6  free emission from the source. Now, as <X > depends on the firmness Z  with which the Mossbauer nucleus i s bound to i t s lattice site (this may be an anisotropic function), and on the temperature, i t i s apparent that cooling the absorber to as low a temperature as i s feasible w i l l increase the resonance fraction.  Similar considerations apply to the source,  although i n general i t i s possible to choose a suitable source matrix so that the nuclei  are very strongly bound and the temperature effect  i s correspondingly diminished. To understand how the Mossbauer effect can be used as a probe to investigate structure and bonding, i t i s necessary to understand how a nucleus i s influenced by i t s chemical environment.  There are three  important ways that a nucleus interacts with i t s external environment which may be manifested i n a Mossbauer spectrum.  These are the electric  monopole interaction, the electric quadrupole interaction and the magnetic dipole interaction.  These interactions give rise to the isomer shift (I.S.),  the quadrupole splitting (Q.S.) and the magnetic hyperfine splitting, respectively.  The Isomer Shift The isomer shift, due to the electric monopole interaction, arises because the nuclear volume i s penetrated by some of the charge density of the electrons.  The total energy of the nucleus then i s subtly  influenced by variations i n the charge density of the s-electrons.  This  variation would be unimportant except that a nucleus undergoing a y-transition  7  w i l l normally change in size and so the magnitude of the nucleus-selectron interaction energy w i l l be slightly different in each of the two states.  Although these changes are only a very small part of the total  nucleus-electron interaction energy and would be nearly impossible to measure on an absolute scale, a relative energy scale may be readily constructed. If a nucleus B is chosen as a reference nucleus and the relative energy of nucleus A i s measured with respect to B then the difference i n their relative energies or the chemical isomer shift, 6, may be approximated  as^ 6  (2)  = frr ZeV f k | ^ (0)J - | ^ 2  <0)  B  |  2  }  where Z i s the nuclear charge, e the electronic charge constant, r the mean nuclear radius, 6r = r - r the difference in the nuclear radii of ex gr 2 2 the excited and ground states, and (0).| and |¥ (0) | are the probS  A  S  U  ability densities for s-electrons in the nuclear volume for A and B respectively. A  2 2  The isomer shift i s the product of a nuclear term (!<jr Z e r «r) r which is essentially constant for a given nucleus, and a chemical term 2  2  {|Y (0) | -|y (0) | } which varies with the electronic environment of the A  S  i3  2  nucleus.  Each of the s-orbitals of the atom w i l l contribute to I ¥ (0)1 ' s 1  but in lesser amounts as the principal quantum number, n, increases. Normally i t i s considered that the core electrons are only slightly influenced by the behaviour of the valence electrons, so their contribution to |y (0)|  remains essentially constant.  The principal contribution to the  8  isomer s h i f t outermost  comes from changes  occupied s - o r b i t a l s .  i n the number o f s - e l e c t r o n s i n the S i n c e p, d and f e l e c t r o n s e x e r t  screening  e f f e c t s on the outermost s - e l e c t r o n s , they a l s o can i n f l u e n c e the magnitude of  1^(0)1 2 ,  but t o a l e s s e r  degree  12  6r The s i g n o f —  f o r a g i v e n n u c l e u s determines t h e t r e n d o f isomer  s h i f t with s-electron density.  F o r b o t h "* Fe and ^ ^ S b  i s n e g a t i v e so  7  7  t h a t an i n c r e a s e i n s - e l e c t r o n d e n s i t y a t the n u c l e u s l e a d s t o a d e c r e a s e i n the isomer s h i f t .  In c o n t r a s t , f o r ^"^Sn — • i s p o s i t i v e . 7  The isomer s h i f t factors.  The f i r s t  and i s a consequence  i s temperature independent e x c e p t f o r two  of these i s known as t h e s e c o n d - o r d e r D o p p l e r o f the temperature dependence o f t h e l a t t i c e  shift vibration  •2  term, <X >.  T h i s g i v e s r i s e t o a s m a l l change i n t h e y-ray  energy o f t h e  form"*""*  (3)  S  .. ^ V 2c  •2  S i n c e <X > i s e x c e e d i n g l y s m a l l r e l a t i v e very small.  For a s e r i e s of s i m i l a r  to c  2  t h i s term i s c o r r e s p o n d i n g l y  compounds measured at the same  .2  temperature, <X > s h o u l d be n e a r l y c o n s t a n t and so the d i f f e r e n c e i n t h e second-order D o p p l e r s h i f t w i l l be e s s e n t i a l l y The second temperature dependent c h e m i c a l o r p h y s i c a l changes of  effect  i n the system i t s e l f .  t h e r m a l l y a c c e s s i b l e energy s t a t e s o r phase  l a t t i c e w i l l l e a d t o changes s h o u l d l e a d t o changes  zero. from  F o r example, d e p o p u l a t i o n  changes  i n t h e isomer s h i f t .  i n t h e Q.S. as w e l l . )  i s that a r i s i n g  i n the c r y s t a l  (Both o f t h e s e phenomena  9  In electron  general,  the  isomer s h i f t  configuration  or,  in  the o x i d a t i o n  state  of  high-spin  Fe  the  may b e u s e d a s a p r o b e  case of  highly  atoms w h i c h c o n t a i n  ionic  for  the  compounds,  a Mossbauer n u c l e u s .  for For  2+ example, s p i n Fe  3+  compounds may b e e a s i l y d i s t i n g u i s h e d  compounds o w i n g t o  However, f o r  low-spin iron  t h e more p o s i t i v e  compounds t h e  isomer s h i f t s  changes i n  from  in  the  isomer s h i f t  o c c u r on a d e c r e a s e i n o x i d a t i o n n u m b e r a r e v e r y much s m a l l e r ,  highformer  which  as an  extra  14 electron  is  covalent  tin  falls  the  at  ligands  isomer s h i f t s  be c h a r a c t e r i z e d as e i t h e r density  the  .  Similarly,  a r e many c a s e s i n w h i c h t h e  r e g i o n between the  electron  t h e s e two  d e l o c a l i z e d onto  compounds, t h e r e  i n t o the  definitely that  largely  the  tin  Sn(ll)  or  nucleus is  of  environment  Sn(lV).  This  intermediate  indicates between  Splitting  is  via its  s p i n quantum number, distribution. the n u c l e a r  electric  I,  greater  The m a g n i t u d e  of  q u a d r u p o l e moment.  charge d i s t r i b u t i o n  about  distribution.  non-cubic extranuclear  these w i l l  shift  compounds w h i c h may  T h e s e c o n d way i n w h i c h a n u c l e u s i n t e r a c t s  splitting  isomer  highly  extremes.  The Q u a d r u p o l e  a prolate  for  of  this  charge d i s t o r t i o n  A positive  field  of  doublets.  e.f.g.,  a  charge  i s measured by Q,  a negative  a distorted  gradient,  energy l e v e l s .  of Kramers  chemical  which have  Q corresponds to  spin axis whilst  electric  Nuclei  its  than h have a n o n - s p h e r i c a l n u c l e a r  The i n t e r a c t i o n  the n u c l e a r  consist  the  q u a d r u p o l e moment.  with  an  oblate  Q corresponds  nucleus with gives  For h a l f - i n t e g r a l  rise  nuclear  to  a a  spins  to  7  .  10  The e.f.g. at the nucleus is defined by the tensor V e.f.g.  (4)  V  V  XX  V  yx  V  xy  zy  xz  V  yy  V  zx  V  V  15  yz zz  where V „ = 8 V/3i8j and V is the electrostatic potential.  Since V has  a continuous second derivative in the domain under consideration we have  2 3V  2  9i3j  and the Laplacian vanishes;  3j3i  (5)  vv  V + V + V xx yy zz  2  =  0,  so that the tensor is symmetric traceless.  Such a tensor may  be  diagonalized by suitable choice of axes (the so-called principal axes), and since the Laplacian is zero, only two parameters are necessary to specify the e.f.g. completely.  In the principal axis system (denoted X,  Y, Z) , the following convention has been adopted; (6)  where  v l^|v ^|v |, z z  Y Y  and the asymmetry parameter  v (7)  x x  n=  YY  -v  XX ( o $ n a )  ZZ  have been chosen as the independent parameters. The nuclear quadrupole coupling Hamiltonian for a nucleus of 15 spin I may be expressed as'  11  where eq = V , z z  I  i s the spin operator and f , I  z  are the s h i f t operators.  For both "^Fe and ^""^Sn the ground state has spin I = "*7 2 and thus no quadrupole moment.  The excited state has I =  /2  that when the  s 0  nucleus i s subjected to a non-cubic e.f.g., two d i f f e r e n t substates a r i s e . In the a x i a l l y symmetric case (n = 0) I  z  = ± ^~12 respectively.  these substates have I  z  = ±  12 and  The energy separation between the two substates i s  c a l l e d the quadrupole s p l i t t i n g , A E ^ , and i s given i n the general case by  (9)  AEQ =  1  /  e qQ ( 1 + § ) Z  2  For a randomly oriented p o l y c r y s t a l l i n e diamagnetic absorber containing e i t h e r of these n u c l e i , and an unpolarized s i n g l e - l i n e source of Y r a y s , the absorption spectrum w i l l consist of two l i n e s of equal -  i n t e n s i t y (with the exception of a few s p e c i a l cases when, f o r example, the Gol'danskii-Karyagin e f f e c t i s o b s e r v e d ^ - see below).  The separation of  the two l i n e s i s equal to A E ^ and the centroid corresponds to the centre of gravity of the unsplit case.  In such a case, neither the sign of eq(=V ) zz  nor the magnitude of n can be deduced.  Only i n a series of experiments  with oriented c r y s t a l s or with magnetic perturbation techniques (see below) can the sign of eq or the magnitude of n be estimated (Figure 2 ) . 121  For  Sb the quadrupole-split spectra are f a r more complex.  Under the influence of a non-cubic e.f.g., the ground state which has I = s p l i t s into three sublevels, having I  z  = ±  /» 2  12 and ±  "V2  12* respectively,  12a  FIGURE 2.  An Approximate Energy Level Diagram f o r a "^Fe Nucleus Subjected to an e.f.g. (n = 0) and Then to an Applied Magnetic F i e l d at an Angle 6 to the P r i n c i p a l Component of the e.f.g..  12  FIGURE 2.  13  in the axially symmetric case. I =  Similarly the excited state with  splits into four sublevels, with  = ± ^^2, - " 2' 1  ± ^^2» P c t i v e l y , in the axially symmetric case. r e s  e  - ~^2  anc  *  As before, the  energy separation of these sublevels is given by the quadrupole coupling Hamiltonian appropriate to each state (eqn. 8, Figure 3). In addition to the parameters of the e.f.g. (i.e., V  z z  and n),  the relative energies of the transitions between ground and excited states are determined by two constants: the quadrupole moment of the ground state 17 Q (-0.28±0.1 barn) (one could just as well use Q except that the gr ex x  J  x  v  value of Q is inherently more precise), and the ratio of the quadrupole §r 18 moments of the excited and ground states, R = Q /Q (+1.34±0.01) . The ' ex ^gr other parameters which determine the spectrum are the linewidths at halfheight (assuming Lorentzian lineshapes), T, for each component, and the transition probabilities which connect the various substates.  Normally  the linewidths are assumed to be equal for a l l the components, and usually l i e in the range 2.1 to 3.1 mm/sec.  The transition probabilities are  considered to be the theoretical ones as deduced from the Clebsch-Gordan coefficients and the appropriate angular factors (Appendix I ) . The magnitude of Q  and R ensure that in a typical antimony  spectrum most of the component lines w i l l overlap.  Thus the spectra have  the appearance of one or more asymmetric lines due to this overlap (Figure 3), The information obtained on computer f i t t i n g a spectrum (Appendix I) consists 2 of the sign and magnitude of e qQ(Q = Q  ), the linewidth T, the isomer 2  shift 6, and, in favourable cases (large e qQ), the value of n as well. If the e.f.g. is small, so that je qQ|-£ 5 mm/sec. then the sign and magnitude  14a  FIGURE 3.  Energy diagram for an  Sb  Nucleus Subjected to Non-Zero Axially Symmetric e.f.g..  FIGURE 3.  m  RELATIVE ENERGY  15  of e qQ may  be i n some doubt. There are two  factors which can cause changes i n the  t h e o r e t i c a l i n t e n s i t y r a t i o s derived for randomly oriented  polycrystalline 2  materials.  The  t h e o r e t i c a l i n t e n s i t y r a t i o s depend on terms l i k e 1+ cos 6  C w h e r e 8 i s the  angle between the  e.f.g.) integrated  incoming y-ray and  over a l l the possible  orientations  the  (which are  equally probable) of the c r y s t a l l i t e s ( i . e . from 0 to TT) . p r e f e r e n t i a l orientation orientations  the  Z axis of  considered  I f there i s  any  of the c r y s t a l l i t e s i n the sample then a l l  are no longer equally probable and  there are departures from  the t h e o r e t i c a l i n t e n s i t y r a t i o s . The  second factor which may  r a t i o s i s the presence of anisotropics (eqn.  1).  The  lead to changes i n the  intensity  2 i n the Debye-Waller f a c t o r , <X >  i n t e n s i t y r a t i o terms should r e a l l y be written as  say  2 f (1+ cos 8) but  if f  (eqn.  1) i s i s o t r o p i c then the function  is simplified  2 to (1+ cos 8) as above. then f  However, i f the Debye-Waller factor i s  w i l l be described by some function  8 as w e l l . integrated  Thus the i n t e n s i t i e s w i l l now over a l l possible  orientations  f (6) which depends on the angle depend on terms l i k e f (8)(1+ cos a of the c r y s t a l l i t e s , and  departures from the i n t e n s i t y r a t i o s calculated formulation are to be expected.  anisotropic  using the  This i s the so-called  2  8)  so  isotropic  Gol'danskii-Karyagin  effect ^. 1  The  Gol'danskii-Karyagin e f f e c t i s independent of the angle  between the y-beam and integrated  out.  the absorber since this dependence has been e x p l i c i t l y  On the other hand, the e f f e c t s of p r e f e r e n t i a l  orientation  16  of the c r y s t a l l i t e s i s not independent of the angle.  This provides a  simple means of d i s t i n g u i s h i n g the two e f f e c t s since a change i n the i n t e n s i t y r a t i o s observed on a l t e r i n g the absorber-y-beam angle means the Gol danskii-Karyagin 1  e f f e c t may be ruled out - the converse i s not  always true, of course.  The  other way i n which these two e f f e c t s are d i f f e r e n t i s i n  t h e i r temperature dependence. temperature independent.  Orientation e f f e c t s are expected to be  The Gol'danskii-Karyagin  e f f e c t which depends  on the anisotropies i n the Debye-Waller factor i s expected to be temperature dependent since these anisotropies should  increase as the  temperature i s r a i s e d .  To r e l a t e the magnitude and sign of the quadrupole s p l i t t i n g exhibited by a Mossbauer nucleus to the chemical environment which produces the e.f.g. i s not a simple problem.  Indeed, even i f the orien-  t a t i o n of the e.f.g. with respect to the c r y s t a l axes ( i . e . laboratory frame of reference)  i s determined i t i s apparently  necessary to have d e t a i l e d  knowledge of the c r y s t a l structure i n order to r e l a t e the e.f.g. to the  19 molecular axes  . Nevertheless,  a knowledge of the sign and magnitude  of the e.f.g. w i l l i n many cases allow us to make some deductions about the d i s p o s i t i o n s of the ligands i n the molecule.  Some assumptions as to  how the ligands influence the d i s t r i b u t i o n of electrons i n a molecule and thus how the e.f.g. at the nucleus arises are e s s e n t i a l i n order to make s t r u c t u r a l p r e d i c t i o n s . In dealing with a molecule the e.f.g. may be separated  to a  17  f i r s t approximation into two main contributions.  One  i s the l a t t i c e  contribution, Q.jjj<> due to charges on the ligands and other ions i n the e  crystal.  The second i s the valence contribution, e q ^ ^ ,  due to the  asymmetric d i s t r i b u t i o n of electrons i n bonding and non-bonding o r b i t a l s . Electrons i n s - o r b i t a l s and electrons i n f i l l e d s h e l l s are not  considered  to contribute to the e.f.g. except f o r induced p o l a r i z a t i o n e f f e c t s . 20 The (10)  conventional picture i s thus eq  where R(0.2  (l-Yjeq  =  L A I  and y (-7  > R > -0.2)  m  +  (l-R)eq^^ > y^ > -100)  are Sternheimer factors 21  accounting  f o r the induced p o l a r i z a t i o n of inner electrons.  /n\ j  Also  ,  2 (3cos 0. - 1) 3  and  / U c o s V - 1)' <> 12  *VAL  et  "  "^PiC  -L-  where q^ i s the charge on ion j whose polar coordinates are 0^,  r^ ( i n  this approximation ions are treated as point charges) while p. i s the 2  3  population of the i t h valence s h e l l o r b i t a l , <(3cos 8^ - l ) / r ^ > being expectation value of t h i s population over the electron coordinates  the  0 , r^.  The summations are over a l l ions j and a l l valence s h e l l o r b i t a l s i . For highly covalent molecular  systems, the contribution from _3  external ions i s expected to be small (owing to the r  dependence) and  thus  18  the q  term to a good approximation arises s o l e l y from charges on LJA.L  the ligands.  S i m i l a r l y , i f there are no non-bonding electron p a i r s * ,  or for t r a n s i t i o n metals with no p a r t i a l l y f i l l e d s h e l l s * * , the term w i l l consist of contributions from each ligand L. the s o - c a l l e d point charge model may  e  q ^ v  L  In t h i s s i t u a t i o n  be applied, i n which the e.f.g.  can  21 be written  as  (13)  eq  =  [ ]  =  Z [ L ] (3 c o s 9 - l ) L 2  L  where eq (lT  (14)  L  )  ep (l-R) T  3 r  q  Y o o  . | 3. L  L  >  i s the charge on ligand L with coordinates Li  r , 9 , and p LI  o r b i t a l population  LI  the e f f e c t i v e LI  i n the hybrid o r b i t a l directed towards L with e f f e c t i v e  electron coordinates  8 , r '. Li  The  l a t t i c e and valence contributions  are  LI  i n sign and i n most covalent compounds | qyAL I > : > I *LAT ^ 3 3 presumably due to the f a c t r >> <r ' >. opposite  e  Li  ec  LI  There also exists a molecular o r b i t a l approach i n terms of the r e l a t i v e a-donor and TT-acceptor strengths  of the ligands.  This approach  has only been developed i n d e t a i l f o r low-spin six-coordinate f i r s t - r o w transition-metal species  *  **  (in particular for F e ( I I ) ) . 2 2  The £ L j  This may not be s t r i c t l y true i f there i s only one which may be treated as a ligand.  value f o r  lone p a i r  This r e s t r i c t i o n can sometimes be relaxed since both the dy and d subsets w i l l have s p h e r i c a l charge d i s t r i b u t i o n s i f they are e i t h e r f i l l e d or empty (see below). £  19  any particular ligand L is taken to be the sua of a a  term for the  contribution to the e.f.g. due to a-donation and a TT term due to the contribution from u-acceptance (TT then i s opposite in sign to a ) . The L  relative TT^ and  Li  values are assumed to be constant from compound to  compound and, again, the contributions due to charges from external ions are neglected.  The relative contribution of each cr to the total e.f.g. Li  is assumed to be proportional to the squares of the coefficients for the appropriate hybrid orbitals directed towards the ligand.  The hybridization  is taken to be d^sp"^ so that for "^Fe 3d „ _, 3d „, 4p , 4p , 4p and 4s x _y z y orbitals w i l l participate i n a bonding. The TT contribution involves the relative TT bonding abilities of the d , d and d orbitals which taken ° xy' xz yz z  z  l  x  z  together are assumed to be equal in the three principal directions of the e.f.g..  The e.f.g. is considered to arise solely from d-orbital augmen-  tation (or depletion) and so the contribution from the 4p orbitals i s neg3 3 lected on the basis <r >, » <r >_, (although no justification for this 4p 3d 22 assumption has been given ). Also, i t i s expected that j a | > | TT | in most Li  LI  cases. The central feature of both the above models for the origins of the e.f.g. i s the assumption that the e.f.g. at the nucleus may be treated as the sum of contributions  L , one from each ligand.  This i s the  23 so-called additive approximation , or as i t is sometimes called the partial quadrupole splitting (p.q.s.) theory.  To what extent this approximation is  valid i s governed by how well the contribution due to a given ligand remains essentially invariant for any series of compounds under 23 consideration  20 '•}..  In this thesis, the value assigned to the contribution to the  total e.f.g. from a ligand L w i l l be expressed in terms of the value of £ as previously defined (eqn. 14)  times e|oJ  (or for "^Fe %e|Q|) so that £ L j  has units of mm/sec. Thus, our values of £LJ w i l l have the same significance as the (p.q.s.)  values of Bancroft . 7  This also means that the V  from p.q.s. calculations w i l l be scaled up by e jQ1 (or  derived  ^setQI) times the  nuclear V^z' This procedure is adopted since the sign of the equivalent ellipsoid of charge, so i f  i s related to  is negative the ellipsoid of  charge is prolate, while i f i t is positive the ellipsoid of charge i s oblate.  2  On the other hand, e qQ is what i s determined experimentally and the relationship between our derived V  2  values and e qQ w i l l just be  121Sb, e2 qQ = -V in 57Fe Q is positive and so i n this case he 2 qQ = V _. For  2  e qQ = sign(Q) V that case.  .  Thus since Q is negative for  As the |LJ values cannot be deduced from f i r s t principles (e.g. 3  for the q„,  T  term in eqn. 14, <r > is unknown) i t is necessary to deduce  them from compounds of known structure. Since the components of the e.f.g. consist of sums and differences of  values i n such a manner that the  addition of an arbitrary constant to each value of £LJ makes no difference  7 25 to the predicted result '  , the normal practice is to assume a value of  0.0 for one particular ligand and to derive  values for the others from  this arbitrary starting point. For molecules with regular geometry, the principal components of the e.f.g. tensor for various combinations of ligands (e.g. for tetra-  21  hedral cases A^BM, £LJ  A2&2^> - ) etc  have been tabulated in terms of the  values^' ^ and also some of them are presented in Figure 4. 2  The values of £LJ are also dependent on the coordination number of the complex, a fact which is not obvious from the derivations. For example, the value of QL^j for the same ligand in octahedral  coordina-  tion has been found to be about 70% of i t s value in tetrahedral coordina-  25 tion  .  This ratio has been deduced theoretically from consideration of  the overlap integrals between metal and ligand orbitals in octahedral  25 and tetrahedral compounds  . At the same time, the suggestion has been  made that the £LJ value of a ligand in the equatorial positions is different from the ^ i f j value of that ligand in the axial positions in trigonal bipyramidal compounds. Thus the £LJ values for a ligand w i l l be "1TET f~ "1 OCT  [ Lj  for tetrahedral coordination, L for octahedral "1TBA coordination, I LJ for trigonal bipyramidal axial coordination, and "l TBE Lj for trigonal bipyramidal equatorial coordination.  r  t  The success of any model for a system is judged by how well the behaviour of the system may be predicted from the model.  Certainly,  the additive model for e.f.g.s has had noticeable success in predicting the sign and magnitude of the e.f.g. in many compounds (particularly of Sn(lV)) which have regular geometry''.  In the converse case, namely  the prediction of the disposition of the ligands about the metal from a knowledge of the sign and magnitude of the e.f.g., the success rate has not been quite so high.  The reason for this i s obvious  since the problem is complicated by the fact that distortions from ideal geometry lead to changes in the relative £LJ  contri-  FIGURE 4.  Some Regular Structures and the Point Charge Expressions for the Components of Their EFG Tensors.  Where the Principal Axes  Are Determined by Symmetry, the Components are Designated by Upper Case Subscripts.  Otherwise, the Principal Axes Can Only  be Found by Diagonalization of the Tensor for Each Case Under Consideration.  CONTINUED/...  23 FIGURE 4 (CONTINUED)  ZZ  V, YY XX  'ZZ  V. YY XX  0 0 0  CONTINUED/...  FIGURE 4 (CONTINUED)  25  butions to the e.f.g. and by the fact that more than one possible structure may lead to e s s e n t i a l l y the same e.f.g. parameters. As yet, no completely s a t i s f a c t o r y treatment has emerged f o r dealing with d i s t o r t i o n s from i d e a l geometry. approaches to this problem.  There have been two  One i s j u s t the straightforward a p p l i c a t i o n  of the simple point charge model i n which the value of £LJ i s assumed to be invariant on d i s t o r t i o n and only the changes i n r e l a t i v e o r i e n t a t i o n of the ligands are c o n s i d e r e d " ^ .  A second approach has been developed  f o r tetrahedral systems assuming that the ligand-metal 25 do not change on d i s t o r t i o n .  orbital  overlaps  Both these methods have some value i n  predicting sign changes on d i s t o r t i o n but have had much less success i n 25 predicting the magnitude of the e.f.g.  (or conversely,  the magnitude  of Besides the additive models, another approach which i s sometimes of use i s based on the Townes-Dailey approximation i n which the e f f e c t i v e electron populations  i n the p- and d - o r b i t a l s are considered  7 22 ' . Thus i f  the e.f.g. i s assumed to a r i s e s o l e l y from p electrons the q VAL may be written as T I A T  <VAL  ( 1 5 )  where K  =  K  p " p {  N  + 1  *  ( N  p  +  N  )  contribution  }  P  _3 i s a constant depending on <r > and N , the e f f e c t i v e o r b i t a l P P P ±  population  i n the appropriate p - o r b i t a l (this formulation i s only useful  when the Z axis i s uniquely defined).  S i m i l a r l y f o r an e.f.g. a r i s i n g  s o l e l y from contributions due to d electrons  26  (16)  q„ VAL AT  = K, {-N d d ?  + N, + N, d ? 2 d x -y xy  1  /o(N, '*  + N, )} d xz yz  d  -3  where K. i s a constant depending on <r >, and N. the effective orbital d d d. population i n the appropriate d-orbital.  l  This approach is particularly  useful for examining the effects due to partially f i l l e d d-orbitals. For example, i f d , d and d are f i l l e d and d o and d o ? are empty as xy xz yz z^x -y in low-spin ferrous compounds, then the e.f.g. should be very small or On the other hand, i f only d 2 i s empty and the other d-orbitals  zero.  f u l l , then a large e.f.g. i s expected which w i l l not vary too much with ligand substitution (provided the N^ for the other d-orbitals remains i essentially constant). This i s apparently exactly the situation i n 27 phosphine and arsine derivatives of Fe(CO)^ The Magnetic Hyperfine Interaction The interaction of the nucleus with a magnetic f i e l d i s known as the nuclear Zeeman interaction.  The magnetic moments of the ground and  excited states interact with an effective magnetic f i e l d at the nucleus so that the degeneracy of the nuclear spin substates i s l i f t e d .  In the case  2 15 when e qQ i s zero, the Hamiltonian describing the interaction i s given by  (  1  7  )  "MAG  -  S * n ^  where g is the gyromagnetic ratio for the state under consideration, $  n  the nuclear magneton, I the nuclear spin operator, and H. the effective magnetic f i e l d at the nucleus.  For this case, without loss of generality  we may choose the z axis to l i e along H and so the eigenvectors of the Hamiltonian are given by  27  (18)  E . MAG M  - g 3 Hm n I  =  r  with m  T  T  =  I  - I , -1+1  where H i s the magnitude of the magnetic f i e l d .  ...  I  Thus 21+1  spaced energy l e v e l s w i l l a r i s e for both the ground and For "* Fe the ground state, with I = ^ 12* ^ 7  separation i s g B H, while the excited o  n  s  s  P^  equally  excited  into two  t  states.  l e v e l s whose  12> i s s p l i t  s t a t e , with I =  into four l e v e l s with a separation of gjj^H between adjacent p a i r s . The  r a t i o between the ground and  excited  state g values, g  and  g^  28 respectively, has  been found to be g /g^  1.750  =  Q  For "* Fe the Y-transitions 7  are r e s t r i c t e d by the dipole  between ground and  t r a n s i t i o n s to s i x at most and The  states  r a d i a t i o n s e l e c t i o n rules"*'"' (Appendix II)  that only Ani-j. = 0, ±1 t r a n s i t i o n s w i l l occur.  pattern for i r o n f o i l .  excited  This r e s t r i c t s the  so  allowed  explains the f a m i l i a r s i x l i n e hyperfine  l i n e i n t e n s i t i e s depend on the squares of  Clebsch-Gordan c o e f f i c i e n t s and  on the angular factors  the  c h a r a c t e r i s t i c of  2 dipole r a d i a t i o n . The angular factors are 1+cos 9 for Am^. = ±1 t r a n s i t i o n s 2 29 and s i n 9 f o r Am^. = 0 t r a n s i t i o n s . For a randomly oriented p o l y c r y s t a l l i n e 30 absorber, averaging over a l l the orientations  ( 1 9 )  sin 4  2  fl  0  =  y-  4TT  / o  / o  gives r i s e to i n t e g r a l s such as  ( s i n 9 ) sin0d9d<j> = 2  | 3  and  ( 2 0 )  and  . . 2FL 1+cos 9  =  74  T  / T  O  O  /  (l+cos 6) sin0d6dct) 2  so the angular factors average to the same value.  =  | 3  Thus the observed  i n t e n s i t y pattern of 3:2:1:1:2:3 of iron f o i l r e f l e c t s the magnitude of  28 31 the squares of the Clebsch-Gordan coefficients for the transitions connecting the various substates. For a  121  2 Sb nucleus in a magnetic f i e l d (e qQ = 0) there would 32  be a total of eighteen allowed transitions would be quite complex.  and the resultant spectrum  Since a l l the antimony compounds dealt with in this  thesis are diamagnets in zero applied f i e l d there is no contribution from the nuclear Zeeman interaction and this interaction w i l l not be  considered  further. The case which is of most interest here is that in which a nucleus is subjected simultaneously  to an electric quadrupole interaction and to a  magnetic hyperfine interaction. In particular for diamagnetic compounds of "^Fe and "^Sn with non-zero e.f.g.'s in moderate (25-50 kG) applied fields, 15 33 the sign of the e.f.g. and the magnitude of n may be determined ' . The Hamiltonian for such an interaction may be written (21)  ^Q •^G  +H Q  -  +  trfcy z {(3t  where H is the applied magnetic f i e l d .  +  K ->> 2+f  2  This is a f a i r l y d i f f i c u l t problem  to solve exactly in that the Z axis defined by the principal component of the e.f.g. w i l l seldom be colinear with the Z 119 field.  For  1  axis defined by the applied  57 Sn and  Fe, the quadrupole moment of the ground state is  zero, so that for this state we simply have excited state the f u l l Hamiltonian (^ Q) +  = H^^.  However, for the  must be used and the problem is  further complicated by the fact that the case which is of most interest is that of a randomly oriented polycrystalline powdered sample subjected to an  29  external field. If the coordinate axes are taken to be the ones defined by the principal axes of the e.f.g. then the applied f i e l d w i l l be at Z' defined 33  by the direction cosines 8, cj> . The magnetic part of the interaction may be written as (22)  YL,. MAG  N  =  -g 3 H (sin8cos<t>I + sin8sin<J>I + cosSl ) In x y z ^  /\  <N  where H is the magnitude of the applied f i e l d (H) and 1^, I , 1^ are spin 29 operators  .  For polycrystalline materials, a l l possible orientations of  the Z' axis with respect to the Z axis as defined by the e.f.g. w i l l occur. Therefore the energy eigenvalues must be found for a l l possible values of 8 and <f>. This i s usually done by computer diagonalization of the appropriate 4x4 matrices for each value of 8 and (j) under consideration. In order to simulate a spectrum, the transition probabilities connecting the various substates are needed as well as the differences in 33  the energy eigenvalues  .  This i s a somewhat more tedious operation since  one needs to know the eigenvectors in terms of the basis vectors for both ground and excited states, the Clebsch-Gordan coefficients for the appro31  priate transitions  , and then sum over the relative amounts of Y-quanta  available with longitudinal and l e f t - and right-hand circular polarizations, These computations are discussed in Appendix II. The result for an "^Fe compound with an axially symmetric e.f.g. is the appearance of a characteristic doublet-triplet pattern in moderate fields.  For a positive V^,  the triplet lies at low velocities and the  30  doublet at higher v e l o c i t i e s .  The order i s reversed i f V  z z  I  s  negative.  When the value of the Q.S. f a l l s below about 1 mm/sec (or the applied f i e l d i s too large) the t r i p l e t - d o u b l e t observation no longer holds since some of the component l i n e s may overlap.  S i m i l a r l y , as n becomes  large, the spectrum i n the applied f i e l d becomes more symmetrical tending to a t r i p l e t - t r i p l e t ) .  (usually  Nevertheless, the spectrum may be  simulated and the sign of the e.f.g. and the magnitude of n estimated f o r most of the cases of i n t e r e s t . 119 Similar considerations hold f o r  Sn except that the s i t u a t i o n  i s not quite so favourable owing i n part to the greater n a t u r a l linewidth of t i n and i n part to the larger magnetic moments of the ground and excited 119 states.  Representative spectra of  Sn i n applied f i e l d s have been  published f o r many cases of i n t e r e s t and serve as a guide i n i n t e r p r e t i n g observed  spectra  31 EXPERIMENTAL  The  121  Sb MBssbauer spectra of two series of compounds were 121 obtained in transmission geometry using a 1 mCi Ba SnO^ source (New England Nuclear) cooled to liquid nitrogen temperature.  The source, which  2 had a cross sectional area of 2.2 cm , was positioned by a phosphor-bronze drive spring mounted on a copper cold finger which dipped into liquid nitrogen.  This assembly was carefully insulated with about 3 cm. of  styrofoam except for a 1 cm. thick portion directly i n front of the source.  The source was driven via a 12 cm. long phenolic plastic drive  rod, by an Austin Associates K.3-K linear motor and an S-3 spectrometer drive unit. The rest of the spectrometer consisted of a Reuter- Stokes RSG-61 proportional counter (Xe - 10% CO^ at two atmospheres), and standard Nuclear-Chicago modules. These included a model 40-9B high voltage power supply, a model 23805 preamplifier, a model 33-15 amplifier-single-channel analyzer, a model 23-4 analog-to-digital converter, a model 021308 timebase generator, and a model 24-2 400-word multichannel analyzer operating i n time mode. The single-channel analyzer was set on the escape peak of the 37 keV  121  Sb ~ r a y Y  32 .  The absorbers consisted of finely powdered neat solids which 2 contained 8 to 10 mg. of antimony/cm . A copper c e l l with mylar windows which 2 had a cross-sectional area of 2.5 cm was employed.  During the course of  each experiment, the absorbers were maintained at 8.5 - 9.0°K i n a Janis  32  model 6DT variable temperature cryostat. by a germanium resistance thermometer.  The temperature was monitored A t y p i c a l spectrum was run f o r  14 - 18 hours and consisted of two mirror image spectra containing 20 - 40,000 counts i n each of the 400 channels (Figure 5).  After each run, the v e l o c i t y scale was calibrated using metallic iron f o i l  28 35 57 ' and a nominally 10 mCi Co(Cu) source.  The zero  of v e l o c i t y was found by using a BaSnO^ absorber and a 5 mCi Ba^^^SnO^ 121 source mounted and run under the same conditions as the Ba  SnO^ source.  As a further check, a number of runs were made using the small amount of r e s i d u a l a c t i v i t y due to Ba^^^SnO^ impurity i n the Ba^^SnO^ source. The difference i n the isomer s h i f t of the BaSnO^ absorber r e l a t i v e to these two sources was found to be less than 0.01 mm/sec. The data points were computer f i t t e d to either a single Lorentzian or to an e i g h t - l i n e Lorentzian pattern appropriate for a non-zero, 32 a x i a l l y symmetric e.f.g.  .  The program was kindly supplied by Dr. L.H. Bowen. 2  In t h i s program f o r non-zero e qQ (n=0), the energies of the t r a n s i t i o n s are determined a n a l y t i c a l l y and the i n t e n s i t i e s of the l i n e s are assumed to be the appropriate combinations of the squares of the Clebsch-Gordan c o e f f i cients f o r a p o l y c r y s t a l l i n e absorber.  The value of Q  /Q  = R = 1.34  was  ex ^gr  J  18 assumed throughout . In the f i n a l data reduction step, a f t e r the parameters for each of the two mirror image spectra were determined f o r comparison (this 2 i s p a r t i c u l a r l y important f o r small e qQ since any prejudice introduced by the f o l d i n g procedure may have a marked e f f e c t on the f i n a l parameters),  FIGURE 5. Schematic Diagram of the Apparati. 121  Employed for Obtaining Mossbauer Spectra.  Sb  LIQUID HELIUM RESERVOIR  LIQUID NITROGEN RESERVOIR  VACUUM SPACE OUTER WALL OF DEWAR STYROFOAM INSULATION  PLASTIC DRIVE ROD  HOLLOW TUBE FILLED WITH HELIUM EXCHANGE GAS  K3K  LINEAR MOTOR  DETECTOR (Xe-C0 ) 2  ABSORBER COPPER BLOCK  SOURCE  LIQUID NITROGEN RESERVOIR  PHOSPHOR-BRONZE SPRING  FIGURE 5.  34  the  spectra were folded using the v e l o c i t y scale determined from the  best least-squares f i t to the iron f o i l c a l i b r a t i o n .  This procedure  generally l e d to an increase of approximately 0.05 to 0.10 mm/sec. i n the  apparent linewidth.  For  spectra which gave poor f i t s f o r r\ = 0, i t was assumed  that n 4 0, and they were f i t t e d using n as a v a r i a b l e . a subroutine was written f o r c a l c u l a t i n g In the  121  For t h i s purpose,  36 Sb spectra with non-zero n  t h i s subroutine, the energies were found by machine diagonalization of appropriate Hamiltonian matrices f o r the ground and excited s t a t e s .  The t r a n s i t i o n p r o b a b i l i t i e s f o r the twelve possible t r a n s i t i o n s connecting the  two states were assumed to be the appropriate combinations of the  Clebsch-Gordan c o e f f i c i e n t s . of  The i n t e g r a l s f o r the angular-dependent part  the above were c a r r i e d out assuming randomly oriented p o l y c r y s t a l l i n e  powders (Appendix I ) .  The at  57  Fe spectra were recorded with the 10 mCi  57  Co(Cu) source  room temperature and the absorbers at 80°K i n transmission geometry 37  with a spectrometer previously described the  (Figure 1).  The centroid of  sodium nitroprusside spectrum was used as an isomer s h i f t standard  and the Q.S. of this spectrum was used at the v e l o c i t y c a l i b r a t i o n .  This  procedure was adopted since the v e l o c i t y range scanned was approximately -4 to +4 mm/sec  A number of runs c a r r i e d out at higher v e l o c i t y ranges  using m e t a l l i c i r o n f o i l , show that the drive i s s u f f i c i e n t l y l i n e a r f o r this purpose All  . the iron compounds used as absorbers were i n the form of  35  finely powdered neat solids.  A minimum of sample thickness was used  consistent with the desire to obtain good signal to noise ratio. The success of this procedure may be judged from the linewidths of the fitted spectra.  For compounds containing a large arsenic to iron ratio, a  number of runs were usually made until the optimal sample thickness was obtained. A l l the ~^Fe spectra were least-squares fitted to Lorentzian components by a program based on one originally supplied by the National 39 Bureau of Standards The magnetic perturbation "^Fe measurements were carried out in a Janis model 11MDT Helium cryostat with a Westinghouse  superconducting  solenoid capable of generating magnetic fields of up to 50 kG. The finely powdered samples were placed in a brass c e l l located at the centre of the applied f i e l d .  The vertically mounted "^Co(Cu) source was driven, via a  drive rod made from thin-walled stainless steel tubing, by an Austin Science Associates K-3 linear motor, located in the common vacuum space with the absorber (Figure 6).  With this geometry, the directions of the  applied field and of the beam of y~ ays from the source were colinear. r  Spectra run at both liquid nitrogen temperature and liquid helium temperature showed no significant differences in parameters. In f i t t i n g the magnetic perturbation spectra, theoretical spectra 33 were generated using a program generously supplied by Dr. George Lang The value of g ^ B was taken as 0.0068 mm/sec/kilogauss^ while the value n  FIGURE 6.  Schematic of Magnetic Perturbation Apparatus  36 FIGURE 6.  DRIVE CAN  •«  *  TO S-3 DRIVE UNIT  K-3 DRIVE  DEMOUNTING FLANGE  TO VACUUM LINE  OUTER WALL OF DEWAR COMMON VACUUM SPACE  LIQUID NITROGEN RESERVOIR  SUPERCONDUCTING SOLENOID INNER VACUUM SPACE  ft  »TO VACUUM LINE  LIQUID HELIUM RESERVOIR STAINLESS STEEL DRIVE ROD SOURCE SUPPORT FOR ABSORBER CELL ABSORBER CELL  MYLAR WINDOWS  DETECTOR  37  -1.750 was  taken f o r the r a t i o (g /g^) of the g values of the ground 28  and excited states In f i t t i n g magnetic perturbation spectra f o r compounds with more than one iron s i t e (iron A and i r o n B), the assumption was made that the contributions to the area under the absorption spectrum would be equal f o r each s i t e .  This i s equivalent to assuming that the resonance  fractions f o r each of the two s i t e s are equal.  That t h i s i s a good  approximation i s shown by the unperturbed spectra at l i q u i d nitrogen and at l i q u i d helium temperature where the differences i n the t o t a l areas under the f i t t e d absorption peaks f o r each of the s i t e s are less than 5% f o r those compounds where a l l the component l i n e s f o r the two s i t e s are resolved.  In the f i r s t step of the f i t t i n g procedure, t h e o r e t i c a l spectra f o r the magnetically perturbed samples were calculated Q.S.  using the observed  f o r each of the s i t e s and using values of n, = 0.0, 0.6, and  0.8.  Appropriate values f o r the r e l a t i v e isomer s h i f t s were employed and a l l possible combinations f o r the signs of the e.f.g. were generated, i . e . S i t e A+, S i t e B+, Site A-, Site B+, S i t e A+, S i t e B-, and S i t e A-, S i t e B-. relationship  that a change i n sign of the e.f.g. gives an i d e n t i c a l spectrum  but i n reverse order was employed  .  At t h i s stage, examination of the  spectra showed which of the combinations was observed spectrum. calculated  The  the best approximation to the  As a refinement, i f necessary, further spectra were  f o r d i f f e r e n t values of n. or f o r d i f f e r e n t values of the assumed  linewidths u n t i l s a t i s f a c t o r y agreement between t h e o r e t i c a l and observed spectra was obtained.  In each of the spectra examined, the sign of the  38  larger e.f.g. could be unequivocally assigned.  The value of n. i s some-  what more i n doubt and i n p a r t i c u l a r f o r the s i t e with the smaller quadrupole s p l i t t i n g the uncertainty i s f a i r l y large.  In at least  one  compound, the signs of the e.f.g.s were obvious from v i s u a l inspection of the spectrum and so no further c a l c u l a t i o n s were c a r r i e d out.  Three series of compounds were employed i n this work. f i r s t s e r i e s of compounds of the type etc.;  Sb(Cp(CO)2^ ) e  n  Cp = TT-cyclopentadienyl, n = 1, 2, or 3 and X  + x n  (  R  The =  halogen,  = large anion) were  prepared by Dr. D.J. Patmore of t h i s department and have been characterized by I.R.  techniques and chemical analyses.  has been reported  The preparation of these compounds  '  The second s e r i e s of antimony compounds of the type Ph^ ^ (X = halogen, e t c . ; n = 1, 2, or 3) have been prepared  ^^^  n  by  Dr. J.N.R.. Ruddick of the Mossbauer research group following published procedures. points  They have been characterized by I.R.  spectra and by melting  .  The i r o n compounds were synthesized by Mr. L.S. department and have been characterized by I.R. 44  analyses  techniques  Chia of t h i s and by  chemical  45  '  .  The preparation of some of these d e r i v a t i v e s has been 46  recently reported  39 RESULTS AND DISCUSSION PART 1  (A)  LFe (CO) 2  LFe„(CO), COMPLEXES AND THEIR DERIVATIVES  6  The structures and bonding c h a r a c t e r i s t i c s of transition-metal carbonyls with metal-metal bonds have been of i n t e r e s t f o r some time.  In  p a r t i c u l a r , a number of i r o n carbonyls with i r o n - i r o n bonds have been studied^ '^ ^^. 7  7 >  49 such as CO  In most cases such systems involve some bridging group  50 , H  51 , SME  , e t c . although systems without such bridges are 2- 52 53 49 not e n t i r e l y unknown (e.g. Fe (C0)g * and Fe.j(C0)^ both contain a 2  2  non-bridged i r o n - i r o n bond). In the past, a number of novel iron carbonyls  containing unsaturated  f l u o r o a l i c y c l i c - b r i d g e d d i ( t e r t i a r y arsines and phosphines) have been synthesized which have the general formula LFe (C0), 2. O o  37 ^6 ' .  The structures of  some of these ligands L, as w e l l as a number of other d i ( t e r i a r y arsines and phosphines) are i l l u s t r a t e d i n Figure 7.  Details of the preparation of some 54  of the f l u o r o a l i c y c l i c ligands have been published 47 been the subject of a recent review  .  and as w e l l , they have  The f l u o r o a l i c y c l i c o l e f i n i c ligands  i l l u s t r a t e d are very v e r s a t i l e i n that they may act as bridging groups, as f o r example i n f ^ f a r s C o ^ C O ) ^ ^ , as monodentate ligands as i n f ^AsPFe (CO) ^ the ligand i s coordinated v i a phosphorus, or as chelating ligands, as f o r example i n f^fosFe(NO) ^ . 7  2  Of course, the unsaturated f l u o r o a l i c y c l i c ligands also have the p o t e n t i a l of coordinating v i a the o l e f i n i c double bond to form a  where  FIGURE 7.  The Structures of Some T y p i c a l Ligands Used i n t h i s Work.  1 f fos  f fos  f fos  D=P(CgH ) ,  E=P(C H )  f AsP  f AsP  —  D=P(C H ) ,  E = As(CH )  f fars  tfars  D = As(CH ) ,  E = As(CH )  4  6  4  8  6  4  5  6  f fars 8  2  H2(  ./As(C H ) 6  5  2  HC 2  ^^As(CH ) 3  ^P(C H )  2  6  5  As(C H )  2  6  diars  arphos  5  2  dpam  H C-P(C H ) HC 2  6  5  2  2  P(C H ) 6  5  h2C  ^P(C H ) 6  5  2  H C-PqH ) 2  dppm  dppe=diphos  5  2  3  6  s(chy  2  5  2  dppp FIGURE 7.  2  6  5  3  3  2  2  2  FIGURE 8.  The Structure of f,farsFe (CO) 0  41  TABLE I . MOSSBAUER PARAMETERS AT 8 0 ^ FOR L F e ( C O ) 2  * f.fars 4  AEq(mm/sec) 1.44 0.64  **  t 6(mm/sec) 0.32 0.28  T(mm/sec)  6  COMPOUNDS IRON SITE  REFERENCE  0.25 0.25  B A  37,47  f. fos 4  + 1.32^ - 0.66  0.32 0.23  0.26 0.26  B A  37,47  f fos  1.19 0.65  0.32 0.22  0.23 0.23  B A  37  0.31 0.27  0.23 0.23  B A  44,47  - 0.83 1.19 0.73  0.32 0.26  0.24 0.30  B A  0.33 0.30  0.28, 0.26 0.24, 0.28  B A  This work  - 0.67 1.38 0.62  0.32 0.28  0.26, 0.28 0.28, 0.28  B A  This work  1.55 0.17  0.32 0.20  0.23 0.25  B A  6  **  f .AsP 4 f AsP 6  ** + - **  fgfars  1  fgfars  f,fos Fe ( C 0 )  7  4 1  45  48  * Experimental uncertainty ±0.01 mm/sec. **  Relative to sodium n i t r o p r u s s i d e , experimental uncertainty ±0.01 mm/sec. o Sign determined with source and absorber at 4.2 K i n this work (See Table VI).  43  TT-complex  with suitable metals.  In f a c t , the X-ray structure of  58 f farsFe2(CO)g  , as i l l u s t r a t e d i n Figure 8, reveals that the ligand  4  i s not only chelating to one i r o n v i a the two arsenics i n the  usual  manner but that the perfluorocyclobutenyl double bond i s linked to the second i r o n atom. In t h i s compound, one i r o n atom l a b e l l e d Fe  i n the f i g u r e ,  i s approximately octahedrally coordinated being surrounded by  three  carbonyls, two  arsenics from the ligand, and the other i r o n atom,  B l a b e l l e d Fe .  The  coordination about Fe  B  i s somewhat more d i f f i c u l t to A.  describe.  I t i s obviously surrounded by three carbonyls, Fe , and  "double-bond".  The  coordination may  be regarded as d i s t o r t e d t r i g o n a l  bipyramidal i f the "double-bond" i s considered 58 a TT-bond with the i r o n moiety  .  the  I t may  as a s i n g l e e n t i t y forming  also be regarded as d i s t o r t e d  octahedral, i f the carbon atoms at each end of the "double-bond" are considered to form two normal cf-bonds with Fe s i t e s i n the octahedron^. 60 r e l a t e d derivatives  and thus to occupy  two  From the c r y s t a l structure data of t h i s " ^ and  i t appears that perhaps the true s i t u a t i o n l i e s  between these two extremes.  In any  case the "double-bond" character of  the C - C l i n k w i l l be severely reduced. The Mossbauer spectra of a number of these d e r i v a t i v e s IJF^CCO)^ 37 4 4 48 have been investigated i n the past ' ' and these data as w e l l as the *  This i s not the only formulation of o l e f i n - i r o n bonding more d e t a i l e d discussion i s given below i n Section E.  59,60  .  A  44  present results are presented i n Table I. As w e l l , a t y p i c a l Mossbauer spectrum of one of these d e r i v a t i v e s , namely fgfarsFe2(C0)g, i s reproduced i n Figure 9.  In t h i s f i g u r e , the absorption l i n e s may be numbered  1, 2, 3, 4 reading from l e f t to right and the two iron s i t e s w i l l be a r b i t r a r i l y l a b e l l e d I and I I without  d i s t i n c t i o n at t h i s point as  A B to which i s Fe and which i s Fe .  Using t h i s procedure,  there are three possible assignments of  the l i n e s , v i z . : (a) l i n e s 1 and 2 to s i t e I, l i n e s 3 and 4 to s i t e I I j (b) l i n e s 1 and 3 to s i t e I , l i n e s 2 and 4 to Site I I * to s i t e I, l i n e s 1 and 4 to s i t e I I .  (c) l i n e s 2 and 3  Parameters derived from these  assignments are given i n Table I I . TABLE I I . Possible Assignments of the Mossbauer Parameters for f f a r s F e ( C O ) . 6  ASSIGNMENT  2  6  LINES  6(mm/sec)  AE (mm/sec)  (a)  1,2 3,4  -0.20 +0.83  0.34 0.30  I II  (b)  1,3 2,4  +0.13 +0.50  1.00 1.06  I II  (c)  2,3 1,4  +0.30 +0.33  0.67 1.41  I II  Q  IRON SITE  The usual method of assigning the l i n e s i n iron carbonyl derivatives which contain two d i f f e r e n t iron s i t e s has been to use an empirical approach so that the assignment of the Q.S. and I.S. of the two  FIGURE 9.  "* Fe Mossbauer Spectra of 7  f farsFe (CO) 6  2  6  and  (PhO) Pf AsPFe (CO) 3  4  2  f  45  f farsFe (CO) 6  2  6  o  o  (Ph0) Pf AsPFe (C0) 3  -2  4  2  0  VELOCITY (mm/sec) FIGURE 9.  5  2  °  46  s i t e s gives parameters which are i n reasonable agreement with the Q.S. and I.S. of related molecules containing only one s i t e . the Mossbauer parameters of ligand v a r i a t i o n may aid i n assigning the l i n e s .  The e f f e c t s on  also be employed as an  For example, i n the previous work of  37 Cullen et a l . on LFe (CO)g compounds the following approach was used. 2  F i r s t , the decision was made that the magnitude of the Q.S. of Fe greater than that of Fe  A  based on the known geometry  58  about Fe  B  was A and Fe .  Secondly, assignment (a) was rejected since the Q.S. was not large enough B for Fe  and the I.S. values l i e outside the range of I.S. which i s 37  c h a r a c t e r i s t i c of iron carbonyls  .  F i n a l l y , assignment (b) was  rejected  since the r e s u l t s did not accord with the predicted magnitude of the changes i n the I.S. i n going from f f a r s F e ( C 0 ) 4  2  6  to f ^ f o s F e ^ C O ) ^ and i n  going from f ^fosFe^CO)^ to f^fosFe (CO)g. Thus only assignment (c) gave 37 r e s u l t s i n accord with predicted behaviour 61 2  A s i m i l a r empirical approach was employed by de Beer ejt a l . i n assigning the four Mossbauer peaks a r i s i n g from the inequivalent i r o n atoms i n some Fe (CO)^L(SR> 2  AsPh^, e t c . ) .  2  compounds (R = a l k y l , a r y l , L = PPh , P(OMe) , 3  3  One possible assignment was rejected on the basis of i t s  unreasonable I.S. values while a second was rejected on this basis as w e l l , although the I.S. evidence was somewhat weaker i n the l a t t e r case.  The  fact that the t h i r d assignment produces one set of parameters which corresponds to the parent compound Fe (CO)^(SR) , as would be expected, i s good 2  2  evidence that t h i s choice i s the correct one. Nevertheless, the assignment of Mossbauer parameters by these  47  e m p i r i c a l approaches can sometimes l e a d to even t o i n c o r r e c t a method o f of  the  avoiding  lines w i l l  applied to  the  dicted  each of  for  results.  p r o d u c e d by  The m a g n e t i c p e r t u r b a t i o n  these d i f f i c u l t i e s .  lead to  sample.  If  the  the d i f f e r e n c e s  lines  Similarly,  if  a magnetic  assignment  final  i n Figure 12. only  assignment  experimental  f i t t e d experimental A s c a n be (c)  (a) (b)  is is  this  The m a g n e t i c  of  correct  this  stage, having is  similar  assignment of 7  confidence  necessary to  i s  prespectrum and  perturbation  purpose.  50kG t y p i c a l  spectra  are reproduced typical  to  (c)  case  compared  as  these that  in  spectra which  may b e  using assignment  3  f i e l d ^is  w i l l be u n i q u e  seen c l e a r l y by a comparison of  Thus the e a r l i e r  l i n e s has been made, i t  field  correct,  gives a pattern which i s  result.  assignments  enough t h e n the  These p a t t e r n s  result  s p e c t r a made o n a m o r e e m p i r i c a l b a s i s  At  large  field  w o u l d b e p r o d u c e d a r e shown i n F i g u r e 1 1 . w i t h the  possible  provides  the Q . S . a n d / o r I . S .  can be c o n f i r m e d .  assignment  technique  the  never before been used f o r  w h i c h w o u l d be p r o d u c e d i f F i g u r e 10.  in  to a magnetic  fgfarsFe2(C0)g in  and p o s s i b l y  p a t t e r n when a m a g n e t i c  the p o s s i b l e c h o i c e s a r e  technique has a p p a r e n t l y  For  Each of  a distinctive  a sample s u b j e c t e d  t h e a s s i g n m e n t of  difficulties  figures,  of  (c)  reproduced  the  to  similar  confirmed.  that  the  right  use q u a l i t a t i v e  pairing  of  the  arguments  in  order  A to  f i n d the  B Fe .  correspondence between s i t e s  Firstly,  in similar  no c a s e s o f  the  coordination  about Fe  I A  and I I is  cases w h i c h have s i x - c o o r d i n a t i o n  anomalously large  splittings  roughly  atoms F e  octahedral  and i r o n - i r o n  52 62 '  and i r o n  even f o r  and  and certainly  bonds t h e r e  systems  are  containing  48  arsines and phosphines^.  Thus, a r e l a t i v e l y small Q.S. i s expected f o r  A  Fe .  Secondly, by analogy to other complexes containing i r o n - i r o n bonds  and o l e f i n i c l i g a n d s ^ ' ^ the Q . S . f o r Fe^ i s expected to be large and 3  c e r t a i n l y f o r these compounds the r e l a t i o n that the |Q.S.| f o r Fe** i  A  i  >IQ.S.I f o r Fe should hold. A B to Fe and s i t e II to Fe .  These arguments imply s i t e I corresponds  As a confirmation of t h i s assignment, inspection of Table shows that the I.S.  of Fe  varies markedly as the ligands are changed from  f^fos to f f a r s while the I.S.:of Fe n  I  hardly v a r i e s .  result expected since s u b s t i t u t i o n of groups on Fe  This i s exactly the  would have only a  second-order e f f e c t on the parameters of Fe so that the assignment of the B A large Q.S. to Fe and the small Q.S. to Fe i s confirmed. As a byproduct of f i t t i n g the magnetic perturbation spectra, the sign of the Q.S. and the value of n at each of the s i t e s i s also found. As a further check on the assignment of the l i n e s and to investigate the systematics of the signs of the Q.S. and the values of r| with ligand s u b s t i t u t i o n the magnetic perturbation technique was also applied to the mixed ligand complex f^AsFFe^iCO)^ and to a phosphine only complex, f fosFe2(CO)g. 4  These spectra are shown i n Figures 13 and 14.  seen from inspection of these r e s u l t s , the sign of ri =0.6 f o r i r o n B while V ^ i s negative f o r iron A.  As can be  i s p o s i t i v e and The spectra are not  too s e n s i t i v e to ri f o r iron A so not too much s i g n i f i c a n c e should be attached to the value derived other than n. i s e i t h e r large or small. Unfortunately, the o r i g i n s of the e.f.g.'s at the two i r o n s i t e s i n these  FIGURE 10.  T y p i c a l Spectra Produced i f Assignment (a) Correct with S i t e I = 0.34 mm/sec, n = 0 and S i t e I I = 0.30 mm/sec, n = 0.  49 FIGURE 10.  SITE I = + 0.34 mm/sec, n = SITE II = + 0.30 mm/sec, n  SITE I = + 0.34 mm/sec, n = SITE I I = - 0.30 mm/sec, n  SITE I.= - 0.34 mm/sec, n SITE II = + 0.30 mm/sec, n  SITE I = - 0.34 mm/sec, n SITE II = - 0.30 mm/sec., n  I -2.0  T  -1.0  I  0.0 0.0  VELOCITY (mm/sec.)  +1.0  +2.0  FIGURE 11.  Typical Spectra Produced i f Assignment (b) Correct with Site I = 1.00 mm/sec, n = 0 and Site II = 1.06 mm/sec, n = 0.  FIGURE  VELOCITY  -  11.  mm/sec.  FIGURE 12„  Fe Mossbauer Spectrum of f^farsFe (CO) 2  an Applied Longitudinal Magnetic Field 50kG Showing Experimental Points and Theoretical F i t .  FIGURE 12.  F e M o s s b a u e rS p e c t r u mo ff fars Fe(C0) i nap a r a l l e l m a g n e t i c field o f 50 k G 6  -1.6  -0.8  0.0  0.8  V E L O C I T Y( m m / s e c )  2  1.6  6  2.4  FIGURE 13.  "*Fe Mossbauer Spectrum of f^AsPFe2(CO)g 7  an Applied Longitudinal Magnetic Field of 50kG Showing Experimental Points and Theoretical F i t .  F e M o s s b a u e r S p e c t r u m o f fA s P F e (C0) i na p a r a l l e l m a g n e t i c field o f 50 k G 4  Site A, Q.S. = -0.83 mm/sec ,77=0.8  V E L O C I T Y( m m / s e c )  2  6  FIGURE  FIGURE 14.  Fe Mossbauer Spectrum of f fosFe2(CO)g i n an 4  Applied Longitudinal Magnetic F i e l d of 50kG Showing Experimental Points and T h e o r e t i c a l F i t .  FIGURE 14.  Fe M o s s b a u e rS p e c t r u mo f ffos Fe (CO) in a p a r a l l e l m a g n e t i c field o f 5 0k G  5 7  4  Site A, Q.S. = -0.66 mm/sec,77= 0.6  -0.8  0.0  0.8  V E L O C I T Y( m m / s e c )  2  6  54  compounds are not known.  This problem i s discussed i n more d e t a i l  i n section E.  There are some i n t e r e s t i n g trends i n Q.S. the data i n Table I shows.  as examination of  F i r s t l y , the magnitude of the Q.S.  at Fe  in  compounds of the type f^AsPFe^CCO)^ i s larger than that of the symmetrical derivatives f fosFe_(C0), and f farsFe„(CO),. n Z o n z o  This sort of behaviour i s  not unexpected i n view of the differences i n a-donor and Tr-acceptor g a b i l i t i e s of phosphines and arsines.  Secondly, the Q.S.  of Fe  decreases  as the value of n increases i n each s e r i e s of compounds containing f f a r s , f AsP, and f f o s . n .n n  This trend correlates with the fact that as  the size of the f l u o r o a l i c y c l i c ring i s increased the s t e r i c requirements g at the carbons bonded to Fe  are reduced.  For example i n a four membered  r i n g , the angles C - C - C are expected to be close to 90° so when the r i n g size i s expanded the angles C - C - C are opened out. Thus changes g i n the geometry at the carbons bonded to Fe are expected to occur as the ring s i z e increases and t h i s may w e l l be r e f l e c t e d i n the Mossbauer paraB meters at Fe . Of course, changes i n the h y b r i d i z a t i o n at the carbon atoms g bonded to Fe may also be influencing the Q.S. i n such cases. The Q.S.  of fgfosFe2(C0)^ i s also of i n t e r e s t . In this compound, g the ligand i s apparently linked to Fe v i a the double-bond i n the usual A 48 manner while only one of the phosphines i s bonded to Fe g Table I shows the Q.S. while that of Fe  of Fe  Inspection of  remains at about 1.5 mm/sec. as expected  i s only about 0.2 mm/sec.  the l a t t e r case i s presumably  .  The decrease i n the Q.S. i n  due to the less severe s t e r i c requirements at  55  Fe  A 48  although  the nature of the i r o n - i r o n bond may change as w e l l 60  since the ligand constraints In a l l L F e ( C 0 ) 2  6  may not be as severe i n t h i s  case.  g complexes studied to date, the I.S. of Fe i s  A A greater than that of Fe . This implies the s-electron density on Fe i s g greater than that on Fe . To s a t i s f y the e f f e c t i v e atomic number (EAN) r u l e , Fe^ i s c o n s i d e r e d  as forming a dative bond to Fe**, so i t i s not s u r p r i s i n g g to f i n d that the electron density on Fe approaches, but i s not equal t o , 37  that on Fe . As mentioned above, there are only small v a r i a t i o n s i n the g I.S. of Fe since there i s no s u b s t i t u t i o n of groups on t h i s i r o n atom. In the compound f^fosFe,,(CO)j  only one phosphine i s bonded to  A 48 Fe  and i n going to fgfosFe (C0)g where one carbonyl i s replaced by a 2  A. phosphine there i s an increase of + 0.02 mm/sec. i n the I.S. of Fe .  This  i s approximately the value usually observed on s u b s t i t u t i o n of a carbonyl by a phosphine i n s i m i l a r compounds^ and r e f l e c t s the d i f f e r e n c e i n C-donor and TT-acceptor strengths between a phosphine and a carbonyl group. S i m i l a r l y , there i s an increase of about + 0.04 mm/sec. i n replacing a phosphine by an arsine i n going from f f o s to f AsP d e r i v a t i v e s . 61 n  this Is the usual e f f e c t observed  n  Again  , and r e f l e c t s the differences i n  bonding c h a r a c t e r i s t i c s between arsines and phosphines.  The only trend  which may be a b i t anomalous i s that observed on replacing the second phosphine by an arsine i n going from ^AsF' to f ^ f a r s complexes, where the  A, increase i n I.S. at Fe  varies from + 0.01 to + 0.04 mm.sec. In t h i s g case, there seems to be a c o r r e l a t i o n between the Q.S. of Fe and the I.S.  56  A A change at Fe , since a small change i n I.S. at Fe i s accompanied by a B A small change i n Q.S. at Fe while a large change i n I.S. at Fe i s B accompanied by a large change i n Q.S. at Fe . Whether t h i s i s a consequence A B of the bonding between Fe and Fe or of the geometry adopted by the ligand A B at both Fe and Fe i s not clear.  The  CO stretching frequencies  i n these compounds are also of  interest and the values f o r the f ^ f o s , f ^ f o s , and f ^ f a r s derivatives have 37 been reported previously  .. As expected, i n a l l LFe2(C0)^ derivatives  there are s i x CO stretching frequencies.  Three of these are r e l a t i v e l y  i n s e n s i t i v e to ligand s u b s t i t u t i o n , the change i n frequency ( A V ^ Q ) being -1 45 less than 7cm i n a l l the compounds reported , while the other three vary markedly with ligand s u b s t i t u t i o n (Av = 12 - 22cm "*"). f fars derivatives have V for f fos d e r i v a t i v e s . n  n  at lower frequencies  The f AsP and R  than the equivalent  V  This correlates with the differences i n a-donor  and ir-acceptor c h a r a c t e r i s t i c s of phosphines and arsines.  In summary, a generally useful a p p l i c a t i o n of the magnetic perturbation  technique to the removal of ambiguities i n the assignment of  Mossbauer s p e c t r a l parameters i n i r o n carbonyl complexes having two d i f f e r e n t i r o n s i t e s has been investigated.  As a consequence the signs of the Q.S. and magnitude of the asymmetry parameters of some LFe^CCO)^ derivatives have been measured.  The  Mossbauer s p e c t r a l parameters of some new LJi^CCO)^ derivatives have also been measured and the r e s u l t s interpreted i n terms of the known structure of f f a r s F e ( C O ) g . 4  2  57  (B)  L LFe (CO) m  2  5  The next class of compounds which i s of i n t e r e s t i s that of the general type L LFe (CO)^, where L™ i s a monodentate ligand such as m  2  Ph^P or (PhCO^P  or a p o t e n t i a l l y bidentate or terdentate ligand such as  f^AsP or dppp (Figure 7) which i s coordinating v i a only one of i t s possible s i t e s .  The compounds are regarded as derivatives of LFe (C0), z o o  where one carbonyl i s replaced by a monodentate group.  These compounds  may be recognized as a class by chemical analysis and by t h e i r d i s t i n c t i v e I.R. patterns i n the carbonyl stretching region which show three bands i n the ranges v^^ = 2035 - 2044 cm , -1 44,45 1970 cm ' -1  v  = 1975 - 1990 cm"  1  2  and v  3  = 1954 -  1 Q 7 n  A t y p i c a l Mossbauer spectrum of one of these d e r i v a t i v e s , namely (PhO) Pf AsPFe (C0) , i s i l l u s t r a t e d i n Figure 9, and the s i m i l a r i t y of the 3  4  2  5  Mossbauer spectra of these derivatives to those of the LFe„(C0), complexes z o i s readily apparent.  For the two i r o n s i t e s , I and I I , the possible  assignments of the s p e c t r a l parameters are the same as f o r those discussed f o r LFe (C0)g complexes 2  (Table I I ) .  Again, the magnetic perturbation  spectrum (Figure 15) confirms that (c) i s the v a l i d assignment.  On the  basis of the s i m i l a r i t y between the Mossbauer parameters of i A F e ^ C O ^ and LFe (C0)g d e r i v a t i v e s , l i n e s 1 and 4 are assumed to a r i s e from Fe 2  l i n e s 2 and 3 from Fe .  and  Using this as a b a s i s , the s p e c t r a l parameters of  a number of such derivatives are presented i n Table I I I .  FIGURE 15.  57  F e Mossbauer Spectrum of (PhO) Pf AsPFe (CO) 3  4  2  5  in an Applied Longitudinal Magnetic Field of 50kG Showing Experimental Points and Theoretical F i t .  FIGURE  15.  F e M o s s b a u e r S p e c t r u mo f [(Ph0) P]f AsPFe (C0) in a p a r a l l e l m a g n e t i c field o f 5 0k G 3  -1.6  -0.8  0.0  0.8  V E L O C I T Y( m m / s e c )  4  1.6  2  5  TABLE I I I . MOSSBAUER PARAMETERS AT 80°K FOR L ^ F e ^ C O ^ COMPOUNDS  **  AE„ (mm/sec.) —Q 1.38 ± 0.03 0.56 ± 0.03  <5(mm/sec.)  r(mm/  0.30 + 0.02 0.35 + 0.02  0.27, 0.31 0.27, 0.32  B A  dppp f^AsP  1.28 ± 0.04 0.59 ± 0.04  0.31 + 0.02 0.32 + 0.02  0.25, 0.39 0.33, 0.35  B A  This work  f.AsP f,fars 4 4  1.35 + 0.02 0.44 ± 0.02  0.30 + 0.01 0.34 + 0.01  0.22, 0.26 0.24, 0.26  B A  This work  +1.51+ ± 0.03 -0.58 ± 0.03  0.29 + 0.02 0.29 + 0.02  0.31, 0.30 0.29, 0.32  B A  This work  (PhO) P fgfars  1.26 ± 0.02 0.50 + 0.02  0.31 + 0.01 0.29 + 0.01  0.26, 0.25 0.23, 0.26  B A  This work  (PhO) P f fars  1.45 ± 0.03 0.64 ± 0.03  0.31 + 0.02 0.32 + 0.02  0.25, 0.28 0.29, 0.25  B A  This work  (Ph) Sb f fos  1.27 ± 0.02 0.17 ± 0.03  0.30 + 0.01 0.38 + 0.02  0.24, 0.23 0.27, 0.28  B A  This work  L L m  Ph.P„m.f.AsP 3 4 m  m  (PhO) P f AsP m  3  4  m  3  m  3  4  m  3  4  sec.)  IRON SITE  Relative to sodium n i t r o p r u s s i d e .  **  t  Experimental uncertainty, ± 0.02 mm/sec Sign determined with source and absorber at 4.2°K (see Table VI).  REFERENCE 44,47  60  Examination of the I.S. data i n Table I I I shows that the I.S. of Fe  increases by approximately + 0.01  of a carbonyl by a phosphine group.  to + 0.08 mm/sec. on s u b s t i t u t i o n  Since the I.S. of Fe  i s less  s e n s i t i v e to ligand s u b s t i t u t i o n (the change i s - 0.01 to - 0.03 mm/sec.) i t i s reasonable to conclude that s u b s t i t u t i o n takes place on Fe . expected, the increase i n the I.S. of Fe (PhO),J? group i s somewhat less (+ 0.01  As  on substituting a carbonyl by a  to + 0.04 mm/sec.) than that  observed on substituting a Ph^P or dppp group  (+ 0.05  to + 0.08  mm/sec).  This i s reasonable since the a , TT c h a r a c t e r i s t i c s of the triphenyl phosphite group are expected to be more s i m i l a r to CO than those of Ph^P or dppp and thus the I.S. should show less change. increases i n the I.S. of Fe  The much larger  observed on replacement of a CO by a phosphine  i n these compounds, r e l a t i v e to that observed between f,fosFe_(CO) and n  b  I  I  fgfosFe2(C0)g, may be ascribed to a saturation of the t o t a l u-bonding capacity of the ligands i n the L LFe2(C0),- complexes, as has been suggested m  for some sulphur bridged iron carbonyls^''". This i s not unreasonable since i n going from LFe (C0)^ to LJj^CCO)^ any buildup of d-electron density at 2  A Fe  due to s u b s t i t u t i o n of a carbonyl may be p a r t i a l l y compensated by  increased d e r e a l i z a t i o n of t h i s d-electron density onto the three remaining carbonyl groups v i a metal*ligand TT-donation. Thus the increase i n the I.S. of Fe^ i s small.  However, i n the L LFe2(C0)^ complexes, A.  CO groups remaining on Fe  m  there are only two  so the degree to which excess d-electron density  may be delocalized i s somewhat reduced and the increase i n d-density at Fe i s larger than f o r the LJi^CCO)^ complexes. increase i n the I.S. at Fe .  This accounts f o r the larger  61  Normally, the increase i n i r o n I.S. for arsenic and antimony complexes are expected to be s i m i l a r to and larger than those for the corresponding phosphorus d e r i v a t i v e s ' ^ .  The increase i n the I.S. of  7  Fe  A  i n going from f ^ f a r s F e ( C 0 > 2  to f A s P f ^ f a r s F e ( C 0 > m  6  4  2  5  i s only + 0.06  mm/sec., compared to an increase of + 0.08 mm/sec. f o r Ph.jP f ^AsPFe^CO) m  r e l a t i v e to f^AsPFe^CCOg, while the increase f o r the Ph^Sb derivative i s + 0.15 mm/sec.  On t h i s b a s i s , i t i s reasonable to suppose f^AsP i s  coordinated through phosphorus and not arsenic.  A d d i t i o n a l support f o r  t h i s conclusion comes from the X-ray study"'*' of f A s P F e ( C 0 ) m  4  4  which has  a t r i g o n a l bipyramidal structure i n which the ligand i s monodentate and coordinated through phosphorus.  Also, the chemical evidence from this  and other s t u d i e s ^ shows that d i t e r t i a r y arsines form weaker chelates than d i t e r t i a r y phosphines  so that any reaction involving competition between  these products i s expected to favour formation of the phosphorus-bonded derivative.  Unfortunately, i t was not p o s s i b l e to prepare enough of a monodentate arsenic d e r i v a t i v e for i t to be characterized by Mossbauer spectroscopy, although I.R. spectra of two such complexes were recorded  45 and are very s i m i l a r to the corresponding phosphorus compounds  . The  i n s t a b i l i t y of such complexes presumably arises because the Fe - As bond i s much weaker than the Fe - P bond i n these compounds.  Interestingly  enough, i t was possible to characterize a monodentate s t i b i n e d e r i v a t i v e , namely P h . j S b f f o s F e ( C 0 ) , whose I.R. pattern corresponded m  4  other derivatives.  2  closely to the  In Ph^Sb"^^fosFe,,(CO),-, the Q.S. i s much reduced over  62  A  the corresponding Ph^P derivative and the increase in the I.S. of Fe on substitution i s much larger (Table III). The increase of electron density on Fe  in  l!\Fe2  ( CO)  5  complexes over that in IJ^faO)^ derivatives should make i t a more effective a-donor.  Molecular orbital calculations on such systems as  M ^ ^ O ) ^ ^ have shown that the metal-metal bond is predominantly 0" in character and i t is reasonable to suppose that such a situation might hold for the Fe — Fe bond in the present series of compounds.  If indeed  a-bonding is important then the hybrid orbitals involved on Fe  should  be dsp  3  2  or d sp  3  or some intermediate hybridization, while for Fe 2  A  they  3  should be close to d sp .  Thus, an increase in the a-donor strength of  A B Fe should lead to an increase in the s-electron density on Fe . decrease in the I.S. of Fe  The  then follows as a natural consequence of the  increase in electron density on Fe . The Q.S.  parameters are also of i n t e r e s t .  From the r e s u l t s of  the magnetic perturbation experiment (Figure 15) i t i s seen that the sign  A  B  of the Q.S.  for s i t e s Fe  and Fe  remains unchanged from the previous case  LFe2(C0)g.  In general the magnitude of the Q.S.  of Fe  i s reduced over the  corresponding complexes of the type LFe2(C0)g and i n p a r t i c u l a r the reduction i s very large i n the Pb^Sb d e r i v a t i v e . the change i n the Q.S.  of Fe  These r e s u l t s imply that  a r i s e s c h i e f l y from the differences i n O - and  TT-bonding a b i l i t i e s of the substituents r e l a t i v e to CO and although geometry changes which follow on ligand s u b s t i t u t i o n play some r o l e i n determining  63  the precise value of AE^ they are not the major factor. the complexes the Q.S. of Fe  B  For most of  B i s somewhat smaller than that of Fe i n  the corresponding LFe^CCO)^ complexes and since  i s p o s i t i v e and  corresponds to electron deficiency along the Z axis, any increase i n electron density along the Fe - Fe bond should lead to a decrease i n g the Q.S. at Fe (provided that i s directed more or less along the Thus the expected increase i n a-donor power of Fe  Fe - Fe bond).  owing  to i t s greater electron density should lead to a decrease i n the Q.S. at g Fe as observed i n most cases. However, geometrical factors may also B play some r o l e since the Q.S. of Fe  i s increased s l i g h t l y i n two cases.  Thus while the I.S. acts as a f a i r l y s e n s i t i v e i n d i c a t o r as to which i r o n i s being substituted, the Q.S. parameters are a much less r e l i a b l e guide. A s i m i l a r s i t u a t i o n apparently exists i n t r i i r o n c l u s t e r complexes with d i t e r t i a r y arsines of the type LFe^CCO).^^. One of the most i n t e r e s t i n g questions about the structures of in, A L LJi^CCO)^ derivatives i s which of the carbonyl groups on Fe has been replaced.  There are two carbonyl groups c i s to the i r o n - i r o n bond and  one carbonyl group trans to this bond.  At this stage, we would not expect  to be able to d i s t i n g u i s h between a substituent replacing e i t h e r c i s carbonyl but we might expect to be able to d i s t i n g u i s h between c i s and trans s u b s t i t u t i o n . In a case i n which c i s substitution i s presumed to occur, namely b f o r L LFe2(C0)  4  complexes (see below) the Q.S. of Fe  increased and i s p o s i t i v e i n sign.  A  i s dramatically  Although there may be geometric factors  64  at work as w e l l i n L LFe2(CO) b  in  4  complexes nevertheless  cis substitution  (CO),- could probably be ruled out on these grounds since the A  Q.S.  of Fe  remains small and negative.  The Mossbauer evidence i s not c  unequivocal on this point, however, since i n the case of L  IJ^^O)^  complexes (see below) there i s both c i s and trans s u b s t i t u t i o n of carbonyls  at Fe^ ^  but no large increase i n the Q.S.  at t h i s i r o n atom.  Other evidence would seem to favour trans s u b s t i t u t i o n . there are only three CO bands i n the I.R. 44 CH2CI2 solution  In p a r t i c u l a r ,  spectra of these derivatives i n  45 *  and, f o r those compounds which have been studied, 45  only three i n the s o l i d state as w e l l  .  This implies that the  two  A. carbonyls on Fe are equivalent or nearly so, and the structure with the highest symmetry puts the two CO's c i s to the i r o n - i r o n bond. The s t r u c t u r a l study of f A s P f A s P F e 2 ( C O ) ^ shows that phosphorus i n the C  4  4  4  c f^AsP  ligand i s substituted trans to the i r o n - i r o n bond while arsenic  is c i s .  Since i t i s u n l i k e l y that there i s any major rearrangement i n  going from f A s P L F e ( C O ) m  4  2  to f A s P L F e ( C O ) C  5  4  2  4  this implies that the phos-  phorus substitutes trans (as stated above) and then the arsenic displaces an a d d i t i o n a l CO to form the tetracarbonyl complex.  Thus, chemical evidence  also favours trans s u b s t i t u t i o n . In summary, Mossbauer spectroscopy has established that i n m complexes of the type L LFe2(C0)^ one  carbonyl on Fe  LFe2(C0)g i s replaced by a monodentate group. I.R.  A  i n the parent  Further, both Mossbauer and  evidence favour s u b s t i t u t i o n trans to the i r o n - i r o n bond and both  Mossbauer and chemical evidence e s t a b l i s h that i n f A s P f f a r s F e 2 ( C O ) ^ m  4  f^AsP  i s bonded' through phosphorus,  4  the  65  (C)  L°LFe (CO) 2  4  The next class of derivatives of LFe„(CO), which has been Z  studied i s  b  c c of the general formula L LFe (CO)g where L i s a chelating 2  bidentate ligand (Figure 7) which i s not necessarily the same as L. The structure of one member of this class of compounds, namely f AsP°f AsPFe (CO) , has been determined by X-ray d i f f r a c t i o n ^ 4  4  2  4  i s i l l u s t r a t e d i n Figure 16.  and  A comparison of t h i s structure to that of  58 f farsFe (CO)g 4  2  (Figure 8) i s of i n t e r e s t .  iron bond i s f a i r l y long, being 2.869 A* d e r i v a t i v e * ^ compared to 2.89  A*  In both compounds, the i r o n -  i n the fAsP°f^AsPFe,, (CO)^ 4  in f ^ f a r s F e ( C O ) T h e 2  equivalence  of the i r o n - iron bond lengths i n these two derivatives has been i n t e r preted as i n d i c a t i n g that l i g a n d constraints play a major r o l e i n determining the i r o n - iron distance The major s t r u c t u r a l d i f f e r e n c e , other than the s u b s t i t u t i o n of an f^AsP f o r an f ^ f a r s bridge between the two i r o n s , i s the replacement of two  carbonyls on Fe  by a chelating f^AsP.  As mentioned above and as can  be c l e a r l y seen i n Figure 16 the phosphorus i s bonded trans to the i r o n iron bond, while the arsenic i s c i s to t h i s bond.  Compounds of the type L LFe^CO)^ may  be recognized as a group  by t h e i r chemical analyses and by t h e i r d i s t i n c t i v e I.R.  patterns i n the  45 CO stretching region  .  There are two closely related classes of  I.R.  frequencies one of which shows three absorption bands and one of which shows four absorption bands.  Each class has two high frequency bands i n  FIGURE  The S t r u c t u r e o f  16.  f.AsP f,AsPFe,(CO) C  FIGURE 16.  FIGURE 17.  57  b Fe Mossbauer Spectra of f^fos f^fosFe^ (CO)^ and f AsP f farsFe (CO)^ C  4  4  2  67  (f fos)f fosFe (CO) 4  4  2  4  o  o  CO CO CO < cr  (tAsP) t f a r s F e X O l 1.00  0  1  VELOCITY (mm/sec) FIGURE 17.  OCD o  J?o°cfL&oo  TABLE IV. M'6SSBAUER PARAMETERS A  80°K FOR L° L Fe (C0). COMPOUNDS o  IRON SITE  A,EQ (nm/sec)  cS (mm/sec)  r (nm/sec)  LINE ASSIGNMENT  1.07 + 0.01 0.61 + 0.01  0.28 + 0.01 0.50 + 0.01  0.28, 0.28 0.27, 0.28  1,3 2,3  B A  45  • 1.05 + 0.02* 0.61 + 0.02  0.28 ± 0.01 0.50 + 0.01  0.30, 0.30 0.29, 0.30  1,3 2,3  B A  This work  f.AsP f.AsP 6 4  1.07 + 0.01 0.67 + 0.01  0.29 + 0.01 0.49 + 0.01  0.27, 0.27 0.27, 0.27  1,3 2,3  B A  f AsP f AsP  0.97 + 0.01 0.60 + 0.01  0.28 + 0.01 0.47 + 0.01  0.28, 0.33 0.28, 0.33  1,3 2,3  B A  45  0.96 ± 0.02 0.59 ± 0.02  0.29 ± 0.01 0.47 ± 0.01  0.23, 0.28 0.27, 0.28  1,3 2,3  B A  This work  f,AsP f,fars 4 6  1.15 ± 0.02 0.73 ± 0.02  0.30 ± 0.02 0.52 ± 0.02  0.24, 0.29 0.26, 0.29  1,3 2,3  B A  This work  f,fars°f.AsP 6 4  0.91 ± 0.03 0.40 ± 0.03  0.27 + 0.02 0.53 ± 0.02  0,31, 0.36 0.26, 0.36  1,3 2,3  B A  This work  fgfars fgfars  0.99 ± 0.02 0.65 + 0.02  0.32 ± 0.02 0.49 + 0.02  0.25, 0.28 0.22, 0.28  1,3 2,3  B A  This work  diars f .AsP  0.86 ± 0.02 0.45 ± 0.02  0.28 ± 0.02 0.50 ± 0.02  0.29, 0.32 0.32, 0.32  1,3 2,3  B A  This work  L L C  f,AsP f,AsP 4 4 C  T  C  C  6  6  C  c  REFERENCE  44,47  CONTINUED/....  TABLE IV (CONTINUED) MOSSBAUER PARAMETERS AT 80°K FOR L° L F e ( C O ) 2  *  4  **  COMPOUNDS  6 (mm/sec)  r(mm/sec)  LINE ASSIGNMENT  IRON SITE  0.95 + 0.02 0.36 + 0.02  0.31 + 0.01 0.46 + 0.01  0.25, 0.22 0.22, 0.30  B A  This work  2,3  0.80 + 0.02 0.51 + 0.02  0.24 + 0.01 0.53 + 0.01  0.25, 0.30 0.22, 0.22  1,3 2,4  B A  This work  f.AsP f f a r s 4 8  1.01 + 0.02 0.64 + 0.02  0.30 + 0.02 0.46 + 0.02  0.23, 0.26 0.24, 0.26  1,3 2,3  B  This work  A  diphos f^AsP  1.08 + 0.04 0.59 + 0.04  0.27 + 0.03 0.51 + 0.03  0.34, 0.46 0.36, 0.46  1,3 2,3  B  1.06 + 0.02 0.64 + 0.02  0.28 + 0.02 0.49 + 0.02  0.21, 0.32 0.27, 0.32  1,3 2,3  B A  1.10 + 0.03 0.62 + 0.03  0.25 + 0.02 0.49 + 0.02  0.26, 0.36 0.24, 0.36  1,3 2,3  B A  AEQ  L L C  dppm f^AsP  dppm f AsP C  +  +  4  Q  fgfos f AsP 4  f.fos f.AsP 4 4  (mm/sec)  Relative to sodium nitroprusside. Experimental uncertainty, ±0.02 mm/sec. Sign determined with source and absorber at 4.2°K, see text and Table VI. Other l i n e assignment.  REFERENCE  This work  A  This work  This work  70  the range v^^ = 2002 - 2013  cm"  and v  1  2  = 1939 - 1952  cm  1  4 5  .  For the  majority of compounds, there are only three absorption frequencies with the t h i r d one l y i n g i n the range V  3  = 1903 - 1920  cm"'". However, there 1  are a few compounds which have four absorption frequencies, the components at low frequency l y i n g i n the range and  = 1906 - 1908  cm  —1  .  = 1917  two  - 1921 cm  Since the l o c a l symmetry at Fe  B  1  i s not very  high and should be easily broken i t i s not s u r p r i s i n g that this sort of behaviour i s observed.  The Mossbauer spectra of these d e r i v a t i v e s are also very distinctive  since f o r most of the compounds there are three component  peaks of r e l a t i v e area 1:1:2.  A t y p i c a l spectrum i s i l l u s t r a t e d i n  Figure 17 while the Mossbauer parameters are tabulated i n Table IV.  For  some of the compounds, the most intense peak i s somewhat broadened and i n one case (dppm f^AsPFe^CO)^) the two components have been resolved.  For those compounds with three peaks there are two possible assignments f o r the three l i n e s , v i z . :  (a) l i n e s 1 and 2 to s i t e I,  l i n e 3 to s i t e I I , i f l i n e 3 i s assumed to be a s i n g l e t ;  (b) l i n e s 1 and  3 to s i t e I, l i n e s 2 and 3 to s i t e I I , i f l i n e 3 consists of two lapping components.  Again, magnetic perturbation techniques allow us to  decide which of these p o s s i b i l i t i e s i s v a l i d .  From the appearance of the  magnetic perturbation spectrum of f^AsP f^AsTFe^iCO)^ reject  over-  i t was  possible to  assignment (a) and the parameters i n Table IV have been derived on  the basis of assignment (b).  Unfortunately, i t was not possible to achieve  a very good f i t of the spectrum.  For some reason, two of the components of  71  the magnetic perturbation spectrum appear to be too intense r e l a t i v e to the t h e o r e t i c a l ones, so i t was not possible to f i n d values of the Q.S., T and n which would f i t the spectrum exactly.  However, the f i t  became noticeably worse except when the sign of the larger Q.S. was p o s i t i v e and n. f o r this s i t e was zero.  S i m i l a r l y , for the smaller Q.S.,  the sign was apparently negative although the f i t of the spectrum d i d not improve s i g n i f i c a n t l y as X] was varied.  Despite these problems, the decision as to whether the paraA B meters of s i t e I should be assigned to Fe or Fe i s r e l a t i v e l y easy to make.  Since i n f^AsP f^AsPFe2(CO)^ we know that only the carbonyls on  A. Fe Fe  A  are being replaced then i n view of the changes i n the parameters of m i n ^ ^ ( C O ) ^ complexes and L LFe2(C0)^ complexes on ligand  substitution we expect rather large changes i n Fe , p a r t i c u l a r l y i n the I.S., while f o r Fe , the changes i n the parameters should be somewhat smaller.  Thus, the assignment of the larger Q.S. to Fe  gives A E ^ = + 1.05  mm/sec. and 6 = 0.28 mm/sec. and so both the I.S. and Q.S. are i n good agreement with our previous r e s u l t s f o r LFe2(C0)g and L LFe2(C0)^ compounds, m  A The assignment of the smaller Q.S. to Fe  i  i  gives | A E Q | =  0.61 mm/sec. and  6 = 0.50 mm/sec. with a large increase i n I.S. as expected. The Mossbauer parameters of these compounds, excluding dppm f AsPFe (C0) , C  4  2  4  f a l l i n the range | A E | = 0.40 - 0.67 mm/sec,  6 = 0.46 - 0.53 mm/sec. f o r F e while f o r F e , | A E | = 0.86 - 0.15 mm/sec, A  6 = 0.25 - 0.32 mm/sec.  B  I n t e r e s t i n g l y , f o r dppm f A s P F e ( C O ) where the 4  2  4  four component l i n e s could be resolved, the parameters are such that i t i s  72  d i f f i c u l t to choose between the two possible assignments of the l i n e s since both give parameters (Table IV) which are i n reasonable agreement with the ranges given above.  A number of features of these spectra are worthy of comment. A.  At  this stage, since there i s only one CO l e f t on Fe  increase i n the isomer s h i f t of Fe  a rather large  over the corresponding i r o n atom i n  related LFe2(C0)g and L LFe2(C0)^ derivatives i s expected and indeed t h i s m  i s observed.  This phenomenon arises because the CO groups, which are very  e f f e c t i v e TT-acceptors, act as electron "sinks" i n withdrawing electron density from the metal.  When CO i s replaced by a less e f f e c t i v e TT-acceptor,  t h i s electron density now becomes l o c a l i z e d on the metal atom.  This sort  of behaviour accounts f o r the f a i l u r e of a p a r t i a l isomer s h i f t model f o r 7 A compounds of t h i s type . I t i s of i n t e r e s t to note that the I.S. of Fe seems to be independent of whether the ligands are phosphines or arsines. For  A c c example, the I.S.'s of Fe i n fgfos f AsPFe (CO)^ and fgfars fgfars 4  Fe2(C0)  4  are i d e n t i c a l despite the fact that there are three phosphines and  one arsine bonded to Fe to Fe  2  i n the l a t t e r .  i n the former while there are four arsines bonded  The large increase i n I.S. (+.10 to .15 mm/sec.)  expected on replacement of the penultimate carbonyl i n going from L LFe (C0) . m  o  to L°LFe2(C0) , as. mentioned above, may swamp out the smaller v a r i a t i o n s 4  due to the differences i n a- and TT-bonding c h a r a c t e r i s t i c s of phosphines and arsines.  The I.S. of Fe  i n t h i s series of compounds varies much more  than i t does i n e i t h e r I J ^ ^ O ) ^ or if^LFe^W)$  compounds.  The values  1  73  range from 0.32 mm/sec. i n fgfars f g f a r s F e ^ ( C O ) ^ ( p r a c t i c a l l y unchanged from LFe (C0)g compounds), to 0.25 mm/sec. i n f^fos f^AsPFe^CO)^. 2  Since  the decrease i n 6 g i n L LFe„(C0) . compounds was a t t r i b u t e d to a-bonding Fe . m  [  between the metal moities i t i s rather s u r p r i s i n g to f i n d that there are compounds of the type L LFe^CCO)^ which have p r a c t i c a l l y no decrease i n g the I.S. at Fe r e l a t i v e to the corresponding LFe„(C0),. compound even L o though the I.S. of Fe  becomes very large and i t s a-donor power i s  increased. There are a number of possible r a t i o n a l i z a t i o n s f o r t h i s behaviour, but as Table IV shows, f o r every compound f o r which t h i s . apparent behaviour i s observed, there i s s i g n i f i c a n t l i n e broadening of the most intense component.  I f the e f f e c t of t h i s l i n e broadening i s to g increase the apparent value of the I.S. of Fe then c o r r e c t i o n f o r this B behaviour gives I.S. f o r Fe which are less than or equal to 0.30 mm/sec. This argument also implies that the second assignment f o r the s p e c t r a l parameters f o r dppp f AsPFe (CO)^ i s the correct one since i t gives an g 4  2  I.S. f o r Fe which i s less than 0.30 mm/sec.. The constraints on the Fe Fe bond length imposed by the bridging ligand l i m i t s the effectiveness of the o r b i t a l overlap between the two irons and probably plays a major role g i n determining the extent of the I.S. decrease at Fe . As w e l l , there B A may be h y b r i d i z a t i o n changes i n the o r b i t a l s Fe i s using to bond to Fe . B 58 A c l o s e r inspection of the geometry at Fe i n f f a r s F e ( C 0 ) ^ 4  (Figure 8) and f ^ A s P ^ A s P F e ^ C O ) ^  6 0  2  (Figure 16) reveals that there are no  74  major changes i n the bond angles of the groups bonded to Fe • example, the angle Fe  A  - Fe  B  For  o - midpoint C = C which d i f f e r s by only 3  between these two compounds represents one of the major changes i n B geometry at Fe .  The angle C - Fe  B  - C, where the C's o r i g i n a t e from  the double bond, remains v i r t u a l l y unchanged between these two compounds 37 and so the assumption  that these angles play a major r o l e i n deterg  mining the Q.S.  at Fe  i s apparently untrue.  „ , J 58,60 . . From these c r y s t a l structure data then i t i s apparent g that the changes i n geometry at Fe are too small to account f o r the g large reduction i n the Q.S. at Fe i n these compounds r e l a t i v e to those i n equivalent complexes of the type LFe^CCO)^ and L LFe2(CO),.. m  m discussed above f o r L LFe2(C0)^ complexes, since  As  B i s p o s i t i v e f o r Fe ,  and i f the z-axis defined by V„„ i s more or less i n the d i r e c t i o n of the Fe - Fe bond then any increase i n electron density along the Fe - Fe bond g w i l l tend to reduce the Q.S.  at Fe .  This would seem to be the most  l i k e l y explanation of the observed behaviour. The Q.S.  at Fe  i n these compounds has about the same range of  values as observed f o r L UJ^CCO),. complexes (except f o r Ph Sb f f o s F e ( C 0 ) ) 3  Again, the sign of the Q.S. Fe (C0) 2  I A E Q |i A As"~A^s.  4  at Fe  as i t i s i n L F e ( C 0 ) 2  A  2  5  c i s apparently negative i n f^AsP f^AsP  and L L F e ( C 0 ) ^ complexes. m  6  4  2  Interestingly,  c c S 0.4 mm/sec. i n complexes L f AsPFe2(G0)^ when L i s of the type 4  In these same complexes | A E Q | ^ i s also quite small.  The explanation  for this behaviour i s not clear e s p e c i a l l y when i t i s considered that i n  75  fgfars fgfarsFe (CO)g, | A E Q | = 0.65 mm/sec. 2  I A E , Q. i A i n complexes 1  c of the type L f AsPFe_(C0), r e l a t i v e  n  to complexes of the type f AsPFe (CO)g. n  There i s also a n o t i c e -  2  /  4  The explanation of t h i s  phenomenon probably l i e s i n the somewhat anomalously high values f o r the corresponding f AsPFe (CO)^ d e r i v a t i v e s . n  2  Considering the Q.S. at Fe  for dppm f AsPFe (CO)^ we see that a l l the complexes of the type 4  2  L f AsPFe (CO) C  n  2  4  where L° i s a P"P or As~P then  |AE^|  l i e s i n the range  A  i  0.59  i  A  - 0.67 mm/sec. so i t i s extremely u n l i k e l y that | A E Q | f o r  dppm f AsPFe (CO)^ could be 0 . 3 6 mm/sec.. Thus, the other assignment of 4  the  2  s p e c t r a l l i n e s must be the v a l i d one - the same conclusion reached  above on the basis of I.S. data. Fe  So, although the o r i g i n of the Q.S. at  i n these compounds i s not clear there are c e r t a i n l y no large changes  i n the Q.S. at Fe  on s u b s t i t u t i o n both c i s and trans to the i r o n - i r o n  bond. In summary, the Mossbauer and I.R. data show that a l l the complexes of the type L L F e ( C O ) 2  4  have structures s i m i l a r to the known  structure of f A s P f A s P F e (CO)^^. C  4  4  2  Magnetic perturbation techniques  have helped to e s t a b l i s h which i r o n s i t e gives r i s e to which absorption 58  peaks.  The known c r y s t a l structures of f ^ f a r s F e ^ C O ) ^  and  f A s P f A s P F e ( C 0 ) ^ have been used t o demonstrate that i t i s the buildup C  4  4  2  4  A B of electron density at Fe and not geometry changes at Fe which deterB mines the decrease i n Q.S. at Fe . remain  A The origins of the Q.S. at Fe  unclear, although a few trends are noted.  76  (D)  L LFe (CO) b  2  4  There remains one other type of derivative of LFe„(CO)> /  which has not yet been discussed.  o  These compounds have the general  formula l^LFe^iCO) ^ and may be distinguished from complexes of the type L LFe^CCO)^ by the fact that they have four I.R. active CO stretching frequencies which occur at distinctly different frequences from those of L LTe^iCO)^. v  The ranges of the CO frequencies are as follows:  = 1982 - 1988 cm" ,  v  = 1870 - 1902  The Mossbauer spectra of these complexes (Table V)  1  cm . 1  = 1932 - 1948 cm" , 1  £  V  = 1914 - 1923 cm" , 1  3  and  are also very different from those of any other derivatives reported here. Most of the spectra consist of two broadened lines as illustrated in Figure 17 for f ^ f o s f o s F e 2 ( C O ) ^ although occasionally three or four lines b  could be resolved.  Details of the line assignments in Table V w i l l be  discussed below. Since these fluoroalicyclic olefinic ligands are so versatile 47 and may  act as monodentate, bidentate, or terdentate groups  to examine these possibilities in detail.  i t is necessary  F i r s t l y , since i t i s d i f f i c u l t  to establish the composition of carbonyls of this type exactly by chemical analysis, i t is necessary to establish whether alternative formulations such as L^LFe^CCO)^ or L LFe2(C0) are possible. b  3  F i r s t l y , since there are four  CO stretching frequencies, i t is unlikely that the L acting as terdentate groups.  b  ligands could be  Moreover, ligands such as dpam and arphos,  which have similar I.R. and Mossbauer parameters to the rest of the  TABLE V. MOSSBAUER PARAMETERS AT 80°K FOR L L F e ( C 0 ) b  2  *  4  COMPOUNDS.  + LINE T (mm/sec) ' , r(mm/sec) ASSIGNMENT  IRON SITE  AEQ(mm/sec) AE^ (mm/sec)  6 (mm, 6" (mm/sec) ^sec)  1.48 ± ± 0.02 1.26 ± ± 0.02  0.41 0.30  + 0.01 ± + ± 0.01  0.28, 0.26 0.28, 0.29  1,3 2,3  B A  0.36 0.36 0.36 0.35 0.35  + +  0.32, 0.32. 0.32. 0.34, 0.34,  0.45 0.45 0.45 0.44 0.44  1,2 1,2 1,2  A  f AsP f AsP  1.21 1.21 1.21 1.30 1.30  1,2 1,2 1,2  A B B A  f AsP f fos  1.48 ± 0.03 1.48 ±± 0.03 0.03 1.35  0.39 + 0.02 0.39 ± 0.02 + 0.02 0.32  0.29, 0.26 0.29, 0.30 0.26 0.29,  1,3 1,3 1,2  A  B B  This work  dppp f AsP  1.84 ± 0.02 1.84 ± 0.02 1.28 ± 0.02  0.39 + 0.01 0.39 0.01 + 0.01 0.31 ±  0.23, 0.24 0.23, 0.25 0.24 0.25,  1,4 1,4 2,3  A  B B  This work  dppp f AsP  1.48 ± 0.02 1.48 ± ± 0.02 0.02 1.64  0.21 + 0.01 0.21 ± 0.01 ± 0.01 0.49  0.23, 0.25 0.23, 0.24 0.25 0.25,  1,3 1,3 2,4  B B A  This work  f fars f AsP  1.09 ± 0.03 1.09 $ ± 0.03 0.03 1.09  0.38 + 0.02 0.38 + ± 0.02 0.02 0.38  0.28, 0.31 0.28, 0.31 0.31 0.28,  1,2 1,2 1,2  A  B B  This work  L L b  f,AsP f,AsP b  b  4  6  b  4  4  b  4  4  b  4  4  ± ± ± ± ±±  0.04 • 0.04 0.04 0.04 0.04  0.03 0.03 ± 0.03 + 0.03 + ± 0.03  Tm  B  CONTINUED/ ...  REFERENCE This work  44,47  45  TABLE V  (CONTINUED)  MOSSBAUER PARAMETERS AT 80°K FOR L L F e ( C 0 ) COMPOUNDS. b  2  AEQ(mm/sec)  L L b  4  t  6 (mm/sec)  T (mm/sec)  LINE ASSIGNMENT  IRON SITE  REFERENCE  arphos f^AsP  1.43 + 0.02 1.18 + 0.02  0.43 + 0.01 0.30 + 0.01  0.24, 0.23 0.24 0.26  1,3 2,3  B A  This work  dppp f^fos  1.80 + 0.02 1.54 + 0.02  0.42 + 0.01 0.29 + 0.01  0.25, 0.23 0.25, 0.25  1,3 1,2  B A  This work  dpam^f^AsP  1.22 + 0.07 1.22 + 0.07  0.38 + 0.04 0.38 + 0.04  0.39, 0.33 0.39, 0.33  1,2 1,2  B A  This work  f^fos f^fos  1.31 + 0.05 1.31 + 0.05  0.34 + 0.03 0.34 + 0.03  0.29, 0.38 0.29, 0.38  1,2 1,2  B A  This work .  B A  This work  b  f,fos f,fos 4 4 b  + +  + 1.35 + 1.26  0.38 0.30  Relative to sodium n i t r o p r u s s i d e . * **  Experimental uncertainty i s ± 0.02 mm/sec. Other possible assignment of l i n e s , see text,  tt  From magnetic perturbation r e s u l t s , see text and Table VI.  79  d e r i v a t i v e s , can only act as bidentate or monodentate moieties. the p o s s i b i l i t y of terdentate L L LFe2(C0) b  3  b  Thus  ligands leading to the formulation  can be ruled out.  The second p o s s i b i l i t y i s that the L  b  ligand i s acting i n a  b monodentate fashion to give L LFe^CCO)^ complexes. g i s e i t h e r bonded to Fe  b This means that L  or that i t i s bonded c i s to the iron - i r o n bond  on F e , since compounds of the type L LFe2 ( C O ) h a v e been established to A  m  have trans s u b s t i t u t i o n .  In the case of s u b s t i t u t i o n on Fe , we have  the two series of complexes L LFe2(C0),_ and L ^ ^ ( C O ) ^ i n which there i s g substitution on t h i s i r o n atom, and i n both s e r i e s the I.S. of Fe l i e s i n the range 0.30 - 0.24 mm/sec. Since the the compound f^fars f^AsPFe,,(CO)^ b  B there i s l i t t l e i f any l i n e broadening and the I.S. of Fe  A = I.S. of Fe  =  0.38 mm/sec. no matter which of the possible l i n e assignments i s employed, monodentate s u b s t i t u t i o n f o r CO on Fe The second p o s s i b i l i t y , i f L i s s u b s t i t u t i o n on Fe^.  may be ruled out. i s acting i n a monodentate fashion,  b  In view of the f a c t that i n L^Fe^iCO)^  where  A Fe  i s being substituted there are only small changes i n the Mossbauer B B parameters of Fe we would expect conversely when Fe i s being substituted that there would not be any dramatic change i n the Mossbauer parameters of A Fe .  A So neither the I.S. nor the Q.S. of Fe should be too much changed  from LFe^iCO)^.  Since this i s not the case, the p o s s i b i l i t y of monodentate g s u b s t i t u t i o n at Fe must be discounted. Another p o s s i b i l i t y f o r L L F e ( C 0 ) ^ structures could a r i s e i f the g o l e f i n i c bond to Fe were broken. However, i n that case possible structures b  2  80 g would be based on f i v e coordination of Fe . For a l l known f i v e  coordinate  derivatives of Fe(CO),. which do not involve o l e f i n i c linkages the Q.S. 27 67 l i e s i n the range 2.0 - 3.5 mm/sec. be  *  and so this p o s s i b i l i t y too can  discarded. We have therefore established that the ligands L  b  are acting as  normal bidentate ligands and that the possible structures f o r the products must be based on the L L F e ( C 0 )  formulation with the b a s i c LFe^CO)^ g skeleton i n t a c t . Possible modes of l i g a t i o n at Fe now must take the form A B of intermolecular bridging, intramolecular bridging between Fe and Fe , A B or chelation at e i t h e r Fe or Fe . b  2  4  Chelation at Fe would have to be between the two positions cis to the i r o n - iron bond since the other possible structure i s adopted by L L F e ( C O ) 2  4  complexes,  This type of chelation would be expected to  produce a large increase i n the I.S. of Fe  - the range of I.S.  observed  A c B for Fe i n L LFe„(C0). i s 0.46 - 0.53 mm/sec. while the I.S. of Fe 2 4 i n the range 0.24 - 0.30 mm/sec meters of f f a r s f A s P F e ( C 0 ) b  4  4  2  4  lies  The examination of the Mossbauer para-  (Table V) shows the I.S. i s 0.38 mm/sec.  for both iron atoms which i s w e l l outside the above ranges f o r e i t h e r Fe B A or Fe , and so chelation at Fe can be ruled out. Fe  B  S i m i l a r l y , chelation at  A should have only a minimal e f f e c t on the parameters of Fe . As there  are large increases i n the Q.S. at Fe  (no matter which l i n e assignment i s  employed - see f o r example f ^AsP^f ^AsPFe^CO) ^ (Table V)) and i n p a r t i c u l a r , b A for f ^ f a r s f^AsP there i s a large increase i n the I.S. of Fe as w e l l , such a p o s s i b i l i t y must be discounted.  81  Intermolecular remains a p o s s i b i l i t y .  bridging between d i f f e r e n t I J ^ C C O ) ^ moieties It i s u n l i k e l y that this i s true since these  compounds are r e a d i l y soluble i n C I ^ C ^ , and a molecular weight determination i n t h i s solvent gave values very close to the formula weight. spectra of (Ph Sb)2 LFe2(C0) derivatives gave values for  which l i e  b  3  I.R.  4  i n the ranges reported above f o r the remaining L^IJ^CCCO^ d e r i v a t i v e s , so these data tend to substantiate the absence of intermolecular bridging. Owing to t h e i r I n s t a b i l i t y , i t was  not possible to p u r i f y s u f f i c i e n t  amounts of the b i s ( t r i p h e n y l s t i b i n e ) derivatives for them to be  characterized  by Mossbauer spectroscopy.  Thus, at this point, the possible structures f o r these complexes are narrowed down to ones i n which the c u l a r l y between Fe  A  B  and Fe .  ligands are bridging  We w i l l r e f e r to the carbonyls  Fe - Fe bond i n LFe2(C0)g as the a p i c a l carbonyls  trans to the  and the ones c i s to the A  Fe - Fe bond as the equatorial carbonyls  intramole-  f o r both Fe  B  and Fe .  Consideration  of the structure of a ligand such as f g f a r s (Figure 7) shows that i t i s f a i r l y r i g i d owing to the perfluorocyclobutene  r i n g and there i s no  p o s s i b i l i t y of bridging from e i t h e r a p i c a l p o s i t i o n to any equatorial p o s i t i o n on the other iron atom nor from one a p i c a l p o s i t i o n to another since the distances to be spanned are too large.  Thus, there are only four  possible bridging structures remaining - those between the various equatorial positions. If we denote the two equatorial CO*s on Fe respectively, then the equivalent  carbonyls  on Fe  as C0(1)  and  CO(2)  are; ( i ) the one which  82  A l i e s roughly i n the plane C0(1) - Fe  B - Fe , denoted CO(l') (Figure 8); A B  and ( i i ) the one which l i e s roughly i n the plane CO(2) - Fe denoted C0(2') (Figure 8).  There are four possible ways i n which the  carbonyls may be replaced, v i z . : (c)  C0(2), CO(l');  (d)  - Fe ,  (a)  C0(1), CO(l');  C0(2), C0(2').  (b)  C0(1), CO(2');  Of course, (b) and (c) are much  less l i k e l y than (a) and (d) but they cannot be discounted even though the d i s t o r t i o n s at the iron atoms are expected to be quite large and such structures should be less favourable. The magnetic perturbation technique was applied to two of these derivatives namely f ^ f o s f ^ f o s F e ( C O ) ^ (Figure 18) and b  2  f^AsP f AsPFe (CO)^. b  4  2  In both cases the sign of the Q.S. was p o s i t i v e f o r both s i t e s , so there i s a sign reversal f o r the Q.S.  of Fe  A  b i n going from LFe (CO)^ to L L F e ( C O ) 2  2  4  derivatives.  To discover which Q.S. belongs to which i r o n atom i n those cases where three or four l i n e s were resolved, i t i s necessary to investigate the I.S. of the two s i t e s .  From our work with L L F e ( C O ) m  2  A we have a good idea how the I.S. of Fe A both Fe  5  and  L LFe (CO) C  2  4  B and Fe  w i l l vary.  F i r s t l y , since  B and Fe  are being substituted we expect that there w i l l be an  increase i n the I.S. of both i r o n atoms.  Secondly, the change i n the I.S.  on substituting an arsenic should be somewhat greater than that on substituting a phosphorus, i . e . Al.S.(As) > Al.S.(P) on the same iron atom. This i s based on the observations above f o r LFe (C0), and o  complexes.  L LFe_(CO) m  c  FIGURE 18.  57  b Fe Mossbauer Spectrum of f^fos f^fosFe2(CO)^  i n an Applied Longitudinal Magnetic F i e l d of 50kG Showing Experimental Points and T h e o r e t i c a l F i t ,  oo  FIGURE 18.  Fe Mossbauer Spectrum of (f fos ) f fos Fe (C0) in a parallel magnetic field of 5 0 kG D  4  4  2  4  Site A , Q.S.  = +1.29  m m / s e c , 77=0  Site  = +1.35  m m / s e c , 77 = 0  B, Q.S.  0.8 VELOCITY (mm/sec)  84  With these two facts i n mind i t i s i n s t r u c t i v e to examine the I.S. of these compounds (Table V) r e l a t i v e to the I.S. of LFe (C0), L o o  derivatives.(Table I ) . of linkage of the L  ligand as (As) or (P) depending on the s i t u a t i o n .  b  For f f a r s f A s P F e 2 ( C O ) ^ b  4  Denoting the change i n I.S. as A l . S . and the mode  4  there i s only one possible l i n e assignment and  one mode of linkage of the ligand L , so we have b  Al.S.  (As) =  + 0.07 mm/sec, f o r Fe , and A  Al.S.  (As) =  + 0 . 1 1 mm/sec. f o r Fe .  S i m i l a r l y , f o r dpam^f^AsPFe2(CO)^ we have B Al.S.  (As) =  + 0.07 mm/sec. f o r Fe , and  Al.S.  (As) =  + 0 . 1 1 mm/sec. f o r F e . A  The l a t t e r values are somewhat less accurate owing to the l i n e broadening i n this compound. For f f o s f f o s F e 2 ( C 0 ) , although b  4  4  4  only two l i n e s could be resolved  i n the z e r o - f i e l d spectrum, d e t a i l e d f i t t i n g of the magnetic perturbation spectrum indicated that the r e l a t i v e I.S. of Fe  (assumed to have the B smaller Q.S.) was 0 . 0 8 mm/sec. lower than the I.S. of Fe . I f we take the (common) I.S. obtained from the f i t of the zero f i e l d spectrum to be A B the arithmetic mean of the I.S. values of Fe  and Fe , the following para-  meters are obtained f o r f , f o s f . f o s F e _ ( C O ) . : b  4  4  2  4  A  AEq  =  + 1 . 2 9 mm/sec,  I.S. =  0 . 3 0 mm/sec. f o r Fe , and g  AEQ  =  + 1.35 mm/sec,  I.S. =  0 . 3 8 mm/sec. f o r Fe .  Using this assignment, we have Al.S.  ( P ) = + 0.06 mm/sec. f o r Fe , and  Al.S.  ( P ) = + 0 . 0 7 mm/sec. f o r F e , A  85 A B Interchanging the assignment of the lines to Fe and Fe leads to a g decrease i n the I.S. at Fe and so may be eliminated. For dpppkf fosFe2(CO)^, 4  i f the assignment i n Table V i s  employed, we have AI.S.  (P) =  + 0.10 mm/sec. f o r Fe , while  AI.S.  (P) =  +0.06 mm/sec. f o r F e which A  i s i n only moderate agreement with the results f o r f^fos^f^fosFe^CCO)^. A B Again, interchanging the l i n e assignments to Fe and Fe gives a g negative value f o r AI.S. (P) f o r Fe and i s rejected. For dppp f^AsPFe2(CO) , four l i n e s were resolved and there are b  4  two possible combinations  of s p e c t r a l data.  Using the f i r s t set of data  from Table V, we have AI.S. (P) =  + 0.07 mm/sec. f o r F e and  AI.S.  + 0.04 mm/sec. f o r F e .  (P) =  B  A  A Interchange of the roles of Fe at Fe  B and Fe  leads to a decrease i n the I.S.  and i s rejected. The second combination of the s p e c t r a l l i n e s f o r dppp^f^AsPFe2(C0)^  (Table V) gives AI.S.  (P) =  - 0,10 mm/sec. f o r F e , and  AI.S.  (P) =  + 0.22 mm/sec. f o r F e ,  B  A  A which i s c l e a r l y unreasonable.  I f the roles of Fe  then we f i n d a decrease i n I.S. at Fe  B and Fe  are interchanged  so this s o l u t i o n may also be rejected.  Thus, the f i r s t combination of the s p e c t r a l l i n e s i s the correct one,  86  We consider now the cases where L  b  i s a mixed ligand such as  arphos or f^AsP, so that there i s a p o s s i b i l i t y of e i t h e r phosphorus or arsenic being bonded to Fe**.  For arphos^f^AsPFe^(CO)^ using the assigng ment i n Table V and assuming arsenic i s bonded to Fe we have g A l . S . (As) =  + 0.12 mm/sec. f o r Fe , and  A l . S . (P)  + 0.03 mm/sec. f o r F e .  =  A  Another possible assignment f o r the structure puts the phosphorus of B A arphos on Fe and the arsenic on Fe . This gives the following parameters: A l . S . (P) =  + 0.11 mm/sec. f o r Fe , and  A l . S . (As) =  + 0.03 mm/sec. f o r F e . A  This assignment i s rejected since AI.S.(P) i s greater than Al.S.(As) B b on Fe ( c f . A l . S . (As) f o r f g f a r s f^AsPFe^CO)^) , while A l . S . (As) f o r A Fe  ID.  i s u n r e a l i s t i c a l l y small i f the I.S. values f o r the L LFe2(C0),-  complexes are considered. Another p o s s i b i l i t y e x i s t s , namely that the assignment of the s p e c t r a l lines i s i n c o r r e c t .  However, interchanging  A B the roles of Fe and Fe i n Table V leads to a decrease i n the I.S. of g Fe  r e l a t i v e to i t s value i n f AsPFe2(C0)g 4  and so this p o s s i b i l i t y can be  discounted. For f A s P f A s P F e ( C 0 ) b  4  4  2  4  following the assignment i n Table V  and considering that arsenic i n f^AsP  b  B i s bonded to Fe we have g  A l . S . (As) =  +0.10 mm/sec. f o r Fe , and  A l . S . (P)  + 0.03 mm/sec. f o r F e .  =  A  Arguments s i m i l a r to those f o r arphos^f^AsP lead us to reject other possibilities.  87  For f^Aspk^AsPFe^CO)^ using the assignments from Table V B and assuming As i s bonded to Fe we have TJ  AI.S. (As) = AI.S. (P)  +0.09 mm/sec. at Fe , and  =- + 0.03 mm/sec. at F e . g A  However, i f phosphorus i s bonded to Fe we have AI.S. (P)  =  AI.S. (As) =  + 0.09 mm/sec. at F e , and B  + 0.03 mm/sec. at F e  A  i n poor agreement with our previous r e s u l t s , and so t h i s s o l u t i o n i s unacceptable.  In this compound, there i s substantial l i n e broadening  and so the uncertainties involved i n the parameters are f a i r l y large. However, i n a l l three of these "mixed L^" complexes i t i s clear that the A B phosphorus end of the ligand i s bonded to Fe and the arsenic end to Fe . F i n a l l y , f o r f^AsP^f^fosFe2(CO)^ we have, i f the arsenic of f^AsP  b  B i s bonded to Fe , AI.S. (As) =  +0.07 mm/sec. at Fe , and  AI.S. (P)  +0.09 mm/sec. at F e . g  =  A  I f we consider phosphorus bonded to Fe we get AI.S. (P)  =  AI.S. (As) =  +0.07 mm/sec. at F e , and B  +0.09 mm/sec. at F e . A  Either of these p o s s i b i l i t i e s i s acceptable.  Again, interchange of the A B assignments of the s p e c t r a l parameters to Fe and Fe leads to a decrease i n g the I.S. at Fe and can be rejected. In view of the r e s u l t s above f o r other f^Aspk compounds i t might be reasonable to suppose arsenic i s bonded to B  Fe  but i n this case the conclusion i s by no means d e f i n i t e .  88  The Q.S.  at Fe  i n these compounds i s somewhat of a puzzle.  only are there s u b s t a n t i a l increases i n the magnitude of the Q.S.  Not  but, i n  those cases i n which i t has been measured, there i s a sign change as w e l l . Some of this behaviour may be r a t i o n a l i z e d by the fact that s u b s t i t u t i o n occurs c i s to the i r o n - i r o n bond.  If  i s d i r e c t e d more or less along  this bond then increases i n a-donor strength and decreases i n TT-acceptor strength on ligand s u b s t i t u t i o n w i l l tend to change the sign of V„„.  As  w e l l , there must be other factors at work to explain such large changes i n A  V^2«  In. p a r t i c u l a r , the geometry at Fe  should be quite s e n s i t i v e to the  nature of the bridging ligand L , although there does not appear to be b  A anomalous increase i n the Q.S. perfluorocyclobutene r i n g . also be  at Fe  any  b f o r the L  ligands which contain the  As w e l l , the strength of the i r o n - i r o n bond may  changing. There i s evidence f o r the weakening of the i r o n - i r o n bond i n at A  least some of these complexes.  For example, the I.S. at Fe  i n these  compounds does not seem to increase as much r e l a t i v e to LFe2(C0)g on ligand substitution as i t does i n the I^LJ^CCO)^ complexes (compare the I.S. data of Table V to that i n Table I I I ) . This could a r i s e i f less s-electron B 2 3 density i s donated to Fe v i a the d sp hybrids and consequently more s-density A  i s l o c a l i z e d on Fe .  Again, i f there i s a weakening of the metal-metal bond  then the Mossbauer parameters l}  3  of F e  A  should be dependent on the nature of  and L while those of Fe^ should be quite i n s e n s i t i v e to changes at F e . A  Moreover, as X-ray s t r u c t u r a l studies s h o w ^ * ^ , the geometry of the o l e f i n g Fe linkage i s not too s e n s i t i v e to the nature of the ligand L. Both these g facts imply that the Q.S. at Fe should be nearly independent of the rest of  89  the molecule and should be primarily dependent on the nature of ligand L^. B As an i l l u s t r a t i o n of t h i s , the Q.S. and f A s p k f f o s F e 2 ( C O ) 4  4  4  at Fe  i s equal to 1.48  b  b i n both f^AsP f^AsPFe^CO)^  mm/sec.  (Unfortunately,  the  45  spectrum of f^AsP fgAsPFe (CO) 2  was  4  not s u f f i c i e n t l y w e l l resolved f o r  i t to be f i t t e d to three peaks, although there i s s u b s t a n t i a l l i n e broadening of one of the component l i n e s which implies the Q.S. at Fe could be somewhat larger than reported.) For dppp^f AsPFe2(CO)^ and b B dppp f f o s F e 2 ( C O ) nearly i d e n t i c a l Q.S. at Fe are found as well (Table 4  4  4  while the Q.S.  parameters of Fe  V)  are s u b s t a n t i a l l y d i f f e r e n t .  In summary, the Mossbauer spectra of the l^LFe^CCO)  ^ complexes  are quite d i f f e r e n t from those of the other derivatives of LFe„(CO) -. In 2. o p a r t i c u l a r , the Q.S. parameters of Fe are much larger and opposite i n sign <  to those of Fe  A  i n other d e r i v a t i v e s .  The Q.S.  of Fe  B  v a r i a t i o n i n magnitude than i t does i n other complexes.  shows much more This i s consistent  B with s u b s t i t u t i o n at Fe . We have demonstrated that only structures with ligand the two  bridging  iron atoms are consistent with the Mossbauer parameters and  physical evidence such as I.R.  spectra.  For cases i n which the  other  bridging  ligand contains both arsenic and phosphorus, the I.S. data lead to a A formulation  i n which phosphorus i s bonded to Fe  B and arsenic to Fe .  Distortions from i d e a l geometry, s u b s t i t u t i o n c i s to the i r o n - i r o n bond, and a weakening of the i r o n - i r o n bond have been postulated the sign and magnitude of the Q.S.  i n these compounds.  to explain  A number of conse-  quences of this behaviour have been investigated and are consistent with this i n t e r p r e t a t i o n .  90  (E)  THE IRON-OLEFIN BOND IN LFe_(C0), AND THEIR DERIVATIVES. z o The nature of the iron - olefin bond in these complexes i s  not well understood. 59 In the f i r s t  There are two extreme formulations of such bonding.  there i s assumed to be a transfer of TT-electron density  from the olefinic double bond to iron v i a essentially a a-bond with a 3 dsp hybrid orbital, accompanied by TT back-donation from f i l l e d iron * d-orbitals of appropriate symmetry to vacant TT -orbitals of the olefinic linkage.  As a result of this transfer of electron density into anti-  * 59 bonding TT -orbitals the carbon - carbon.bond w i l l be lengthened . In B  this description, Fe  in our compounds would be considered to be essen-  t i a l l y five-coordinate.  In our present series of complexes, the X-ray  structural studies have shown that the plane of the perfluorocyclobutane ring i s not at a particularly favourable angle for TT-bonding to Fe^ The second formulation of the bonding implies that iron i s six-coordinate i n these complexes and that the olefin forms two normal  68 CT-bonds from carbon to iron  . The carbon - carbon bond would therefore  be lengthened to approximately the normal single bond distance. The f i r s t problem encountered in trying to discover which of these bonding schemes gives the most adequate description in this particular case is the fact that similar systems containing iron - iron dative bonds  37 are not at a l l common  .  In most examples of dinuclear iron carbonyl -  olefin complexes which have been reported the iron - iron bonds are considered to be "normal" shared pair a-bonds and the olefinic linkages to be  91  three electron donors.  An example of such a system i s c y c l o h e p t a t r i -  e n y l d i i r o n hexacarbonyl*' . 9  The magnitudes of the Q.S.  are not very d i f f e r e n t from those i n the present cases.  i n such systems*'  4  However, the  best systems with which to compare the present compounds would appear to be o l e f i n i c derivatives  of Fe(CO),..  These compounds have Q.S.  imately the same magnitude as i n the present case*^' ^. 7  bonding should be quite s i m i l a r to that at Fe  of approx-  Moreover, the  except f o r the formation  of a o~-bond along the Fe - Fe bond axis i n our case, versus the aTT-bonding i n the case of a x i a l carbonyls. compounds there does not  seem to be any  and  Interestingly enough, i n our  s i g n i f i c a n t shortening of  the  i r o n - a x i a l carbonyl bond length"^'*^ which one might have anticipated A i f there i s no TT-bonding to Fe . 27  Fe(CO),. and  i t s n o n - o l e f i n i c derivatives such as Ph^PFeCCO)^  have | A E Q | i n the range 2.0 - 3.5 mm/sec.  The  Z  empty.  Variations  i n the magnitude but not  on a x i a l s u b s t i t u t i o n and  '  p r i n c i p a l contribution  the s p l i t t i n g has been a t t r i b u t e d to the f a c t that d 2  67  to  i s nominally  the sign of AE^ are expected  are attributed to some p a r t i c i p a t i o n of d 2 Z  in  '  27  the formation of a-molecular o r b i t a l s . In addition, the e f f e c t s of equatorial s u b s t i t u t i o n have been investigated both t h e o r e t i c a l l y and 27  experimentally.  The  r e s u l t s of this work  show that f o r ligands which  are s u f f i c i e n t l y l i k e CO i n t h e i r bonding c h a r a c t e r i s t i c s that the  substi-  tution may  d2  be treated as a perturbation,  then the e f f e c t i s to mix  Z  with  varying amounts of the other d - o r b i t a l s , so that although the magnitude of A E Q remains approximately constant, V ^  may  have e i t h e r sign and n any  value  92  in i t s permitted range.  Thus, f o r our compounds we can see that the  B Q.S. at Fe  should be dominated by the contribution from the e s s e n t i a l l y  non-bonding o r b i t a l and the sign of the e.f.g. w i l l be dependent on the exact  B d e t a i l s of the bonding at Fe . The model i n which the two carbons from the o l e f i n form only  a-bonds with the i r o n ^ can also be used to make predictions about the e.f.g..  Again, f o r this theory the e.f.g. w i l l be dominated by the f a c t  that the d-orbitals are not f i l l e d and the exact ordering of the energies of the d-orbitals w i l l depend on the d e t a i l s of the O and IT contributions from each of the ligands.  Thus, again, the sign of the e.f.g. and the  magnitude of H w i l l vary depending upon which of the d-orbitals i s highest i n energy and e s s e n t i a l l y unoccupied.  That the e.f.g. i s r e a l l y dominated by the "hole" i n the d-orbitals i s amply demonstrated by a series of experiments with o l e f i n F e ( C O ) c o m p l e x e s , where controlled reduction gives r i s e to r a d i c a l anions whose Q.S. i s approximately one h a l f the value f o r the neutral complex ^. 7  This i s exactly the result one would predict i f one electron  were introduced into the non-bonding  orbital . 7  g It i s apparent from the foregoing that the e.f.g. at Fe  is  not going to be a s e n s i t i v e probe f o r the nature of the iron - o l e f i n bond since both extreme formulations of t h i s linkage give r i s e to q u a l i t a t i v e l y g s i m i l a r r e s u l t s , namely the e.f.g. at Fe d-orbitals.  i s dominated by the u n f i l l e d  One thing which might be of some value i n 'this connection,  93  TABLE VI. MAGNETIC PERTURBATION RESULTS* COMPOUND  (mm/sec)  f AsPFe (CO)  6  f fosFe (CO)  6  4  2  A  2  f farsFe (CO) 6  2  6  (PhO) P f AsPFe (CO) m  3  f AsP  4  2  f AsPFe (CO)  4  4  2  f AsP f AsPFe (CO)  5  4  b  4  4  2  f fos f fosFe (CO)  4  b  4  4  2  4  n  SITE  + 1.45 - 0.83  0.6 0.8  B A  + 1.32 - 0.66  0.6 0.6  B A  + 1.41 - 0.67  0.6 0.0  B A  + 1.51 - 0.58  0.8 0.0  B A  + 1.07 - 0.61**  0.0 o.ot  B A  + 1.48tt + 1.26tt  o.ot o.ot  B A  + 1.35 + 1.29  o.ot o.ot  B A  Source and absorber at 4.2 K, 50 kG magnetic f i e l d applied p a r a l l e l to y-beam. t Not e x p l i c i t l y  f i t t e d as a parameter.  ** Apparent sign, but not conclusive. tt Sign determined on inspection.  94  however, would be an i n v e s t i g a t i o n of the sign of the e.f.g. and  the  value of n f o r i r o n i n a series of o l e f i n - FeCCO)^ and o l e f i n Fe(CO),jL complexes where the nature of both the o l e f i n i c substituents and the ligand L were systematically varied.  The r e s u l t s of our present series of magnetic perturbation experiments show that  i s p o s i t i v e f o r Fe , i n every case we have  measured, although there appears to be some v a r i a t i o n i n n. (Table VI). This result i s not unexpected f o r L F e ( C O ) , L L F e ( C O ) 2  6  2  5  and L LFe^CO)^  A  complexes since  s u b s t i t u t i o n occurs at Fe , and a v a i l a b l e X-ray data  Indicate there are only small s t r u c t u r a l changes at Fe^ ^8,60^ L LFe (CO) b  2  4  t  ^  e  complexes, a sign change might conceivably occur since there  i s s u b s t i t u t i o n at Fe .  However, no such change was  case i n which the sign was measured.  This may  observed i n e i t h e r  imply that the p a r t i c u l a r  substituents employed do not perturb the system s u f f i c i e n t l y f o r a sign change to occur.  95  PART 2 Ph  5-n  c  SbX  Derivatives,  n  X-ray c r y s t a l l o g r a p h i c studies of compounds such as S b C l , j , 71  Ph SbCl 3  7 2 2  , Me SbCl 3  7 3 2  , Ph^bOMe , P h ^ b ( O M e ) 74  74 £  and Ph^SbOH  75  have  shown that these compounds adopt t r i g o n a l bipyramidal structures with the electronegative groups i n the a x i a l p o s i t i o n s .  I.R.  and Raman data are  also consistent with t h i s type of structure f o r compounds of the type R SbX 3  and R ^ S b X ' 7 6  2  which i s apparently when such exceptions  7 7 , 7 8  .  There are a few exceptions  i o n i c , but both I.R. occur.  The  77  such as Ph^SbClO^,  and Mossbauer studies  lack of a Q.S.  8  show  (as expected f o r a t e t r a -  hedral Ph^Sb cation) and the high resonance f r a c t i o n s support i o n i c +  structures f o r both Ph.SbClO. 4 4  8  and Ph.SbBF. 4 4  79  .  In p a r t i c u l a r , the s t r u c t u r a l parameters of Ph^SbOMe and Ph Sb(OMe) 3  2  are of i n t e r e s t since they show there are no large deviations  i n the 0 - Sb bond lengths or i n the equatorial Ph - Sb bond lengths 74 between these two compounds  .  These data lend some confidence  a p p l i c a t i o n of an additive model f o r the Q.S.  i n the  to r e l a t e d compounds since  the theory depends c r i t i c a l l y on the assumption that the ligand parameters do not change appreciably from one compound to another. The i n t e r p r e t a t i o n of the Mossbauer parameters f o r compounds of 8 18 80 this type has been f a i r l y well established ' ' and recently there has been some attempt to apply the additive model to some of these compounds . 7  121 results of the  Sb Mossbauer measurements are summarized i n Table VII  The  96  and a t y p i c a l spectrum i s shown i n Figure  In general, our present  19b.  r e s u l t s f o r the Ph.jSbX2 and Ph^SbX  8 18 compounds are i n good agreement with the values previously reported ' for compounds of this type which have been summarized i n Table VIII. 2 In p a r t i c u l a r , the magnitudes of e qQ and the zero n values  (except for  Ph^Sb(OCOCH^)2 - see below), show that these compounds adopt t r i g o n a l bipyramidal Previous  structures with e f f e c t i v e  I.R.  or C^ symmetry about antimony. 76 81 82  studies on these compounds  v  '  *  and the s t r u c t u r a l study  of Ph^SbOH ^ have come to s i m i l a r conclusions. From the data i n Tables VII and VIII i t i s possible to derive 7  p.q.s. values f o r various ligands and then to employ these p.q.s. values 2 to predict e qQ f o r other derivatives as a test of the additive model. As a s t a r t i n g point for these p.q.s. c a l c u l a t i o n s , the parameters of Ph^SbCl and Ph^SbC^ w i l l be examined.  This was  selected as a s t a r t i n g  72 point because the structure of Ph^SbC^ i s known  .  A l l the bond angles  about Sb are within 3° of those f o r a regular t r i g o n a l bipyramid, so we safely treat this molecule as being  undistorted.  f  i TBA  Cl  in  TBA TBH 7 Ph^SbC^ and use the r e l a t i o n for a B2 A^ M case (Figure 4, #6) 2 and the value of e qQ for Ph^SbC^ from Table VIII, i t i s found that V  ZZ  =  ~ [  A  3  Ph  =  The p o s i t i v e sign for V ^  P b  ]  =  + 20.6 W s e c ,  - 6.9 mm/sec. indicates that the electronegative C l groups  can  TABLE VII. Sb MOSSBAUER PARAMETERS AT 9 K (THIS WORK). COMPOUND  I.S,. (mm/sec.)  Ph.SbCl 4 Ph.SbOH 4 Ph,SbNCS 4 Ph Sb(NCS)  2  Ph Sb(N0 )  2  3  3  3  (Ph Sb) OCr0 3  2  Ph Sb(OAC) 3  Ph Sb(O)OH 2  Ph SbCl 2  3  2  4  -  LINEWIDTH (mm/sec.)  2 ** X  - 6.4 + 0.7  0.0  2.8  151  4.1 + 0.1  ~ 5.3 + 0.5  0.0  2.9  172  5.2 + 0.1  - 6.4 + 0.6  0.0  2.9  188  5.6 + 0.1  - 20.4 + 0.7  0.0  2.6  147  5.7 + 0.1  - 21.3 + 1.0  0.0  3.0  147  4.3 + 0.1  - 16.6 + 0.8  0.0  2.8  143  5.1 + 0.1  - 21.6 + 0.5  0.0  2.6  192  5.2 + 0.1  - 20.1 + 0.5  2.7  153  1.8 + 0.1  - 10.9 + 0.5  0.0  3.1  148  7.0 + 0.1  +. 25.9 + 0.7  0.0  3.0  194  7.0 + 0.1  + 25.2 + 0.7  3.0  189+T  Sn0  6.46 ± 0.04  0.22 ± 0.05  3<  Constrained to n. = 0.0, except where otherwise noted. Approximately 180 degrees of freedom,  tt  nt  5.2 + 0.1  Relative to source Ba t **  2 e qQ (mm/sec.)  No s i g n i f i c a n t improvement i n f i t f o r n 4 0»  TABLE VIII. 1 2 1  S b MOSSBAUER SPECTRA AT 4.2°K  I.S. (mm/sec.)  COMPOUND  (PREVIOUS STUDIES).  2 e qQ (mm/sec.)  n  LINEWIDTH (mm/sec.)  REFEI  Ph.SbF 4  - 4.56  - 7.2  0.0  2.62  8  Ph^SbCl  - 5.26  - 6.0  0.0  2.73  8  Ph.SbBr 4  - 5.52  - 6.8  0.0  2.75  8  Ph.SbNO4 3  - 5.49  - 6.4  0.0  2.57  8  Ph SbF  - 4.69  - 22.0  0.0  2.66  8  - 6.02  - 20.6  0.0  2.55  8  - 6.32  - 19.8  0.0  2.75  8  - 6.72  - 18.1  0.0  2.58  8  - 6.11  - 24.0  0.0  2.74  8  - 6.40  - 22.1  0.0  2.58  8  (+ 17.5)  (0.0)  (2.82)  8  3  2  Ph SbCl 3  2  Ph SbBr 3  2  Ph3sbi2 (CH ) SbCl 3  3  (CH ) SbBr 3  3  2  2  (- 9.69)  (Ph.Sb)  *  7  121 Relative to source Ca Sn0 . 3  99a  FIGURE 19.  The  121 Sb Mossbauer Spectrum of P h S b C l Showing F i t 121 2  with n. = 0 and the  3  Sb Mossbauer Spectrum of a  T y p i c a l Compound of the Type R SbX , 3  Namely P h S b ( N 0 ) . 3  3  2  2  100  withdraw charge along the Z axis and so the equivalent e l l i p s o i d of charge i s oblate owing to the excess of electron density i n the XY plane , 7  Turning now  2 to Ph^SbCl i f we assume i t s e qQ value to be the g  mean of the present measurement and the one due to Long e_t al , and use the applicable equations (Figure 4, # 5), we have V  zz  -  2[ci]  T B A  - 3[ph]  [Cl] TBA  f  1  jj?hJ  TBA  and = - 7.2 mm/sec..  T B E  + 2[ph]  =  T B A  +  6.2  mm/sec.  "1TBE  Ph  t  are substituted, i t i s found that  ~lTBA X I values can be found from compounds  [ XJ1TBA  derivatives can be calculated as a check.  2 values e qQ f o r R^SbX (same X) The results are summarized i n  Table IX.  With the exception of the Br compounds the agreement between 2 observed and calculated e qQ values i s very s a t i s f a c t o r y considering the g  quoted  experimental error l i m i t s of about ± 0.3 to ± 0.7 mm/sec. f o r  compounds of the type Ph^SbX (Tables VII and V I I I ) .  For Ph^SbBr, the  agreement i s somewhat less s a t i s f a c t o r y , and i n view of the fact that I e qQ | f o r R^SbB^ i s less than that f o r R.jSbCl i t i s s u r p r i s i n g to f i n d 2 2 Ie qQ] Ph^SbBr i s greater than |e qQ| Ph^SbCl. 2  2 A comparison of the e qQ values derived from nuclear quadrupole 83 84 85 2 resonance (NQR) studies ' ' (these give values of e qQ/h) and those derived from Mossbauer spectroscopy i s of i n t e r e s t .  The Mossbauer  101  parameters are consistently + 0.5 to + 2.0 mm/sec. larger i n magnitude than those derived from NQR studies (compare Tables VII and VIII to Table X).  This systematic difference could arise from some error i n  18 the Mossbauer velocity calibration, or from the value of R , or perhaps from the conversion factor from NQR frequencies to mm/sec. (Table X 86 E  could be wrong).  The most likely explanation, however, i s a  temperature dependence of the quadrupole coupling constant.since the NQR values are measured at room temperature while the Mossbauer values are measured at 4.2°K. For the compounds of the type Ph.jSbX2 studied in this work and 8 18 for those previously reported ' , there i s a f a i r l y good linear correlation between |e qQ| and the I.S. (Figure 20), with the exception of the point due to (Ph^Sb^OCrO^.  The best least squares f i t to these points  (excluding (Pb^Sb^OCrO^) gives e qQ = - 1.74 (I.S.) - 30.5 mm/sec. From the slope of this l i n e  7  i t i s possible to t e l l that in this series  of compounds O bonding effects are dominant and the ligand polarities 2 govern the trends in both I.S. and e qQ. Thus F which has the highest electronegativity produces the most positive I.S. (most like Sb ~* ) and +  the largest |e qQ| since i t s cf-donor ability should be the least, relative to a phenyl group.  Similarly, I has the smallest electronegativity of the  X groups studied here and leads to the most negative I.S. and the smallest |e qQ|. 2  The large deviation of (Ph^Sb)£0(^0^ from this linear correlation is probably due to the fact that i t is the only compound in the series which  102 TABLE IX. APPLICATION OF THE ADDITIVE MODEL TO PREDICT THE e qQ VALUES FOR R SbX COMPOUNDS, 5-n n TBE Values Assuming ^ C l J ^ = 0.0 mm/sec. c  B A  Value (mm/sec.) - 6.9  Ph SbCl  - 8.0  Me SbCl  3  3  ± 0.9 TBA  C  Values Assuming  SbCl "1  2  2  *  r  TBA  Cl  = 0.0 mm/sec.  Value (mm/sec.)  3.  Source Compound  Source Compound  - 0.2  Ph SbBr  + 0.3  Ph SbF  - 0.7  Ph SbI  - 0.1  Ph Sb (NCS)  + 0.2  Ph Sb(N0 )  - 7.2  Pb^SbCl  - 0.3  Ph.SbOH 4  3  3  3  2  2  2  3  3  3  2  2  Predicted and Observed Values of e qQ. Compound  Predicted e qQ (mm/sec.)  Ph.SbBr 4 Ph.SbF 4 Ph.SbNO, 4 3 Ph.SbNCS 4 (CH.)_SbBr, 3 3 i  - 5.9  - 6.8  - 6.9  - 7.2  - 6.7  - 6.4  - 6.5  - 6.4  - 23.2  - 22.1  * See Text.  Observed e qQ (mm/sec.)  TABLE X. NQR • DATA - AT ROOM TEMPERATURE. COMPOUND  e qQ (MHz) 2  121  Sb  EQUIVALENT e qQ + (mm/sec.) (Signs Assumed) 2  REFERENCE  Me SbCl  2  660.393  - 22.04  83  Me SbCl  2  662.18  - 22.10  84  Me SbBr  2  631.127  - 21.06  83  Me SbBr  2  630.57  - 21.04  84  Ph SbCl  2  592.35  - 19.77  84  Ph SbBr  2  565.78  - 18.88  84  603.91  - 20.15  84  (509.00)  (+ 17.0)  84  3  3  3  3  3  3  Ph SbF 3  2  (Ph Sb) 3  S b C l at 210°K  84.67  2.83  85  S b C l at 249°K  84.63  2.82  85  5  5  1 MHz = 3.337x10"  mm/sec. using E (  Sb) = 37.15 Kev?  and the relationship 6E = E.  FIGURE 20.  121  Correlation of the  Sb I.S. and  2  e qQ for a Number of Derivatives of the Type Pb^SbX^  104  105  may be formulated as Ph^SbXY.  Thus i f one of the groups  (oxygen i n  this case) i s more e f f e c t i v e l y able to compete f o r electron density than the other (CrO^) then there w i l l be deviations from regular behaviour. That this i s probably the s i t u a t i o n i s i l l u s t r a t e d by the structure of (Ph SbN ) 0, where the Sb - 0 distance of 1.985A 3  3  shorter than the Sb - 0 distances i n Ph Sb(0Me) 3  Ph^SbOMe  is significantly  8 7  2  (2.061A )  7  and Pb^SbOH  4  (2.0481 )  7 5  .  (2.033^ , a v g . ) \ 7  2  Moreover, the I.S. and  2 e qQ values f o r (Ph SbCl) 0 and (Ph SbBr> 0 measured at l i q u i d nitrogen 3  2  3  2  g temperature  also show s i g n i f i c a n t departures from regular behaviour  (the e qQ values at 78 K are inherently l e s s accurate, however).  As w e l l ,  departures from predicted I.S. behaviour may be c h a r a c t e r i s t i c of Sb - 0 bonding  (see discussion of Ph Sb(0)0H below).  The Ph^SbX derivatives also 2 f a i l to show a l i n e a r c o r r e l a t i o n of the I.S. and e qQ. There i s , however, a general trend to more p o s i t i v e I.S. with increasing e l e c t r o n e g a t i v i t y 2  i n Ph.SbX d e r i v a t i v e s . 4  There are two other I.S. trends i n these compounds which are worthy of comment. than Me SbX 3  2  The f i r s t i s that the I.S. of Ph SbX 3  2  i s more p o s i t i v e  as expected i n view of the better o donating power of a l k y l  t  [  1 TBG  Mel  l TBE Ph .  Similar behaviour has been found i n t i n  chemistry where the p.q.s. value f o r Me i s more negative than that f o r Ph 25 i n both tetrahedral and octahedral complexes has also been observed f o r Ph^bCp (C0) FePF 2  n-C^Hg) (see Part 3, below).  6  ,  Further, t h i s trend i n I.S.  and Bu Sb(CO) FePF 3  2  6  (Bu =  The second trend i s that f o r a given X the  106  I.S. of R^SbX i s more p o s i t i v e than R^SbX^.  Again, a s i m i l a r trend to  higher s-electron densities at the nucleus i n R^SnX2 compounds r e l a t i v e to R^SnX compounds has been observed  i n t i n chemistry .  This phenomenon  7  i s discussed i n more d e t a i l below. I f the structure of Ph^SbCl^ i s assumed to be t r i g o n a l + — 88 bipyramidal l i k e the s i m i l a r t i n compound, Et^N Me2SnCl , then i n 2 order to c a l c u l a t e a value f o r e qQ"using the additive model, i t i s f "IT BE necessary to have a value for C l 3  A reasonable choice f o r a model compound from which to derive  [  "1TBE ClJ might appear to be S b C l y  Crystal structure data at -30  o  C show  SbCl^ to adopt a regular t r i g o n a l bipyramidal s t r u c t u r e , although 7 1  the  fact that the a x i a l Sb - C l bond lengths of 2.34A , avg., are somewhat -  72  shorter than those i n Ph^SbC^ (2.48A. , avg.)  might make the a p p l i c a t i o n  of the additive model to t h i s compound somewhat questionable. The Mossbauer parameters at 77°K for SbCl,. were measured by Bowen 89 et a l . and the value of -4.4  mm/sec. was  data to a quadrupole-split pattern.  2 found for e qQ on f i t t i n g  However, there i s some evidence  the from  85 NQR  studies  that SbCl,. undergoes a phase t r a n s i t i o n , and i n fact i t may 85 — + exist as a dimer or as an i o n i c species such as SbCl, SbCl, at low 6 4 90 temperatures.  In view of this f a c t , Stevens and Bowen  have r e i n t e r p r e t e d  121 their earlier  Sb data for SbCl^ and found that an equally good f i t could  be obtained f o r two single l i n e Lorentzian components, consistent with the existence of two s l i g h t l y d i f f e r e n t s i t e s i n the compound. 80 S u r p r i s i n g l y , Bancroft et a l . have recently employed the  107  89 o r i g i n a l values of Bowen et a l . , c i t i n g the NQR data of Schneider 85 and D i Lorenzo  as showing that there are no appreciable changes i n the  NQR frequencies above and below the t r a n s i t i o n temperature (presumably o 80 the NQR frequencies at 210 and 249 K form the basis of this statement ) . In f a c t , i n the NQR study, the authors were unable to obtain consistent values of n and eQq ( s i c ) f o r antimony below 195°K, and the s i x resonances which were obtained imply the presence of at least two d i f f e r e n t kinds of antimony^.  Since the data of Bowen et al.^ were obtained at 77°K, this  2 — 80 means that the assignment of a negative sign f o r the e qQ of SnCl,. 2 based on i t s equivalence with the o r i g i n a l l y reported sign of e qQ of SbCl^ must be considered as doubtful at best. It i s possible to convert the NQR frequencies of Schneider and Di Lorenzo  85  2 to give a value of 2.82 mm/sec. f o r the |e qQ| (the sign i s  indeterminate) of SbCl^ above 249°K where i t i s t r i g o n a l b i p y r a m i d a l . 71  80 This r e s u l t indicates that the c o r r e l a t i o n of Q„ and Q„, by Bancroft et a l . Sn Sb 2 J  i s not n e c e s s a r i l y i n e r r o r , provided that the sign of e qQ f o r SnCl5 i s the same as that f o r SbCl,.. f ~\ TBE Thus, the s i t u a t i o n f o r deriving a ^ClJ value from SbCl,. i s not too favourable.  Nevertheless, an estimate f o r the possible range of  r XBE values for C l j can be made i f the two possible signs and the magnitude 2 2 of e qQ from NQR studies are employed. For e qQ = + 2.82 mm/sec,  ^ClJ^ -0.9  B E  =  + 0.9 mm/sec. while for e^qQ  mm/sec..  =  - 2.82 mm/sec, £ c i J ^  BE  =  108  Using these values i t i s possible to calculate the e.f.g. components for Ph^SbCl^.  An e a r l i e r X-ray s t r u c t u r a l study had character-  ized Ph^SbCl^ as being t r i g o n a l bipyramidal with two a x i a l chlorines, two 91 equatorial phenyl groups, and one equatorial chlorine atom  but r e f i n e -  92 ment  of the structure showed the. compound under study was r e a l l y  Ph^SbCl.j'l^O which i s s i x coordinate.  In our present  case the compound  has been properly characterized as Ph SbCl.j . Thus, as previously 2  mentioned, we s h a l l assume the structure of Pb^SbCl-j i s very much l i k e + — 88 that of the corresponding t i n species, Et^N M e ^ n C l ^ I f t h i s i s indeed the structure then a p p l i c a t i o n of the additive model gives the following components of the e.f.g. for Ph^SbCl^ (Figure 4, # 8) using £ c i J v  Thus, V  *  zz  =  - 0.9 mm/sec:  zz " I W  V  V  TBE  TBE  " [ J "[ ] C1  YV  =  - 2[ph]  XX  "  -iH"'' +  =  T B E  TBE  2  - [Cl]™ +  2[d]  - 16.4 mm/sec, H. =  T  B  E  C1  4[d]  -  TBA  16  =  T B A  2[d]  = " -*  T B A  2 0.8 and e qQ  =  + 14.7 mm/sec.  -  + 1.7 mm/sec.  + 16.4 mm/sec.  The analysis of Ph SbCl.j was performed by P. Borda of t h i s 2  department.  Anal.  C l , 27.85.  Found;  Calculated f o r P h S b C l : 2  C, 37.52;  3  C, 37.67;  H, 2.42; C l , 27.78.  H, 2.49;  109  f" 1TBE Similarly, i f C l  =  Vjg£ whence V  =  - 1 8 . 2 mm/sec.  =  + 1 2 . 9 mm/sec.  =  + 5 . 3 mm/sec,  - 1 8 . 2 mm/sec,  =  z z  + 0 . 9 mm/sec. i s employed:  2 n = 0 . 4 and e qQ  =  + 1 8 . 2 mm/sec.  These values may be compared with the experimental r e s u l t 2 which shows e qQ  =  + 2 5 . 2 mm/sec. (Table V I I ) .  Although the sign of  2 e qQ i s predicted c o r r e c t l y , there i s a lack of agreement i n the magnitude. 1 TBE Cl values but i s most  [  l i k e l y a r e s u l t of d i s t o r t i o n s from regular geometry.  In Me2SnCl.j the  o 88 angle C - Sn - C i s about 140  and so i f P l ^ S h d ^ i s also t r i g o n a l  bipyramidal a s i m i l a r d i s t o r t i o n of the C - Sb - C angle would not be unreasonable.  Considering the e f f e c t of such d i s t o r t i o n s using the simple  point charge approach shows that as the angle C - M - C i s increased from 120° (assuming  the d i s t o r t i o n i s i n the C^SbCl plane), the magnitude of 21 V becomes larger . The equation describing this behaviour i s ZZ " [ ] ~ [ c i ] ™ + 2 ( 3 s i n V l ) [ p h j , where <f> i s the angle z z  V  =  2  C  1  C - Sb - C l . \C1J  =  T  B  T B E  A  I f we use c> j = 110° corresponding to C - Sb - C = 140° and  - 0 . 9 mm/sec, we f i n d V  f  ~l TBE Clj  =  z z  =  - 2 1 . 8 mm/sec, n = 0 . 3 .  + 0 . 9 mm/sec, then V  z z  =  -  On the  2 3 . 6 mm/sec,  n =  0 . 1 , i n more s a t i s f a c t o r y agreement with the experimental result  V  =  z z  - 2 5 . 2 mm/sec. , ( r w 0 . 2 ? ) . 119 A recent  Sn Mossbauer study using the magnetic perturbation 2 technique has established that e qQ i s p o s i t i v e and n l i e s i n the range  110  + - 93 0.5 - 0.7 in Et.N Me SnCl . 4 2 3 0  0  In the present work, i t was not possible to establish the value of n with certainty for Ph^SbCl^ (see below), although i t i s possible to estimate that O^n^O.6. The structure of Pt^SbCl^ cannot be definitely assigned as trigonal bipyramidal from the present work, however, although the Mossbauer parameters are consistent with this formulation. The reason for this discrepancy is that the formulation of the structure i n terms of a six-coordinate moiety with bridging Cl groups and trans phenyl groups i s also consistent with the observed parameters.  The possibility of strong  interactions v i a chlorine bridges and a six-coordinate structure has been recently advanced for Me2SbCl  3  94  2 • A very crude estimate of the e qQ value  [Ph1 OCT [C1]° V  ZZ  =  =  CT  4  (jk)  [ci] 0 C T  B R I D G I N G  ~4[ci]0Cr  = [ci]  T B A  .  f"  =  1 TBE  Ph  and  We have (from Figure 4, # 12),  - -27.6 mm/sec.  If the bridging Cl's are  cis to each other then, n. ^ 0 (but small) , while i f they are trans, r) = 0. This result is not inconsistent with the experimental data.  To this time,  there has been no Mossbauer I.S. data published for organoantimony compounds with six-coordinate structures nor indeed any data on other organoantimony compounds of the type R^SbX^ and so i t is not possible to choose between the five- or six-coordinate structures on the basis of the I.S. data.  Thus,  no definite conclusions may be reached regarding the structure of Pt^SbCl^ at  9°K on the present Mossbauer results. Unfortunately, we were unable to establish with certainty the  value of n for Ph^SbCl^.  When the spectrum was fitted with n as an adjustable  Ill  parameter, the value derived was 0.22 ± 0.05 which for  121  Sb Mossbauer  90 spectra i s l i t t l e different from zero  . Further, there was essentially 2  no improvement in the goodness of f i t (x  dropped from 194 to 189) and  visually, there was also no apparent improvement.  It i s possible that  the (visually) poor f i t of the spectrum (Figure 19a) i s a result of departures from theoretical intensity ratios due either to the Gol'danskii18  Karyagin (G.K.) effect as previously reported by Stevens and Ruby  for  compounds of the type R.jSbX2, or to non-random orientation of the crystallites in the powdered sample.  It i s surprising that none of the  other compounds of the type R^SbX^ which were examined in this work and for which one might expect the G.K. effect to be at a maximum, showed any evidence for G.K. asymmetries (see for example Figure 19b). Indeed, the whole question of observing the G.K. effect i n antimony compounds at 4.2°K is very intriguing.  F i r s t l y , one might expect  that the anisotropics in the Debye-Waller factors at 4.2°K would be very small in any case, and certainly for any t i n compounds in which the effect has been observed, i t i s quite temperature dependent and is nearly absent at or below 77°K.  However, the G.K. effect i s also somewhat dependent on  16 the y-energy  and this could conceivably account for differences between  tin and antimony compounds. The other possible explanation for deviations from theoretical intensity ratios i s non-random orientation of the crystallites i n the powdered sample.  This phenomenon can be distinguished from a genuine G.K.  effect by changing the relative angle of incidence of the Y beam and the -  112  absorber.  I f the i n t e n s i t y r a t i o s are observed to vary then the  G.K,  e f f e c t may be ruled out - the converse i s , of course, not necessarily true. Stevens and Ruby do not report whether such a test was  carried out on  t h e i r compounds'^.  It i s i n t e r e s t i n g that f o r the antimony compounds where  G.K.  18 e f f e c t s have been claimed to occur the published spectra  show a super-  f i c i a l resemblance to those i n which the asymmetry parameter, n, takes on 90 small, non-zero values.  From the work of Stevens and Bowen  , i t is  obvious that i f n>0.4 then the energies and the t r a n s i t i o n p r o b a b i l i t i e s are s u f f i c i e n t l y d i f f e r e n t from the case when n = 0 that there w i l l be no doubt that these two e f f e c t s may be distinguished.  From the work of  18 Stevens and Ruby  i t i s not clear whether n was  allowed to vary as a  parameter before or a f t e r correction f o r the apparent G.K.  e f f e c t and so  i t i s not possible to judge whether they could d i s t i n g u i s h unequivocally between these two e f f e c t s . Unfortunately, i t i s not possible f o r us to examine t h i s problem i n more d e t a i l since there i s only one compound i n this work f o r which the anomalous behaviour was  observed and at present we have no computer subroutine  to hand f o r the analysis of possible G.K.  effects.  Also, our present  experimental set up (Figure 5) does not allow f o r any a l t e r a t i o n of the source-absorber geometry and so i t was not possible to carry out an orientation study. I.R.  studies on Ph Sb(0C0CH ) =(Ph Sb(0AC) ) and Pb^Sb(0C0CD ) 3  3  2  3  2  3  2  as w e l l as t h e i r trimethylantimony analogs show the presence of e s t e r - l i k e  113  acetate groups  95  .  Further, there i s no appreciable change i n the  frequencies v(C=0) and v(C-O) between s o l u t i o n and s o l i d s t a t e spectra nor any s i g n i f i c a n t differences i n the behaviour between t r i p h e n y l - and trimethylantimony  of v(C=0) and v(C-O)  derivatives.  The I.R.  spectra  of the trimethyl derivatives show only an asymmetric Sb-CH^ s t r e t c h i n g frequency  (v  i n the I.R.  (Sb-CH^));  the symmetric Sb-CH_ mode (v  (Sb-CH_)) i s absent  spectrum but appears as a strong band i n the Raman^.  Thus  t r i g o n a l bipyramidal structures with planar SbC^ groups have been assigned to these compounds^' "'. 9  The I.R.  and Raman spectra of the t r i p h e n y l  analogs show both V (Sb-C) and V (Sb-C) v i b r a t i o n s , but breakdown of the a  s  l o c a l symmetry approximation  was  invoked to r a t i o n a l i z e these r e s u l t s and 76  planar SbC^ groups have been assigned to these compounds as w e l l  .  Since  V(C=0) and V(C-O) are very s i m i l a r i n both series of compounds such a s t r u c t u r a l assignment does not seem  unreasonable.  I f Ph^SbCOAC^ were t r i g o n a l bipyramidal with a x i a l acetates and equatorial phenyl groups at 9°K then the value of n from the Mossbauer spectrum should be zero, or at least very small.  Thus the f a i r l y large  value of n (0.46) derived f o r Ph.jSb(0AC)2 (compare Figures 21a and implies that i t does not have this structure at 9°K.  21b)  The most l i k e l y s o r t  of s t r u c t u r a l change would be to a s i x coordinate s t r u c t u r e , probably one bidentate acetate group and one e s t e r - l i k e acetate group.  with  A bidentate  acetate group has been postulated to occur i n Ph^SbOAC^ ' *' and the existence of both bidentate and e s t e r - l i k e acetate groups has been postulated i n 97 f ~|OCT Ph Sb(OAC) . A p.q.s. c a l c u l a t i o n f o r Ph Sb(0AC) assuming [PhJ 1  2  3  3  2  9  114  [Ph]  =  T B E  - 6.9 mm/sec. and [ 0 A C ]  B I D E N T  =  [oAc]  E S T E R  =  0.0 mm/sec.  and a regular mer - octahedral structure (Figure 4, // 15) gives | e qQ | = 20.7 nm/sec, ri = 1.0.  Small d i s t o r t i o n s from regular geometry could  2 conceivably lead to the observed values of e qQ ri  =  =  - 20.1 mm/sec. and  0.46. Of course, a s i m i l a r six-coordinate structure with bridging  2 acetate groups would also be i n good agreement with the observed e qQ and 98 ri. Such bridging carboxylates are commonly found i n organotin chemistry Calculations f o r the corresponding fac - octahedral structures (Figure 4, 2 # 14) give e qQ - 0 and so such a formulation must be rejected as incompatible with the observed  values.  Another p o s s i b i l i t y would be a t r i g o n a l bipyramidal structure with one a x i a l and one equatorial acetate group, as i n Figure 4, # 8. I f f  "1TBA  we use OACI  =  f~  "1 TJ3FJ  OAC]  =  0.0 mm/sec, and the appropriate phenyl  values, then we derive |e qQ| - 11 mm/sec. and n - 1.  I t i s d i f f i c u l t to 2  conceive of d i s t o r t i o n s from t h i s structure which would lead to e iQ - 20 mm/sec. and n, - 0.5 and so this structure, too, must be rejected as unreasonable. 99 Further studies  are being conducted i n these laboratories i n  order to elucidate more c l e a r l y the bonding i n t e r a c t i o n s i n these carboxylates, In p a r t i c u l a r , low temperature I.R. studies of v(C=0) d i s t i n g u i s h bidentate from e s t e r - l i k e carboxylates.  and v(C-0)  should  FIGURE 21.  Sb Mossbauer Spectrum of Ph Sb(OAC) 3  2  Showing Improvement of F i t  f o r n = 0.46 (b) Over That f o r n = 0.0 (a).  TRANSMISSION  116  The Mo'ssbauer parameters f o r Ph^SbCOjOH are q u i t e d i f f e r e n t from  those of Ph^SbCl^,  whereas i f t h e s e compounds were i s o s t r u c t u r a l i t  might be a n t i c i p a t e d t h a t the d a t a would be f a i r l y s i m i l a r . expect  the s t r u c t u r e of P h ^ S b ^ O H to c o n s i s t  m o i e t i e s w i t h b r i d g i n g 0-atoms.  The  ready  One  might  of t r i g o n a l b i p y r a m i d a l  solubility  of the  related  97 alkyl  compounds i n o r g a n i c s o l v e n t s  s t r u c t u r e were adopted  i n d i c a t e s t h a t i f such a b r i d g e d  then i t would p r o b a b l y  c o n s i s t of o l i g o m e r s  perhaps  b e i n g the f i v e - c o o r d i n a t e antimony analog  of BuSn(0)OH (Bu = n-C^Hg).  s t r u c t u r e o f t h i s t i n s p e c i e s i s proposed  t o be  a cyclic  The  trimer with  Sn - 0 - Sn l i n k a g e s ^ . 1  I f indeed  the i n d i v i d u a l Pb^SbCOjOH u n i t s have a  trigonal  b i p y r a m i d a l s t r u c t u r e then t h e r e are a number of p o s s i b l e arrangements o f the l i g a n d s about antimony.  A p p l i c a t i o n of the a d d i t i v e model f o r the  Q.S.  shows t h a t some of t h e s e arrangements a r e i n c o m p a t i b l e w i t h the e x p e r i -  f  "I TBE  From our p r e v i o u s d i s c u s s i o n of the [OH]™  - [ci]™  we  expect  Cl  that [ o ] OH  L  B  R  I  D  G  E  will  l i e i n the range - 1.0  l i e i n the same range.  assume t h a t £OHJ w i l l have a v a l u e i n i t s range such t h a t  ^ZZ  v  a  l  u  e  ke produced.  s  Using  "IBRIDGE  v a l u e s and the knowledge t h a t  will  maximum or a minimum as the case may  0  We  to  also  will  w i l l be a t a  be, i n o r d e r t h a t the l a r g e s t range of the a p p r o p r i a t e p h e n y l v a l u e s  from  2 T a b l e IX we  first  examine the range o f p o s s i b l e e qQ v a l u e s f o r a s t r u c t u r e  w i t h a x i a l p h e n y l groups. V  Z Z  -  4[ph]  T B A  From F i g u r e 4, # 6, we -  {o]  B R I D G E  -  [OH]  T B E  have,  117  Thus, V  z z  should l i e i n the range  while n i s small.  4 - 25.8 nm/sec.  - 31.8 mm/sec. <  From this treatment we see that V  i s far too large  in magnitude and has the wrong sign, so t h i s structure may be rejected.  A structure with equatorial phenyl groups i s also possible and this gives (Figure 4, #8)  '» - ! M - M™.- W IBE  In this case  BA  has values i n the range - 20.2 mm/sec. ^  mm/sec. while n l i e s i n the range 0.5 - 0.9.  •< - 14.2  Departures from regular  t r i g o n a l bipyramidal geometry could conceivably lead to the observed r e s u l t of V^  z  =  +10.9 mm/sec , n = 0 but i n view of. our previous  discussion of d i s t o r t i o n s i n such systems the angle C - Sb - C i s anticipated to become larger than 120° on d i s t o r t i o n and so the value of should become even more negative.  Thus, although t h i s structure  cannot be eliminated, i t must be regarded as extremely u n l i k e l y .  F i n a l l y , f o r a structure with one a x i a l and one equatorial phenyl group (Figure 4, #9) we have,  v  z z  = |[o]  -  B R I D G E  which gives + 17.6 mm/sec. ^ V  z z  [p ] h  T B E  -[ph]  T B A  -  [OH]  TBA  > + 10.6 mm/sec, n. = 0.3 - 0.4.  This  i n much better agreement with the experimental r e s u l t s . Thus, i t i s possible to f i n d a five-coordinate structure for 2 P l ^ S n ^ O H whose calculated e qQ i s i n reasonable agreement with the observed value.  In l i g h t of the d i s t o r t i o n s which should be produced by  the bridging oxygen atoms, i t i s u n l i k e l y that a regular structure w i l l  118 be adopted by this compound.  The large difference i n magnitude and  the opposite sign observed f o r V^  z  i n Ph^SbCl^ and Ph^SbCCOOH may thus  be r a t i o n a l i z e d by assuming these compounds are not i s o s t r u c t u r a l .  It i s the I.S. of Ph^SbCOOOH, however, which i s r e a l l y somewhat anomalous as compared to that of Ph^SbCl^.  In addition to Ph^SbtCOOH,  other compounds containing antimony-oxygen linkages have I.S. values which are somewhat more p o s i t i v e than might have been anticipated on the basis of e l e c t r o n e g a t i v i t y arguments.  For example, the I.S. of Ph^SbOH  i s more p o s i t i v e than that of Ph^SbF while the I.S. of (Ph^b^OCrO^ i s more p o s i t i v e than that of Ph.jSbF2. probably  These data suggest that i t i s  the Sb - 0 linkage which i s responsible f o r at least part of  the large increase i n the I.S.  In this regard, i t would be of i n t e r e s t to  measure the I.S. of the polymer Ph^SbO.  There are other factors which may influence the I.S. as w e l l . From t i n chemistry, we may extrapolate a number of trends.  F i r s t l y , an  increase i n the coordination number i s generally associated with a decrease i n the I.S. at the t i n nucleus"^ . 1  119 Sn and  Since 6r/r i s opposite i n sign f o r  121 Sb, we would expect a more p o s i t i v e I.S. at the antimony 119  nucleus on increasing the coordination number.  Secondly, the  values of compounds of the type R^^SnX^ i n i t i a l l y increase  Sn I.S.  as n i s  increased, reaching a maximum at n = 2, and then decrease a g a i n ^ . 1  1  On  simple e l e c t r o n e g a t i v i t y grounds, however, one might expect the I.S. to decrease monotonically.  The explanation advanced f o r this phenomenon i s  that when the organic R group i s replaced by a more electronegative ligand,  119  some rehybridization takes place, leading to more s-character i n the Sn - C bonds and more p-character i n the Sn - X bonds.  Thus, although  the X groups are more electron withdrawing, the electrons withdrawn are primarily p i n character and i n fact the net result  i s less shielding  of the s-electrons by p-electrons and an increase i n the L S . ^ . Of course, t h i s process must reach a maximum at some stage and then a decrease i n the I.S. should be observed.  Thus, f o r antimony compounds of the type  ^SbX^ we would  expect an i n i t i a l decrease i n the I.S. as n increases and indeed t h i s i s observed f o r n = 1, 2 (Tables VII and VIII).  For antimony, however, the  value of n which minimizes the I.S. has not been established but i t i s obvious from our discussion of the mechanism involved that i t may w e l l depend on the e l e c t r o n e g a t i v i t y of the X-group and should be p a r t i c u l a r l y s e n s i t i v e to the h y b r i d i z a t i o n at antimony.  For example, i n compounds of  the type R^SbX^ which adopt t r i g o n a l bipyramidal structures, the I.S. should be d i f f e r e n t f o r the case when the two organic groups are equatorial r e l a t i v e to the cases i n which they are both a x i a l or one a x i a l and one equatorial.  Thus, the much more p o s i t i v e I.S. of Pb^Sb^OH r e l a t i v e to P l ^ S h d ^ may be explained i n part by the nature of the Sb - 0 linkage and in part by assuming that these two compounds are not i s o s t r u c t u r a l .  In summary, the Mossbauer data f o r compounds of the type R^SbX and R,SbX  are consistent with t r i g o n a l bipyramidal structures with the  120  X groups i n a x i a l p o s i t i o n s .  The additive model f o r Q.S. was applied  2 to these derivatives and the calculated e qQ values were i n good agreement with the experimental ones.  For compounds of the type Ph.jSbX2 a l i n e a r 2  r e l a t i o n was  found between the I.S. and e qQ, the slope of which was  consistent with a-bonding e f f e c t s being the dominant factor i n determining the Mossbauer parameters. above l i n e a r r e l a t i o n s h i p .  (Ph^Sb^OCrO^ was  found to be an exception to the  Possible structures were suggested f o r  Pt^SbCl^* but on the basis of present evidence no choice could be made between a five-coordinate structure with equatorial phenyl groups and six-coordinate structures with trans phenyl groups and bridging chlorines. The Mossbauer parameters of Ph^SbCOAc^ were found to be inconsistent with the formulation of t h i s compound as a t r i g o n a l bipyramidal species.  Octa-  hedral structures with one e s t e r - l i k e and one bidentate acetate group are 2 compatible with the observed parameters.  For Pb^SbtWOH both the e qQ  value and the I.S. are quite d i f f e r e n t from those of Pt^SbCl^ but these differences may be r a t i o n a l i z e d by assuming that the two compounds are not i s o s t r u c t u r a l . Sb - 0  I t has been found that i n compounds containing the  linkage the I.S. values are apparently more p o s i t i v e than would be  predicted on the basis of e l e c t r o n e g a t i v i t y arguments.  121  PART 3  R Sb(Fe(CO) Cp) _ X COMPOUNDS. n  2  4  n  The nature of heteronuclear metal-metal bonds has been the subject of much recent discussion, particularly in compounds where a group IV element is bonded to a transition metal such as manganese, iron or cobalt.  Compounds containing Fe-Sn bonds have been rather extensively  A v v . 102-109 , , 57^ studied by Mossbauer spectroscopy , where use of_ both Fe and, 119_Sn  resonances has led to valuable insights concerning the nature of the Fe-Sn a-bond and the question of possible (d-d)" bonding.  From data on  a number of derivatives of the types CpFe(CO)L.SnR.j and CpFeL2.SnR.j (Cp = 7T-C^H^;  R = Cl, CH^, C,H-;  i t has been suggested ^ 1  character.  2  L = tertiary phosphine, arsine, stibine) ,  that the Fe-Sn bond i s essentially pure a in  Greenwood ^, Donaldson ^ 1  1  and their respective co-workers have  7  2  reported signs of the quadrupole coupling constants e qQ for both iron and 2  t i n in CpFe^O^SnCly  The positive e qQ for t i n indicates an excess of ?  electron density on the t i n atom, from which Donaldson ^ 1  the Fe-Sn bond must have predominantly G  g  character.  7  z  has concluded that  Bryan *" has arrived 11  1  at similar conclusions on the basis of X-ray crystallographic studies. To date the only Mossbauer study of the Fe-Sb bond using both "* Fe and 7  121  Sb  resonances is of the two compounds Ph.jSbFe(C0) and 4  (Ph^Sb^Fe^O) ^> i donor . 111  The  121  Sb  n  which the group V element acts as a two-electron data were i n t e r p r e t e d  111  as indicating very l i t t l e ,  i f any, TT back-donation from f i l l e d 3d orbitals on iron to vacant 5d orbitals on antimony.  122  The preparation of a number of group V derivatives of t r a n s i t i o n metal carbonyls i n which the group V element acts as a one41 electron donor has recently been reported  and i t seemed a t t r a c t i v e to  investigate the Mossbauer spectra of a f a i r l y extensive series of such 42 compounds containing Fe-Sb a-bonds  .  Since i s o e l e c t r o n i c t i n and antimony  derivatives having i d e n t i c a l ligands are expected to show e s s e n t i a l l y l i n e a r correlations f o r both isomer s h i f t s and quadrupole s p l i t t i n g s , we have chosen f o r the present study cations of the type (X = c l , Br, I, CF^, C g ^ , n - C ^ ;  X Sb(Fe(CO) Cp) n  2  + 4 n  n = 1, 2, 3, but not a l l combinations).  The corresponding neutral t i n species have been widely studied by Mossbauer 102-107,112,112 .„ . , _ ., v , • spectroscopy ' and X-ray s t r u c t u r a l data are available i n ,,110,114-116 _ „. , . . many cases as w e l l ' . Furthermore, although there i s a paucity t  of X-ray data f o r compounds with Fe-Sb bonds, the structures of two of the present d e r i v a t i v e s , v i z .  Cl Sb(Fe(CO) Cp) 2  2  2  SbCl  7  and  ClSb(Fe(CO) Cp) 2  3  118 FeCl .CH Cl 4  2  cations  2  , have been published.  Both compounds consist of discrete  i n which the antimony atom i s i n a very d i s t o r t e d tetrahedral  environment,  associated with large, replaceable anions (Sb-jCl^  and  2FeCl^  , respectively).  The corresponding t i n compounds consist of four-  coordinate neutral s p e c i e s  1 1 4  11  *'.  In the case of Cl Sb(Fe(CO) Cp) SbCl^ 2  2  2  there are two  independent  cations i n the unit c e l l , but they have e s s e n t i a l l y the same geometry about antimony ^. 11  2  123  121 (A)  ..  121  Sb Mossbauer parameters.  The  Sb Mossbauer spectra of compounds  containing tetracoordinate antimony species such as those reported here are expected to follow certain trends, which may  be i l l u s t r a t e d by a pre-  liminary consideration of the data given i n Table XI.  F i r s t l y , since  these derivatives should be i s o e l e c t r o n i c with the corresponding n e u t r a l 21 25 t i n species, an additive model ' as discussed above f o r the quadrupole 2 24 coupling constants e qQ should be applicable , and there should be a 119 more or less d i r e c t c o r r e l a t i o n with on the basis of  1 1 9  Sn  Sn quadrupole s p l i t t i n g s .  data [e qQ| i n the series XSb(Fe(CO) Cp> 2  2  + 3  Thus,  is  expected to increase with changes i n X i n the order Bu, Ph < I < Br, C l , as observed.  Secondly, i n compounds containing cations of the type  X Sb(Fe(CO) Cp) 2  2  i f the X^Sb^X angle i s considerably less than 109.5°  + 2  (as indeed i t i s i n  Cl Sb(Fe(CO) Cp)  the asymmetry parameter 2 H should be less than unity but considerably greater than zero, and e qQ should be opposite i n sign but of approximately the same magnitude as i n + 19 25 2  2  the corresponding XSb(Fe(C0) Cp) 2  2  Sb Cly  1 1 7  2  derivatives  3  ,  '  .  Table XI shows that  t h i s i s i n fact the case (see also Figures 22 and 23). that the n values observed  (0.44, 0.46)  f o r the C l S b ( F e ( C O ) C p ) 19  are s l i g h t l y smaller than that reported (0.65) compound C l S n ( F e ( C 0 ) C p ) . 2  > Fe^Sn^Fe  2  1 1 4 , 1 1 7  .  2  It should be noted  2  2  + 2  species  for the corresponding t i n  This i s consistent with the fact that Fe^-Sb^Fe  T h i r d l y , the e qQ value for S b ( C o ( C O ) P P h ) ^ i s zero 2  3  3  within experimental error, as anticipated for a tetrahedral antimony derivative with four i d e n t i c a l ligands. 121 A c l o s e r look at the systematics of the  Sb quadrupole coupling  TABLE XI. 1 2 1  COMPOUND  S b MOSSBAUER PARAMETERS FOR COMPOUNDS OF THE TYPE R Sb(Fe(CO)„Cp), X* n I 4-n 2 T(mm/sec.) 6* (mm/sec.) n e qQ (mm/sec.) b  2  X  c  -9.3 + 0.2 (-9.4 + 0.2)  +29.0 ± 1.0 (+31.4 ± 1.1)  0.46 ± 0.05 (0.0)  2.8 + 0.1 (2.7 + 0.1)  206 (263)  (ci Sb(Fe(CO) Cp)[Cr(SCN) (NHj)  -9.1 + 0.2 (-9.2 + 0.2)  +28.7 ± 0.5 (+30.2 ± 0.6)  0.44 ± 0.04 (0.0)  2.9 + 0.1 (2.9 + 0.1)  183 (213)  [fir Sb (Fe(CO) Cp) ^ [PFg]  -9.6 + 0.3 (-9.7 + 0.3)  +26.6 ± 0.4 (+28.6 ± 0.4)  0.44 ± 0.06 (0.0)  3.0 + 0.1 (3.0 + 0.1)  203 (334)  -8.3 + 0.2 (-8.4 + 0.2)  +18.3 ± 0.7 (+20.1 ± 0.9)  0.68 ± 0.08 (0.0)  2.4 + 0.1 (3.4 + 0.2)  187 (265)  (ciSb(Fe(CO) Cp) £ J JFeCl^  -8.8 + 0.1  -23.9 ± 1.7  0.0  3.1 + 0.1  198  (BrSb(Fe(CO) Cp) j[PF J  -8.6 + 0.3  -23.8 ± 0.7  0.0  3.1 + 0.1  225  "lSb(Fe(CO) Cp) ~|[l j  -8.8 + 0.3  -20.5 ± 1.0  0.0  3.1 + 0.1  191  _ISb(Fe(CO) Cp) J j [ l » F g J  -8.8 + 0.2  -22.4 ± 0.2  0.0  2.9 + 0.1  205  fphSb(Fe(CO) CPg [ P F ^ J  -7.9 + 0.1  Unresolved  0.0  3.2 + 0.1  205  -7.0 ± 0.2 (-7.0 ± 0.2) (-7.0 + 0.2)  -7.0 ± 0.4 (-6.8 ± 0.4) (+3.4 ± 1.4)  0.43 ± 0.16 (0.0) (0.0)  2.8 + 0.1 (2.9 + 0.1) (3.4 + 0.1)  [ci Sb(Fe(C0) Cp) J|PF^| 2  2  2  2  2  2  4  2  ((CF ) Sb (Fe(CO) Cp) Jj jjCr(SCN) (NH ) J 3 2  2  4  2  2  6  3  2  3  3  2  2  [ph Sb (Fe (CO) Cp) 2  2  JPF^j  3  116 d (118)° (158) d  G  CONTINUED/. to  TABLE XI(CONTINUED). 121, Sb MOSSBAUER PARAMETERS FOR COMPOUNDS OF THE TYPE R^Sb^e (CO) Cp) ^ X 2  COMPOUND  2  r(mm/sec.)  ± 0.5 ± 1.3)  0.0 (0.0)  2.8 ± 0.1 (3.6 ± 0.2)  149 (208)  +9.4 (-2.9  ± 0.4 ± 0.8)  0.0 (0.0)  2.9 ± 0.1 (4.0 ± 0.2)  72 (147)'  +6.9 (-4.2  ± 0.5 ± 0.8)  0.0 (0.0)  2.9 ± 0.1 (3.3 ± 0.1)  169 (200)  0.0  2.9 ± 0.1  185  e qQ (nun/sec.)  -6.7 ± 0.2 (-6.5 ± 0.2)  +9.6 (-4.5  -6.7 ± 0.2 (-6.5 ± 0.2)  [Bu SbFe(CO) Cp)j [ P F J  -6.9 ± 0.1 (-6.8 ± 0.1)  [sb(Co(CO) PPh ) J jPFg]  -8.7 ± 0.2  [jh SbFe(C0) Cp] 3  (PFg'J (Run 1^  2  (Run 2 )  3  f  2  3  8L  3  4  Samples contained ca. 10 mg. Sb/cm  2  0.0  o and gave absorption i n t e n s i t i e s of ca. 10-25% at 9 K.  Values i n parentheses are alternate f i t s of the data.  In most cases we f e e l there i s a,  2  s i g n i f i c a n t reduction i n X f °  o r  u  r  preferred  solution.  c  Isomer s h i f t s are r e l a t i v e to a Ba S n 0 source at 80°K. Approximately 180 degrees of freedom unless otherwise noted.  d  Approximately 100 degrees of freedom.  e  No s i g n i f i c a n t improvement i n f i t f o r n 4 0.  f  Two independent measurements.  b  1 2 1  3  2C  n  6^ (mm/sec.)  X  d  126a  FIGURE 22.  121 Sb Mossbauer Spectrum of Cl2Sb(Cp(CO)2Fe) PFg Illustrating Improvement +  -  2  of F i t for n = 0.46 (b) Over That for n = 0.0 (a).  i  126  FIGURE 22. o  Oj  1.00-  .98i  Cl Sb(Cp(C0) Fe)2 P F  .96-  2  2  with 77 = 0 . 0 .94-  Q)  0  0  o°  0 0 0  Cl Sb (Cp(C0^Fe^P^ 2  with 77 = 0 . 4 6  -20  -10  0  10  V e l o c i t y( m m / s e c )  6  FIGURE 23.  121 Sb McSssbauer Spectrum f o r CISb(Cp(CO) Fe) PF ~• +  2  3  6  127  CISb (Cp(CO)FeSPfT 2  -24  -16 -8 0 8 Velocity (mm/sec) FIGURE 2 3 .  16  FIGURE 24.  Sb Mossbauer Spectrum of Ph^SbFe(CO) Cp PF 2  Alternate F i t s to the Data.  fi  Showing  In (a) the F i t t i n g  2 Parameters Were 6 = -6.7 mm/sec, e qQ = +9.4 mm/sec, T = 2.9 m / s e c , n. = 0.0. In (b) , <5 = -6.5 mm/sec, 2 e qQ = -2.9 mm/sec, T = 4.0 mm/sec, n = 0.0. 2 The F i t with e qQ>0 i s Clearly Preferable.  Ph SbCp(CO) Fe PI^"  P h S b C p ( C O ) F e PF '  e qQ = -f 9.4 mm/sec  e qQ = - 2.9 mm/sec  +  3  +  2  3  2  2  FIGURE 24.  2  6  129  constants i s of i n t e r e s t . contributions  As discussed  to the e l e c t r i c f i e l d gradient  In the compound Sb (Co (CO) ^PPh^) ^ PF^ should contribute  at antimony,  one expects that  +  only q  above, there are two major and  ^y^-  - 0 so that  2 Since e qQ = 0, with a l i n e -  to the e.f.g..  J_iA-L  width of 2.9 mm/sec, an upper l i m i t of a few mm/sec. may contribution from q j ^ j *  Similar conclusions  may  be set for any  be drawn from data f o r  •+• — 2 8 Ph^Sb ClO^ , where again e qQ = 0 with a linewidth of 2.6 mm/sec. . i s thus clear that i n the present derivatives q,  TAT  It  makes the dominant  contribution to the e.f.g at the antimony nucleus. 121 Perhaps the most i n t e r e s t i n g  2 Sb e qQ r e s u l t s reported  are those for the organoantimony d e r i v a t i v e s .  here  For both R^SbFe(CO)^Cp  +  2 complexes (R = Bu, Ph), e qQ i s apparently p o s i t i v e (see Figure 24). contrast, the sign of e^qQ( ^Sn) i n Bu^SnFe(CO) Cp i s reported"*"*^  In  to be  11  2  2 119 109 112 negative, and a negative e qQ( Sn) has been predicted ' for 119 Since the sign of the quadrupole moment Q f o r Sn 121 i s the same as the sign of Q (and Q ) for Sb, these results indicate gr ex  Ph SnFe(CO) Cp also. 3  2  that the p r i n c i p a l component of the e.f.g. tensor, V„„, i s opposite i n sign i n the antimony and t i n compounds.  For the antimony derivatives  here, i t i s apparent from trends i n the isomer s h i f t  reported  (vide i n f r a ) that  the  electron density i n the region of the bonds decreases i n the order Sb-M  > Sb-R  > Sb-X,  and the same ordering i s deduced from data on  ponding t i n compounds ^. 1  corres-  On this basis alone one would expect e^qQ  p o s i t i v e f o r both antimony and t i n i n the triorgano d e r i v a t i v e s .  to be  However,  because of the f a i r l y small difference i n electron density at Sb between  130  Sb-M and Sb-R bond directions (and at Sn between Sn-M and Sn-R bond directions), relatively minor changes in p-electron donor and acceptor properties between R. and M groups vis-a-vis tin and antimony could presumably account for the observed sign reversal. 2 The additive model for e qQ should apply to these derivatives as i t does to the R SbX„ derivatives previously discussed. 5-n n tj c  these compounds, X-ray structural s t u d i e s  11  Since for  how large deviations  s  from regular tetrahedral bond angles we shall not carry out p.q.s. calculations explicitly since the theories for describing such distortions 7 25  are inadequate at present '  From data on a variety of tetrahedral t i n compounds the magnitudes 25 112 of the p.q.s. values (which are negative quantities) are found ' to decrease in the order Bu > Ph > Fe^O^Cp, which i s consistent with the 2 negative e qQ i n Bu^SnFe^O^Cp and the fact that the magnitude of the Q.S. i s greater for this compound than for the corresponding triphenyltin species.  For our R^SbFe^O^Cp"*" derivatives, the positive coupling  constants require that the p.q.s. value for Fe^O^Cp i s more negative than that for either R group. 2  This, together with the observation that  2  e qQ(Ph) > e qQ(Bu), means that the p.q.s. values for the antimony complexes decrease in magnitude in the order Fe^O^Cp > Bu > Ph, the same order expected from isomer shift data, since the I.S. of the butyl derivative is more negative than that of the phenyl derivative. It i s interesting to note that in the compounds Ph.jSbFe(CO)  4  131  and (Ph^Sb^FeCCCO-j, 121  of  e  2 121 111 qQ( Sb) i s also p o s i t i v e .  In f a c t , the values  2 2 Sb e qQ and isomer s h i f t 6 f o r these compounds (e qQ = + 9.0, +  mm/sec; & = - 6.62,  - 6.65  mm/sec, respectively) are nearly i d e n t i c a l  v/ith those of Ph^SbFe(CO) Cp . +  2  10.9  Thus, while i t i s usual to consider that  i n one case antimony i s acting as a two-electron donor to i r o n and i n the other as a one-electron donor, the o v e r a l l electron configuration at Sb and the electron density i n the Sb-Fe bonds are e s s e n t i a l l y the same i n both cases and the d i s t i n c t i o n i s a purely formal one. In the series Ph S b ( F e ( C O ) C p ) ( n  = 1, 2, 3), |e qQ j at Sb  2  decreases with decreasing n.  I f we assume the bond angles are such that  Fe-Sb-Fe > Fe-Sb-R > R^-Sb^R, as expected on the basis of X-ray data f o r s i m i l a r t i n  1 1 4  structural  and p h o s p h o r u s ^ derivatives and the known 1  121 structure of (CH^) bF (CO)^ , we f i n d that the molecular o r b i t a l 25 2 treatment of Clark et a l . predicts just the reverse order of |e qQ| s  e  3  25 values.  I t should be noted that Clark's model  of the ligands are constant throughout  assumes that p.q.s. values  such a s e r i e s .  However, there i s some  evidence from "^Fe Q.S. data discussed below that this i s not the case, since (AE^)^ i s found to decrease i n the order Ph„SbFe(CO)_Cp > Q Fe 3 2 +  Ph Sb(Fe(CO) Cp) 2  2  + 2  > PhSb(Fe(CO) Cp) 2  + 3  (Table X I I I ) , i n d i c a t i n g changes i n  the electron d i s t r i b u t i o n i n the Fe-Sb bonds.  The observed trend i n |e qQ|  values i s probably best r a t i o n a l i z e d i n terms of t h i s e f f e c t which would produce a decrease i n the difference of r e l a t i v e p.q.s. values of Fe(C0) Cp 2  and Ph. 121 The second parameter of i n t e r e s t i s the  Sb isomer s h i f t ^g^'  132  T y p i c a l ranges of values f o r 6 ^ are -19 to -9 mm/sec. f o r Sb(lII) compounds and -7 to +4 mm/sec. f o r Sb(V) compounds  7  ( r e l a t i v e to  121 Ca at  SnO^).  Normally, 6 ^ f o r organoantimony (III) derivatives  fall  the most p o s i t i v e end of the range f o r Sb(III) while those f o r  organoantimony (V) f a l l at the most negative end of the range f o r Sb(V) . The 6  values f o r the present series of compounds l i e between -9.6  and  -6.7 mm/sec, neatly spanning the range of values from Sb(III) to Sb(V). 103 122 If  the arguments of Zuckerman and co-workers  *  in  assigning oxidation states to the corresponding i s o e l e c t r o n i c t i n species as Sn(IV) are used, our compounds must be regarded as derivatives of Sb(V). However, <Sg^ of organoantimony compounds such as (p-ClC^H^)^Sb and (p-CH^OC^H^^Sb  56  which c e r t a i n l y would be regarded as Sb(III)  d e r i v a t i v e s , are more p o s i t i v e ( i . e . , more "Sb(V)-like") than those of X Sb(Fe(CO) Cp) 2  2  + 2  (X = C l , B r ) .  S i m i l a r l y at the other end of the scale  i f we were to regard the present species as derivatives of Sb(III), where for  example R<jSb: acts as a two-electron donor toward i r o n , we f i n d some g  compounds such as Ph.jSbI  2  and Ph SbCl.j (Table VII) with s h i f t s more 2  negative than those of R.jSbFe(CO) Cp 2  +  (R = Ph, Bu).  This apparent  dichotomy i s perhaps not s u r p r i s i n g i n view of the fact that the isomer s h i f t r e s u l t s from the p a r t i c u l a r e l e c t r o n i c configuration about the metal nucleus which i s e s s e n t i a l l y a continuous function, while the oxidation number i s a discontinuous function.  Thus the assignment  of a  p a r t i c u l a r oxidation state f o r antimony i n compounds of this type has little  justification.  133  It has been s u g g e s t e d ^  that 6 g  1  should be used to  n  assign the valency of t i n rather than the formal oxidation state.  On  this basis the isomer s h i f t s of our tetravalent antimony compounds span the range from t r i v a l e n t to pentavalent antimony d e r i v a t i v e s , behaviour which i n no way seems unusual.  In the compounds X Sb(Fe(CO)2^p)2  o  n  e  2  ^-§b.t expect that on  m  substituting a more electronegative group X the s-electron density at the antimony nucleus would decrease due to more e f f i c i e n t e l e c t r o n withdrawal.  Since the change i n nuclear charge radius between excited and 121  ground states (6r/r) i s negative f o r  Sb, 6g^ should then increase  towards more p o s i t i v e values i n the order Ph < CF^ < Br, C l , whereas the reverse order i s that observed  (Table XI).  The explanation f o r this  e f f e c t p a r a l l e l s that f o r t i n - t r a n s i t i o n metal complexes ^ »107 where 1  3  the r e l a t i v e amounts of p-character used i n the M-X bonds increase i n the order Ph < CF^ < Br, C l , and thus the amount of s-character i n the M-Fe bonds w i l l increase i n the same order.  The fact that the s-electron  density at the Sb nucleus increases with increasing s-character of the Sb-Fe bond shows that the Fe(CO) Cp group i s a better donor than halogen, 2  CF^ or Ph groups. For a series such as X ^ _ S b ( F e ( C O ) C p ) n  2  + n  the greater donor  strength of the Fe(CO) Cp moiety i s expected to dominate the trend i n 2  s-electron density so that  should become more negative as n increases  This trend i s observed f o r Ph^ S b ( F e ( C O ) C p ) , but both X Sb(Fe(CO) Cp) +  n  2  n  2  2  (X = C l , Br) complexes have 6_, values more negative than those of the OD  134  corresponding XSb (Fe(CO) Cp) . j  derivatives.  +  2  This suggests  that i n  the dihalides the s-character i n the X-Sb bonds i s already so low that replacement of an X by Fe(CO) Cp leads to l i t t l e i f any increase i n 2  the t o t a l s-character i n the Fe-Sb bonds, and the isomer s h i f t i s thus dominated by an increase i n p-shielding.  P a r a l l e l i n g this there i s a general trend to smaller increases i n s-electron density at t i n for each increment i n n as n increases i n ~~ i * <-u 103,105,107,125,126 , , • •' complexes of the type X^ SnM (where M represents a v  n  n  transition-metal carbonyl group).  Indeed, f o r the series C l ^ Sn|Mn(C0)^j^ n  as n increases from 2 to 3, and for I _ S n ^ C o ( C 0 ) ^ j n ^ ^ ^ as n 4  n  increases from 3 to 4, the s-electron density at t i n actually f a l l s , an e f f e c t s i m i l a r to that observed  here.  While i t i s clear from the above discussion that the system121 a t i c s of  Sb isomer s h i f t s show many p a r a l l e l s to the corresponding t  119 Sn systems, i t i s worth examining the c o r r e l a t i o n i n more d e t a i l . Using the data of Ruby et a l . (with a correction of + 0.20 mm/sec. 1  f o r a systematic error i n t h e i r  7  119  127 Sn parameters ) one obtains the  l i n e a r c o r r e l a t i o n between  and 5 f o r i s o e l e c t r o n i c antimony Sb Sn and t i n compounds shown i n Figure 26. Ruby's values for <S were 1 1 Q  1  1  ± i y  7  191  121 converted to a scale r e l a t i v e to Ba Sn0„ Ca  121  ^nO^) using the value  (= < 5 , 9 ,  S  b  r e l a t i v e to  17 127 128 ' * - 8.62 mm/sec. for the isomer s h i f t  121 of InSb r e l a t i v e to Ba SnO^- S i m i l a r l y , a value of 1.95 mm/sec. f o r 119 119 the isomer s h i f t of aSn r e l a t i v e to Ba SnO^ (= Sn0 ) as derived 2  129 7 from r e f . 17 (accepted values are 2.0 to 2.1 mm/sec. ) was used to  135  convert the corrected <5^g 119 to a Sn to 3-^9  values of r e f . 17 from  119 r e l a t i v e to Ba SnO^* S  relative  n  S  n  This procedure was adopted  ^ ll9 121 119 121 6Sn, 8Sn and SnC^, ^20^ n  since the points  were  128 available independently as a cross-check  .  We have gathered i n  Table XII the available isomer s h i f t data f o r nominally i s o e l e c t r o n i c pairs of antimony and t i n compounds of the types X^ S b ( F e ( C 0 ) 2 C p ) n  and X _ Sn(Fe(C0)2Cp) , 4  n  n  + n  and these points are also displayed i n Figure 25.  It should be noted that i n the region below the straight  line  2 121 2 119 ( Sb) > ¥ ' ( Sn), while the opposite i s true i n the region above s s the l i n e .  ¥  Consider f i r s t the dichloro and dibromo d e r i v a t i v e s .  As can  be seen from Figure 26 both points l i e below the i s o e l e c t r o n i c l i n e . That this should be so i s a consequence of the s l i g h t l y d i f f e r e n t structures of C l S b ( F e ( C O ) C p ) 2  2  +  1  1  7  2  and C l S n ( F e . ( C O ) C p ) 2  2  114 2  .  The  Fe-Sb-Fe angle (134.7°, avg.) i s s i g n i f i c a n t l y greater than the Fe-Sn-Fe angle (128.6°) i n t h e i r respective compounds.  This indicates that the  Fe-Sb bond should have more s-character than the Fe-Sn bond so that the 121 s-electron density at the  Sb nucleus should be somewhat greater than  119 that at the  Sn nucleus.  Note that although the compounds are  i s o e l e c t r o n i c t h i s does not mean that the charge densities at the n u c l e i 128 w i l l be equal For  only that they w i l l be equivalent. the compounds Ph^ S b ( F e ( C O ) C p ) n  2  + n  and Ph^ S n ( F e ( C O ) C p ) n  2  n  i t appears (Figure 26) that there i s a trend toward somewhat less than equivalent electron densities at the  121 Sb. nucleus as n increases.  It i s  TABLE XII. ISOMER SHIFTS OF NOMINALLY ISOELECTRONIC ANTIMONY AND TIN COMPLEXES. 6 , (mm/sec.) / n i * . T3 c n ^ (Rel. to Ba SnOy  COMPOUND  1  J c i S b (Fe (CO) C ) J Jcr(SCN) (NHg) J  -9.3  ^Cl Sb(Fe(CO) Cp) J p F ^ j  -9.1  2  2  P  2  4  2  2  1  5_  KEF«  (mm/sec) ^ _ 119 . REF. (Rel. to Ba SnO^)  COMPOUND  n  c n  Cl Sn(Fe(CO) Cp) 2  2  2  2  2  2  1.95,  1.98  d,e  Br Sb(Fe(CO) Cp) JpF J  -9.6  a  Br Sn(Fe(CO) Cp)  PhSb(Fe(CO) Cp) J [PF J  -7.9  a  PhSn(Fe(CO) Cp)  jPh Sb(Fe(C0) Cp)2j p F ^ j  -7.0  a  Ph Sn(Fe(CO) Cp)  |l>h SbFe(CO) Cp| Q>F^j  -6.7  a  Ph SnFe(CO) Cp  1.43 ,  1.41  g.h  ph sb][cio J  -5.9 Ph Sn  1.20 ,  1.22  g,l  2  2  2  2  2  6  2  3  A  3  6  2  4  2  2  2  2  2  3  2  -6.0  JBu SbFe(CO) Cpj |PF^J  -6.9  4  3  a  4  2  This work.  C  Ref. 56.  d  Ref. 104.  6  Ref. 106.  3  2.00^  g  1.74  g  J  2  J  1.47  Bu^nFe(CO) Cp 2  Ref. 107. Ref. 105. Ref. 102. i 119 H.A. Stockier and H. Sano, Trans. Faraday S o c , 64, 577 (1968). Converted to Ba SnO3 scale assuming 6(a-Sn) =• +2.10 mm/sec.. b  Ref. 111.  a  f  J  4  [Ph Sb] [BF ]  1.99  J  2  f  8  J  7  h  }  FIGURE 25.  The Correlation of Shifts.  Sn and  Sb Isomer  The Straight Line, Based on the Assumption  of Equivalent Electron Density at the Two Nuclei, i s after Ruby. The Points for the Isoelectronic Pairs Are Labelled Using the Notation M = Sn or Sb and Fe = Fe(C0) Cp. +  9  FIGURE 25.  2.00-  PhMFe  Br* MFe<  3  CUMFe:  Ph MFe 2  o  1.50-  Bu MFe^ 3  CD  2  H  PhJVIFe PK.M  1.00-  4  o _o  .50-  a ) S bi s o m e rs h i f t sr e l a t i v et oB a S n0 s o u r c e b ) S ni s o m e rs h i f t sr e l a t i v et oB a S n O * 3  119  0.0-  119  -10  -8  -6  -4  a ) Velocity (mm/sec)  -2  o  i—*  138  d i f f i c u l t to e s t a b l i s h i f t h i s apparent trend i s r e a l l y s i g n i f i c a n t i n view of the uncertainties involved i n the slope and p o s i t i o n of the isoelectronic line.  It should be noted however that the e r r o r bars f o r  the ^ 2 1 9 values are rather misleading, since the major source of e r r o r Sn arises from a possible systematic error i n converting from a-Sn BaSnO^ reference value.  to  Thus the apparent gradient of the data points  has somewhat more s i g n i f i c a n c e than the i n d i v i d u a l values. If the trend i s a r e a l one i t implies that the difference i n the amount of s-density donated by phenyl and by FeCCO^Cp i s less f o r 121  119 Sb than for  Sn.  This e f f e c t could a r i s e i n two ways.  I f there i s  a change i n h y b r i d i z a t i o n between the two s e r i e s of compounds such that there i s more s-character i n the Sb-C  than Sn-C bonds (and  correspondingly  less i n the Sb-Fe than Sn-Fe bonds), then since F e ^ O ^ C p i s a better 121 donor than phenyl the s-electron density at the Sb nucleus w i l l not 119 change as r a p i d l y as that at the  Sn nucleus.  view of the results f o r the dihalogeno other p o s s i b i l i t y i s an increase i n any bond over that i n the Sn-Fe bond.  This seems u n l i k e l y i n  complexes discussed above.  The  (d-d)TT i n t e r a c t i o n i n the Sb-Fe  This would be expected since the  greater % £ £ of antimony w i l l contract i t s 5d o r b i t a l s and lower them i n e  energy r e l a t i v e to t i n , so that any Fe -»• M back-n-donation should be enhanced f o r M = Sb.  This i n t e r a c t i o n should lead to a lowering of the 121 119 s-electron density at Sb r e l a t i v e to Sn by increased s h i e l d i n g as the number of F e ^ O ^ C p groups i s increased.  C l e a r l y the best pair of  compounds to d i s t i n g u i s h between these two e f f e c t s i s Sn(Fe(C0)2Cp)  4  and  Sb(Fe(C0)2Cp) , but a l l attempts to prepare the l a t t e r d e r i v a t i v e have +  4  139  thus f a r been unsuccessful  42  .  For t h i s pair any h y b r i d i z a t i o n effects  should be minimized so that i f the trend i s due to a-bonding d i f f e r ences only the point should f a l l below or on the i s o e l e c t r o n i c l i n e . Conversely,  i f (d-d)TT i n t e r a c t i o n s are responsible they should be  maximized f o r t h i s p a i r and the point should l i e above the i s o e l e c t r o n i c line.  While there thus remains some uncertainly on the basis of 121 Sb Mossbauer data concerning the ir-character of the Fe-Sb bond (see 121 below), i t i s clear that the  Sb isomer s h i f t s are determined p r i m a r i l y  by a-bonding e f f e c t s and that any possible 7T-interactions play a purely secondary r o l e . 57 (B)  ••  57  Fe Mossbauer Parameters.  The  Fe isomer s h i f t s i n the present  derivatives (Table XIII) f a l l i n the narrow range 0.38  - 0.42  mm/sec.  ( r e l a t i v e to sodium n i t r o p r u s s i d e ) . The narrow linewidths (0.23 -  0.27  mm/sec.) indicate that i n any given compound a l l the i r o n atoms are i n e s s e n t i a l l y i d e n t i c a l environments.  The  range of 6^^  values i s nearly the  same as that reported f o r FeXCO^Cp groups bonded to tin ^ ,104,106^ There may be a trend to s l i g h t l y higher values i n the antimony derivatives 1  2  (see Table XIV) but a lack of published information on the "* Fe resonances 7  130 i n the t i n complexes  and the inherent d i f f i c u l t y of comparing small  differences i n isomer s h i f t s derived from d i f f e r e n t sources  precludes  detailed a n a l y s i s . It  i s worth considering i n some d e t a i l j u s t what changes i n  6^^  140  TABLE XIII.  ••  57  a  Fe MOSSBAUER PARAMETERS FOR COMPOUNDS OF THE TYPE R Sb(Fe(CO)„Cp), X n 2 4-n COMPOUND  5  b  (mm/sec)  AEg  C  (mm/sec.)  (mm/sec.)  0.40  1.83  0.26  0.40  1.86  0.25  0.40  1.81  0.26  0.40  1.83  0.26  (CF ) Sb(Fe(CO) Cp) J jPFgj  0.42  1.80  0.23  ClSb(Fe(CO) Cp) J peCl^j  0.39  1.73  0.25  [BrSb(Fe(CO) Cp) J JPF^j  0.38  1.72  0.26  JlSb(Fe(CO) Cp) JjPF J  0.39  1.71  0.26  [lSb(Fe(CO) Cp) j^I J  0.40  1.74  0.27  [phSb(Fe(CO) Cp)^|  0.41  +1.73  Ph Sb(Fe(CO) Cp) J [PF^  0.39  1.74  0.26  Ph SbFe(CO) CpJ  JPF^J  0.41  1.86  0.23  JBu SbFe(CO) Cp|  JPF J  0.38  1.87  0.24  Cl Sb(Fe(CO) Cp) J JCr(SCN) (NH ) J 2  2  2  4  ~Cl Sb (Fe (C0) Cp) 2  2  2  2  2  2  6  ^Sb Cl J 4  2  3  2  6  2  2  2  3  2  2  3  3  2  6  3  3  2  2  2  3  2  3  14  J [pF *j  [Br Sb (Fe (CO) Cp)2 2  2  JPF J  J  |cl Sb(Fe(CO) Cp) J  3  2  j~PF^  2  6  0.26  6  A l l measurements on neat s o l i d s with absorbers at 80°K and "^Co(Cu) source at room temperature. Isomer s h i f t r e l a t i v e to sodium n i t r o p r u s s i d e ;  estimated error  ±0.01 mm/sec.. c  Quadrupole  d  F u l l width at half-maximum;  splitting;  The sign of e^qQ was  estimated error ±0.01 to ±0.02 mm/sec. average of the two resonance  lines.  determined with both source and absorber at 4.2°K,  and the absorber i n a l o n g i t u d i n a l magnetic f i e l d of 30 kG (Figure 26).  TABLE XIV. Fe MtfSSBAUER PARAMETERS FOR Fe(CO)„Cp GROUPS BONDED TO TIN AND ANTIMONY COMPOUND  6(mm/sec.)  Cp(CO) FeSnCl 2  3  CCp(CO) Fe) SnCl 2  2  2  (Cp(CO) Fe) Sn(NCS) 2  2  Cp(CO) FeSnBu 2  Cp(CO) FeSnMe 2  3  3  Cp(CO) FeSnPh 2  2  3  (mm/sec.)  REF.  0.41  +1.86  a  0.40  1.86  b  0.39  1.84  c  0.39  +1.66  a  0.36  1.68  b  0.39  +1.69  a  0.38  +1.75  a  0.36  1.75  c  0.37  1.83  b  0.35  1,82  c  6(mm/sec.)  COMPOUND  (Cp(CO) Fe) SbCl 2  2  Cp(CO) FeSbBu 2  3  Cp(CO) FeSbPh  3  2  +  +  + 2  0.40  AE„ (mm/sec.) REF. —<J  1.81-1.86  0.38  1.87  d  0.41  1.86  d  Ref. 106. Isomer s h i f t values from this reference have been converted to the sodium nitroprusside scale, by the addition of 0.27 mm/sec. b  C  Ref. 104. Ref. 102.  ^ This work.  ^  d  142  between i s o e l e c t r o n i c t i n and antimony complexes would be expected depending upon whether or not there i s s i g n i f i c a n t Fe-Sb TT-bonding.  In  these compounds antimony may be regarded either as forming a normal (shared) electron p a i r cr-bond to a neutral Fe(CO) Cp group, i n which 2  case one i s e s s e n t i a l l y comparing R.jSb'  with R^Sn* (say), or as forming  +  a dative bond to a p o s i t i v e l y charged Fe(C0) Cp  species, where now the  +  2  comparison would be R^Sb: with R^Sn: .  In e i t h e r case antimony should  -  be a poorer CT-donor than t i n or conversely a better 0-acceptor.  This  means that i f the Fe-Sb bond i s e s s e n t i a l l y pure a i n character, the augmentation of 4s-electron density at iron w i l l be smaller i n the antimony d e r i v a t i v e s , which should consequently show higher ^ F e isomer 7  Any Fe-Sb Tr-bonding,  s h i f t s than the corresponding t i n complexes.  assuming t h i s to be between f i l l e d 3d o r b i t a l s on i r o n and vacant antimony 5d o r b i t a l s , would a f f e c t 6  i n the opposite d i r e c t i o n since the decrease  i n d-electron density at i r o n would decrease 5  by deshielding.  fact that there i s only a very small increase ( i f any) i n 6^  e  The  i n the  antimony complexes appears to argue i n favour of some 7T-interactions i n these compounds.  The "* Fe quadrupole s p l i t t i n g s , (^ Q)p > show two trends which 7  E  e  we f e e l are i n t e r r e l a t e d . (R = Ph, n = 1, 2, 3;  F i r s t l y , i n the compounds R^Sb (Fe (CO) Cp) ^ _ 2  R = C l , Br, n = 1, 2),  (AE  Q  )  P  E  + n  increases as n  increases, and hence as the a-donor a b i l i t y of the R Sb moiety decreases. n  S i m i l a r l y , i n the compounds R S b ( F e ( C O ) C p ) 2  order Ph < CF^  <  2  + 2  (^ g) E  F e  increases i n the  Br, C l , p a r a l l e l i n g the increasing e l e c t r o n e g a t i v i t y of R.  In both cases a decrease i n Sb  Fe cr-donation and thus i n the charge  143  density at Fe along the Fe-Sb bond direction (Z axis) is accompanied by an increase in (AE ) . Q re  Although the nature of the iron bonding  orbitals in these compounds is poorly understood, these results clearly imply a deficiency of electron density at iron along the Z axis and that V^^ should be positive (oblate charge distribution). spectrum of  PhSb(Fe(CO) Cp) .3 ( g) PF  w a s  2  The "^Fe Mossbauer  measured in an applied longitud-  inal magnetic f i e l d of 30 kG, and V^^ was found to be positive as expected (Figure 26).  It might also be noted that ( A E ^ ) ^ is essentially constant  for the compounds XSb(Fe(CO) Cp) 2  + 3  (X = Ph, I, Br, Cl). In these cases  any differences due to alterations in a-donor and TT-acceptor properties of the XSb group are distributed amongst three iron atoms and effectively masked. That the ^Fe  quadrupole interaction in these compounds,  particularly those containing only one or two Fe(C0) Cp moieties, i s 2  reasonably sensitive to details of the electron distribution about iron is apparent from the data in Table XIII.  This, together with the fact  that there are no large changes in (^ q)p between corresponding E  e  t i n and  antimony compounds (Table XIV). implies that the electron distribution in the Fe-Sb bond is quite similar to that in the Fe-Sn bond.  What small  differences there are in ( ^ E ^ ) ^ are consistent with the expected changes  This is exactly the inverse effect to the decrease in (AE ) seen m c above in L LFe„(C0),- and L LFe_(C0), complexes as the a-donor power ? e  A  Fe  B  •*• Fe  increased.  In both cases V„„ is predicted to be positive.  144a  FIGURE 26.  57  Fe Mossbauer Spectrum of PhSb(Cp(CO) Fe) PFg" +  2  3  in an Applied Longitudinal Magnetic Field of 30kG  0  2 The Sign of e qQ i s Clearly Positive.  Fe Resonance of P h S b (Cp (CO) Fe )| P f | " in a parallel magnetic field of 30 kG 2  o  1.001  CD  o ° a- ° < t f c o  0  •2 98'E § -96-  O0>  eft  o o o o o  CO  66  CD  CO  O O  .94^ -2.0  o o o o  •1.0  o  o  CPo  o  o  °o  o oo o o<fcocP  aP o o °>o  o o  o o o o T  0.0  1.0  Velocity (mm/sec) FIGURE 26.  2.0  3.0  145  i n a-donor and i r - a c c e p t o r p r o p e r t i e s between Sn and Sb. r e i n f o r c e s the c o n c l u s i o n s isomer s h i f t  d a t a , namely t h a t  i n t h e s e compounds. positive  drawn above from "^Fe,  1 1 9  S n and  1 2 1  result Sb  antimony and t i n a r e n e a r l y i s o e l e c t r o n i c  Thus i n t h e i o n i c  antimony d e r i v a t i v e s most o f t h e  charge must r e s i d e on antimony r a t h e r than b e i n g  onto t h e l i g a n d s .  This  T h i s agrees w i t h a s i m i l a r c o n c l u s i o n  delocalized r e a c h e d on t h e  118 basis  (C)  of c r y s t a l structure  data  The C o r r e l a t i o n o f "* Fe Mossbauer Parameters w i t h t h e C a r b o n y l 7  S t r e t c h i n g F r e q u e n c i e s i n t h e I.R.  As we have d i s c u s s e d  above, i n our  57 present  s e r i e s o f compounds c o n t a i n i n g  i r o n - a n t i m o n y bonds t h e  Fe  Mossbauer parameters have been found t o be q u i t e i n s e n s i t i v e t o t h e n a t u r e of the other  groups bonded t o antimony  (Table X I I I ) .  o f s e n s i t i v i t y we have i n v o k e d a - and TT-bonding d e c r e a s e d by i n c r e a s i n g donor s t r e n g t h i n TT back-bonding  (M -> L) .  To e x p l a i n t h i s  effects since 6p i s e  (L ->• M) and i n c r e a s e d  by d e c r e a s e s  The Q.S. f o r i r o n i s a l s o a f f e c t e d by t h e s e  f a c t o r s and we e x p e c t the Q.S. t o become more n e g a t i v e  with  increasing  a-donor power and more p o s i t i v e w i t h i n c r e a s i n g TT b a c k - b o n d i n g . e f f e c t s o f t h e s e phenomena on V has  may be a p p r e c i a t e d  The  i f i t i s noted  that  been found t o be p o s i t i v e i n a l l t h e d e r i v a t i v e s , X F e ^ O ^ C p , i n  which i t has been m e a s u r e d * ^ . 1  values  lack  I t i s o b v i o u s from t h e range o f measured  o f t h e Q.S. i n such compounds (1.80 ± 0.20 mm/sec.) ^ 1  2  106,131 133  7 27 t h a t t h e r e i s a l a r g e c o n t r i b u t i o n from an u n f i l l e d d - o r b i t a l ' and t h a t the v a l e n c e c o n t r i b u t i o n (q__._) i s o n l y 10-20% o f t h e t o t a l e . f . g . . Since VAJ-i  H lies  i n t h e range 0 < n < 1 and can g i v e  contributions  t o ( A E ^ ) ^ o f up t o  TABLE XV. Fe MOSSBAUER AND v „ PARAMETERS OF SOME X.MFe(CO)_Cp DERIVATIVES. n  COMPOUND  V  1  ( c m  1  )  v (cm~ ) 1  2  SOLVENT  REF.  6 (mm/sec.) Fe  Ph PFe(CO) Cp PF "  2070  2030  NUJOL  119  0.32  Ph AsFe(CO) Cp PF "  2062  2017  NUJOL  119  -  Ph SbFe(CO) Cp PF ""  2050  2005  NUJOL  119  0.41  Bu SnFe(CO) Cp  1972  1923  CHC1  106  0.38  Cl SnFe(CO) Cp  2048.0  2008.3  CHC1  3  137  Me SnFe(CO) Cp  1985.4  1930.1  CHC1  3  Ph SnFe(CO) Cp  1995.3  1943.5  CHC1  ClFe(CO) Cp  2057.6  2012.0  CHC1  BrFe(CO) Cp  2052.9  2007.3  CHC1  IFe(CO) Cp  2043.8  2000.0  CHC1  +  3  2  6  +  3  2  6  +  3  2  3  2  3  2  3  2  3  2  2  2  2  6  a  REF.  1.92  132  -  -  1.86  THIS WORK  1.75  106  0.40  1.85  b  137  0.36  1.75  102  137  0.36  1.82  c  137  0.50  1.88  131  137  0.51  d  3  1.87  131  137  0.49  d  3  1.83  131  3  3  3  a  d  Converted to sodium nitroprusside by addition of 0.27 mm/sec. b  Q.S.(mm/sec.)  Average value of r e f s . 102, 104, 106 (Table XIV).  C  Average value of r e f s . 102, 104 (Table XIV).  d  Converted to sodium nitroprusside by addition of 0.11 mm/sec.  TABLE XVI. Fe MOSSBAUER PARAMETERS  AND  PARAMETERS OF SOME X M(Fe(CO) Cp) 2  V _(cm ) V (cm" ) V (cm~ )  COMPOUND  1  ]  1  2  1  3  v  ( c m "" ) SOLVENT 1  4  2  REF.  5 Je  2  DERIVATIVES.  (mm/sec,) Q.S.(mm/sec.) REF.  Cl Sb(Fe(CO) Cp) PF "  2065  2053  2020  CH C1 2  2  41  0.40  1.86  THIS WORK  Br Sb(Fe(CO) Cp) PF "  2062  2048  2018  CH C1 2  2  42  0.40  1.83  THIS WORK  (CF ) Sb(Fe(CO) Cp)2?F~  2063  2050  2021  CH C1 2  2  42  0.42  1.80  THIS WORK  Ph Sb(Fe(CO) Cp) PF "  2045  2027  1994  CH C1 2  2  42  0.39  1.74  THIS WORK  Cl As (Fe(CO) Cp) FeCl ~  2071  2057  2030  CH C1  2  41  0.38  1.79  THIS WORK  (NCS) Sn(Fe(CO) Cp)  2022  2007  1983  1970  CHC1  3  106  0.39  1.69  106  2026  1999  1972  1963  CHC1  3  106  0.38  1.67  a  1998  1980  1947  1933  Cyclohexane 142  2036  2010  1985  1962  Cyclohexane 141  0.36  1.66  104  2  2  2  2  2  3  2  2  2  2  2  2  2  2  2  Ph Sn(Fe(CO) Cp) 2  2  a  2  2  Cl Ge(Fe(CO) Cp) 2  6  2  Cl Sn(Fe(C0) Cp)  2  6  2  2  2  6  2  4  2  Average value r e f . 104 and 106 (Table XIV).  1985  2  148  15% of i t s t o t a l value (eqn. 9 with n, = 1) then small variations i n could equally w e l l a r i s e from changes i n n as they could from  ( A E Q ) ^  changes i n q__._.  As w e l l , there could be contributions to ( A E )  changes i n the geometry about iron.  These facts mean that the Q.S.  from of Fe  i n these compounds i s not a good diagnostic t o o l for separating possible a- and TT-bonding e f f e c t s . 133 However, as reported by B u r l i t c h and F e r r a r i  , the  combination  of Mossbauer spectral parameters with data on carbonyl stretching frequencies i s often informative.  S i m i l a r l y , both Mossbauer and I.R.  techniques  have been employed by Dessy and co-workers ^ i n t h e i r study of o l e f i n 7  Fe(CO)  4  complexes. In the analysis of carbonyl stretching frequencies, models i n  which TT-bonding  13^  and models i n which a-bonding  predominate have both been employed.  133  However, now,  '  136  are considered to  i t i s generally consid70 137-139  ered that both a- and TT-bonding e f f e c t s are important  '  , although  i n certain cases e i t h e r a or TT e f f e c t s may be the predominant f a c t o r i n determining CO stretching frequencies.  In f a c t , our present Mossbauer  data and the reported CO stretching frequencies used i n concert are incompatible with a a-only or a  TT-only model.  In the 0 + TT model, i n every case where the electron density on 138 the metal i s increased, V  should decrease  .  For the TT contribution,  the i n t e r a c t i o n i s not a simple one, since the e f f e c t of TT-bonding on V i s normally considered to be very d i r e c t i o n a l and quite d i f f e r e n t i f the TT-bonding substituent i s c i s or trans to the carbonyl group.  However, i n  149  our present complexes, only cis substitution can take place and so there Is only the one type of interaction to consider. So, increases in a-donor strength lead to decreases in v ^ '''" ^ while decreases in TT 1  8  4  r  back-bonding also lead to decreases in V ^ Q . From the foregoing discussion, we may recognize two extreme cases i f a ligand L ' i s substituted for a ligand L in these complexes assuming the O and TT effects to be comparable in magnitude. Case A .  If L ' is a better a-donor than L then 6p  decreases.  If L  1  V ^ Q w i l l increase.  £  decreases and V ^ Q  is a better TT-acceptor than L then 6„ decreases and Fe The net effect of both these interactions i s a  relatively large decrease in the I.S. but l i t t l e i f any change in v  r n  .  In the converse situation, where L ' i s both a worse a-donor and TT-acceptor than L, the I.S. w i l l increase but Case B.  w i l l hardly change.  If L' i s a better CT-donor than L then 6_ decreases and V „ Fe CO n  decreases.  If L' i s a worse TT-acceptor than L then 5„ increases and v„„ Fe CO  decreases.  The net effect w i l l be l i t t l e i f any change in the I.S. but a  relatively large decrease in V ^ Q * In the converse situation, where the a-donor power i s worse and the TT-acceptor power is greater then the I.S. w i l l hardly change but V  w i l l increase.  For most cases, either a or TT effects w i l l dominate so both Mossbauer and I.R. parameters w i l l change.  Nevertheless, i f the two tech-  niques are employed in concert the qualitative picture which emerges is' very satisfying, as illustrated below.  150  F i r s t , there are some experimental problems to be discussed. For example, the values of V  which we are employing normally have been  measured i n solution at room temperature whilst the Mossbauer parameters have been measured i n the s o l i d state at 80°K.  Nevertheless, since the  e f f e c t s which we are looking f o r are r e l a t i v e l y large changes i n the Mossbauer parameters (the change must be more than 0.02 mm/sec. to be s i g n i f i c a n t ) or i n V e f f e c t s on V  138  we may have some confidence that things l i k e solvent  ( s h i f t s of ± 10 cm  our f i n a l conclusions.  —1  are not uncommon  137  ) w i l l not a f f e c t  As w e l l , compounds of the type R^MFe^O^Cp  (M = P, As, Sb, Sn, etc.) normally have two I.R. frequencies i n the carbonyl region (v^ and v^) while those of the type R2M(Fe(C0)2Cp)2 have e i t h e r three I.R. frequencies (v^,  , and v^) or four I.R. frequencies (^>  v^, and V^) i n the carbonyl region.  v  2>  The appearance of three or more bands  i n the l a t t e r compounds arises because rotation of one F e ^ O ^ C p group about the M-Fe bond can give r i s e to three d i s t i n c t structures with symmetries C ^ , C , or C^; g  these correspond to the r e l a t i v e positions adopted by the 141  Fe(C0)2Cp moieties on rotation for  .  Group t h e o r e t i c a l treatments  show that  symmetry there are three bands (A, B^, and B 2 modes), f o r C  there are four bands (2A' and 2A"), and f o r 141 bands (4A) .  g  symmetry  symmetry there are also four  Evidence from band i n t e n s i t i e s shows the organotin d e r i v a t i v e s  f a l l i n t o the class with C symmetry, while the halogen derivatives of s germanium and t i n have symmetry. I t i s apparent from the data that the halogen derivatives of antimony probably have C 2 symmetry while v  + Pl^SbtFeCCO^Cp^ probably has C derivatives.  g  symmetry  42  l i k e the corresponding t i n  151  As inspection of Table XV shows, there i s a wide range of V ^ Q as w e l l as a f a i r l y large range i n 6^  f o r the R^MFeCCO^Cp compounds.  e  We s h a l l pick some t y p i c a l comparisons and discuss them i n d e t a i l . 57 example, comparing the  .. Fe Mossbauer parameters and  For +  of Ph^SbFeCCO^Cp  cation with those of Ph^SnFe(CO) Cp we see there are large changes i n the 2  s p e c t r a l parameters.  Both the decrease i n \)  and i n the I.S. f o r the  Ph.jSnFe(C0)2Cp d e r i v a t i v e may be explained by assuming that Ph^Sn i n a better a-donor than Ph^Sb"*".  This i s exactly the conclusion which was 121 119 reached on comparison of the Sb and Sn Mossbauer parameters above. S i m i l a r l y , comparing the parameters Ph^SbFe^O^Cp* and  XFe(C0)2Cp  (X = halogen) derivatives we see that  hardly changes yet  there i s a large increase i n I.S. f o r the halogens.  Since the halogens  should be worse TT-acceptors than Ph^Sb"*" these results indicate t h i s i s the converse of case A above and so the halogens must be somewhat worse a-donors than Ph.jSb . +  Comparison of the data f o r Ph.jPFe (CO) C p 2  +  with that of  Ph^SbFe^O^Cp* shows only small changes i n V.^Q but a large decrease i n I.S. f o r the Ph^PFe^O^Cp* d e r i v a t i v e .  This i s an example of case A  above and shows Ph^P i s both a better Tr-acceptor and a better a-donor than Ph.jSb.  S i m i l a r l y , comparing the s p e c t r a l parameters of the FeCCO^Cp group i n Cl SnFe(CO) Cp and Ph SbFe(CO) Cp 3  change i n e i t h e r V  2  3  or 6  2  +  shows there i s l i t t l e or no  and t h i s implies the Ph,Sb  +  group and Cl„Sn  CU Fe 5 J are very s i m i l a r i n t h e i r e f f e c t s on the electron density at i r o n i n these  152  two complexes.  This also means that the a-donor power of the Ph^Sb"*" The worse a-donor power  group is roughly equivalent to that of Cl^Sn.  of the halogens relative to Ph.jSb as observed above does not seem unreasonable in view of this situation. In general, both the "* Fe Mossbauer parameters and CO stretching 7  frequencies for the R M(Fe(CO) Cp) 2  2  2  derivatives (Table XVI) would seem to  be less sensitive to the nature of R and M.  This i s exactly the behaviour  which would be predicted since the effects of ligand changes are being distributed between two Fe(CO) groups. However, detailed analysis of 2  the data for the R M(Fe(CO) Cp) 2  2  2  derivatives is precluded at this stage  both by the lack of published information on the ^ Fe Mossbauer parameters 7  of some of the more interesting compounds (Ph Sn(Fe(CO) Cp) for one) and 2  2  2  by the fact that various compounds apparently belong to different , 141,142 symmetry classes .  153  BIBLIOGRAPHY  (1)  R.S. 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Ho and J.J. Zuckerman, J. Organometal. Chem., 49, 1 (1973).  (123)  S.R.A. Bird, J.D. Donaldson, S.A. Keppie, and M.F. Lappert, J. Chem. Soc. (A), 1311 (1971).  (124)  G.M. Bancroft and K.D. Butler, J.C.S. Dalton, 1209 (1972).  (125)  S.R.A. Bird, J.D. Donaldson, A.F. LeC. Holding, B. Ratcliff, and S. Cenini, Inorg. Chim. Acta, j 6 , 379 (1972).  (126)  S. Ichiba, M. Katada, and H. Negita, Bull. Chem. Soc. Japan, 45, 1679 (1972).  (127)  S.L. Ruby and G.K. Shenoy, Phys. Rev., 186, 326 (1969).  (128)  S.K. Ruby, H. Montgomery, and CW. Kimball, Phys. Rev. B, JL, 2948 (1970).  (129)  Ref. 5, p 374.  (130)  A number of tin derivatives of Fe^O^Cp have been reported i n Ref. 105, where the ranges 0.30 - 0.37 mm/sec. (relative to sodium nitroprusside) for "* Fe isomer shifts and 1.60 - 1.83 mm/sec. for 7  "* Fe quadrupole splittings are given. 7  However, these authors do  not quote values for individual compounds. (131)  R.H. Herber, R.B. King, and G.K. Wertheim, Inorg. Chem., 3_, 101 (1964).  (132)  K. Burger, L. Korecz, P. Mag, U. Belluco, and L. Busetto, Inorg. Chim. Acta, 5_, 362 (1971).  (133)  J.M. Burlitch and A. Ferrari, Inorg. Chem. , 9_ 563 (1970).  (134)  F.A. Cotton and C S . Kraihanzel, J. Amer. Chem. Soc , 84, 4432 (1962).  (135)  R.J. Angelici, J. Inorg. Nucl. Chem., 28, 2627 (1966).  (136)  R. Ugo, S. Cenini, and F. Bonati, Inorg. Chem. Acta, 1, 452 (1967).  y  162  (137)  J . Dalton, I. Paul, and F.G.A. Stone, J . Chem. Soc. (A), 2744 (1969).  (138)  W.A.G. Graham, Inorg. Chem., _7» 315 (1968).  (139)  T.B. B r i l l , J . Organomet. Chem., 40, 373 (1972).  (140)  R.E. Dessy and L. Wieczorek, J . Amer. Chem. S o c , 91, 4963 (1969).  (141)  N. F l i t c r o f t , D.A. Harb ourne, I. Paul, P.M. Tucker, and F.G.A. Stone, J . Chem. Soc. (A), 1130 (1966).  (142)  K.N. Anisimov, B.V. Lokshin, N.E. Kolobova, and V.V. Skripkin, Iz. Akad. Nauk., SSSR, Ser. Khim., _5» 1024 (1968), (Engl. T r a n s l . ) .  (143)  Nucl. Data Sheets. 6, 90 (1971).  (144)  Ref. 4, pp 18-1 - 18-3.  163  APPENDIX I  The analysis of the quadrupole-split  121  Sb spectra i n the  case when the asymmetry parameter n i s not zero i s interesting  since i t  i l l u s t r a t e s i n d e t a i l the procedures which are used i n the general case for pure Ml  y-transitions  between states with d i f f e r e n t nuclear spins.  In  p a r t i c u l a r , the cases of interest w i l l involve randomly oriented polycrysta l l i n e absorbers and s i n g l e - l i n e y s o u r c e s . The Hamiltonian f o r a nucleus with spin I and quadrupole moment Q subjected to a non-axially symmetric e.f.g. i s  H  Q " 41(21-1)  < Z 3 1  +  5  (  V  +  * 3 >  using the same conventions as before. 121  = / , denoting A = 5  2  For the ground state of  Sb with spin I  the Hamiltonian matrix may be w r i t t e n as (AI-2)  <m» / 5  |H | / m> = 5  2  2  10A  0  lOriA  0  0  0  0  -2A  0  i8nA  0  0  10nA  0  -8A  0  18nA  0  0  18nA  0  -8A  0  lOnA  0  0  18nA  0  -2A  0  0  0  0  10nA  0  10A  164  S i m i l a r l y , f o r the excited state of  121 7 Sb with spin /  2  e qQ* 2  denoting B = — — ,  where Q* i s the quadrupole moment of the excited  state, the Hamiltonian matrix may be written as (AI-3)  21B  <m /2|H | / m> ,7  =  7  QA  2  21nB  • 0  0  0  0  0  0  0  0  0  0  0  3B  0  21nB  0  -9B  0  60nB  0  0  0  0  45nB  0  -15B  0  60nB  0  0  0  0  60nB  0  -15B  0  45nB  0  0  0  0  60nB  0  -9B  0  21riB  0  0  0  0  45nB  0  3B  0  0  0  0  0  0  21nB  0  2 IB  45n.B  The energy eigenvalues and eigenvectors would be found by machine diagonalization of each of the above Hamiltonian matrices f o r 2 appropriate values of e qQ and n. The energy eigenvalues may be represented A l , A2, A3 f o r the ground state and BI, B2, B3, B4 f o r the excited state. eigenvectors i n terms of the basis kets,  jl  e  The corresponding  m > and | l g nig> f o r the g  excited and ground states respectively are of the form -  -»  —'  bll b  b  l2  b  —  21  b22  1 3  b  23  b  l4  b  24  b  15  b  25  b  16  b  26  b  17  b  27  b  28  (AI-4)  bl8 _ —  >  etc. f o r the excited state and  165 ll  l  *12  l  l  l  (AI-5)  21  22  *23  13  24  *14  l  »15  l  etc. for the ground state.  25  he  26  l  To calculate the i n t e n s i t i e s i t i s necessary to have some 121 An unpolarized source of Sb 143 Y-rays since the Y t r a n s i t i o n i s pure Ml , w i l l emit equal numbers of knowledge of the Y-rradiation a v a i l a b l e . _  l e f t - and right-hand c i r c u l a r l y polarized Y | l , l > ' respectively.  - r a  y s , denoted | l , - l > '  and  The number of l o n g i t u d i n a l l y polarized Y-quanta  i  144  |1,0>' should be zero  .  In general the Y r a y d i r e c t i o n and the Z a x i s , -  defined by the p r i n c i p a l components of Rather the incoming This i s equivalent  y-ray w i l l  the e.f.g., w i l l not be  colinear.  be at an angle 6,<{> r e l a t i v e to that Z axis,  to observing the Y r a d i a t i o n at an angle Q,$ r e l a t i v e -  to i t s d i r e c t i o n of propogation. The numbers of Y-quanta with p o l a r i z a t i o n s  |1,-1>,|1,0> and | l , l >  available i n the basis coordinate system are given by the sum  over the  amounts of Y quanta with l e f t - and right-hand c i r c u l a r p o l a r i z a t i o n emitted _  by the source (since.11,0>' = 0), i . e . |l,-l>'+ | l , l > where  (AI-6)  | l , l > ' = ^(l+cose)e *|l,l> i  J2 - % ( l - c o s e ) e-i<J>l " | l , - l > - ^ s i n 6 | l ,0> 1<P  and (AI-7)  |l,-l>» = -!s(l-cose)e  i<f>  |l,l> + Ji(l+cose)e *|l,-l> - ^psin9|l,0> _:L  166  r e l a t i v e to the basis coordinate system "*. 1  These terms may be rewritten as  (AI-8)  |l,l>'  =  A ( l ) | l , l > + A ( - l ) | l , - l > + A (O)|l,0> +  +  +  and  (AI-9) whe re A ( l ) +  |l,-l>' =  =  A_(l)|l,l> + A_(-l)|l,-l> +  %(l+cos9)e *, e t c . 1  The excited state eigenvectors basis vectors  A_(0)|l,0>  | ^2 7  m  > e  of the basis vectors  »  t  n  e  |^I2m  are written i n terms of the  ground state eigenvectors  are written i n terms  >, and the absorbed photons |l,M>  S  connect the  „ 31  two sets of basis vectors v i a the Clebsch-Gordan coupling of the type (AI-10) where M = m -m e g  < / 5  2  1 m  coefficients  M| / m > 7  g  2  e  takes on the values 1. 0. -1. * •'  Thus the r e l a t i v e i n t e n s i t i e s of the | l , l > ' t r a n s i t i o n s may be found from consideration of matrices of the form  167  f  H  CN  co <r in vo r-»  oo  r H r H r H r H r H i - H r H r H ,  LO|c\l  O  CO|cN 1  •-) I CM —<|CM CJ —<|cN  <|CM  CO | CM  < | CM  CJ CO I CM «» m|cM  CO | CM CO I CM  CJ  CJ  i*  <|CM  I CM CJ  ."  LO|CM  CO CN  i~ m CN  C0|CN I ,* CO CN  COlcN * LO|Csl  1  1  1  r-l|cM CO I CM I CJ  r-<|CM |-<|CM CJ  CO I CM r-l  I CM CJ  lO|CN  m|cN  m|cN  CO | CM  CJ r-|cM A  J  ITllcN CJ  +  I*  r-t  4C  rH  1  CO  cd *  rH  co *  I  rH  «  CN  3  •K r H  1  « J  168  where (AI-12)  < / o 1 m (m -m ) | / g e g' 2  C. (m ,m ) = + g e'  5  7  z  =  </  v  5  2 0  1  m  g  1  M| / 7  1  I 0  9  m >A (m -m ) + e g' v  e  m >A. e +  J  (M)  and A (M) are the angular parts A ( l ) , A (0) and A ( - l ) as defined i n (AI-8). +  +  +  +  Similar matrices a r i s e for | l , - l > ' quanta except C (m , m ) are 8 e replaced by C (m , m ). These have i d e n t i c a l Clebsch-Gordan c o e f f i c i e n t s - g e but the A (M) terms are replaced by the corresponding A_(M) terms (AI-9). +  At this stage, after the matrix m u l t i p l i c a t i o n i s carried out and the results squared for each of | l , - l > ' and | l , l > ' those two terms would be summed and the r e s u l t would be the r e l a t i v e i n t e n s i t y f o r some value of 9,<i>. Since the results f o r a p o l y c r y s t a l l i n e randomly oriented powdered absorber are desired, integrations over the values of 8 and $ are required. In fact i t i s only necessary to integrate over the angular parts such as |Aj_(M)| since none of the other c o e f f i c i e n t s has any angular dependence. 2  Hence only integrals of the form  i (AI-13)  2ir  "4TT O  TT  O |A (l)rsined9d< , e t c . , +  r  need be considered. Closer examination of the integrals a r i s i n g from|l,l>' quanta shows some i n t e r e s t i n g r e s u l t s . 1  (AI-14)  J  v  2TT  £  F i r s t l y , evaluating the integrals  TT  & |A (l)|sineded<f> +  explicitly,  169  2TT 4TT  .TT  4"(1 + 2 cos6 + cos^e) sinGdedcj)  o  O  1 6  CAI-15)  1 j  2 7 r  /  1 =  7 /  2 7 r  4rr o  , 2 |A (0) I sinedBdcj)  11  2  1  J I  ( s i n  9 )  s i n 6 d 9 d (  t>  JL 3 and CAI-16)  ^  =  {  ^  |A(l)r +  , 2TT TT "4TT O £ 4^  1  "  2  sinGdedcj)  c  o  s  + cos 8) sinBdedc})  0  1 6  There are a number of other i n t e g r a l s which represent the cross terms such as (AI-17)  I f f 4TT  2TT  TT  1  o  o 2TT  =  "4TT o  = 0  *  V)  TT  o  V°)  sinSdSdc})  JY 4  _.  A  +  c  o  s  9  )  e  1  sine}sin8d9dc|)  170  2TT  CAI-18)  i  4TT  /  TT  /  O  A  O  $S  +  ( 1 )  1  ^  T« 0  a  =  L  L  A  +  ( _ 1 )  sin  1  9 9 l> d  d(  9-A  9  -T(l-cos e)e~ Z  Zl(P  sineded(}>  and (AI-19)  1  / o  4TT  / o  . =  tlT  =  0  A (-1) A (0) sinBdedt})  2TT O  O  TT  r —  -  • f (  . , 1  -  C  O  S  e  )  s  i  n  9  S i n e d 9 d (  e  r  In f a c t , a l l the cross terms are zero since a l l the i n t e g r a l s involve terms like 2TT  (AI-20)  /  . ,  e % ±ml  =  0  For | l , - l > ' quanta the i n t e g r a l s give the same r e s u l t s , namely  1  i 2 2 1 2 f o r |A_(1)| and |A_(-1) | , and f o r |A_(0) | . This means  that i t i s not necessary for  to carry out the matrix m u l t i p l i c a t i o n  explicitly  | l , - l > ' quanta since the f i n a l r e s u l t s a f t e r performing the  integrations w i l l be i d e n t i c a l to the | l , l > necessary  ?  case.  Thus i t i s only  to multiply the r e s u l t s of the | l , l > ' case by two (after  squaring) to account f o r the contribution of the | l , - l > '  case.  (AI-21)  E  * * * * * l' 12' 13 l4' 15 16j a  a  ,a  a  ,a  C, (1)  2C.(0)  O, (-1)  0  C (l) 2  2C (0)  0  0  0  0  0  0  0  0  0  0  0  0  0  6  0  0  0  0  0  0 0  o  c (D  C (-l) 0  2C (0)  C (-l)  0  c d)  2C.(0)  0  0  0  C (l)  0  0  0  3  o  4  0  5  C.(-l)  2C (0) C (-1) 5 ' 5 Cs  c (D 6  w  c  0  2c co) c ( - i y 6  6  11 12 13 14 15 16 17  where (^(1) = <-| 1 -| 1 |-^- -^->, e t c . , are the Clebsch-Gordan c o e f f i c i e n t s from the terms C^C-j, J") e t c . , i n AI-11.  172  So the t r a n s i t i o n between the states A l and BI which has r e l a t i v e energy BI - A l (ignoring the y-ray energy) has i n t e n s i t y equal to the sum of four terms (owing to the degeneracies involved) each of which i s the sum of three terms l i k e (AI-22)  i |a  (AI-23)  +| | a ^ C ( 0 ) b  (AI-24)  +| | a  (^(1) b  u  1  n  u  1 2  + a  C (l)b  1 2  + a  C (l)b  1 3  + etc.  + a* C (0)b  1 3  + a* C (0)b  l 4  + etc. |  12  2  2  C^-Db^ + a  From consideration  12  2  C (-l)b 2  13  3  3  1 4  + a  13  3  C (-l)b 3  1 5  |  2  + etc. | .  of the foregoing, solutions to other  cases involving pure Ml t r a n s i t i o n s may be developed.  Programs have been  written for the solution of the general case, but i t i s sometimes more convenient and more e f f i c i e n t to develop subroutines f o r a s p e c i f i c case such as t h i s one.  173  APPENDIX I I  The following discussion of magnetically perturbed Mossbauer spectra follows the treatment of C o l l i n s and Travis "'. 1  The case of a nucleus with ground state with nuclear spin 1 I =  3 12 and an excited state with nuclear spin I =  12 subjected to  both an i n t e r n a l non-zero e.f.g. and to an external applied f i e l d i s considered.  To be more s p e c i f i c , the case of a diamagnetic, poly-  c r y s t a l l i n e absorber i s considered. The f i r s t step i s to solve the energy eigenvalue problem. For the ground state, since Q^  = 0 the solution i s t r i v i a l .  r  The  Hamiltonian operator f o r the ground state i n t e r a c t i o n i s (AII-1)  H ^  =  =  -g $ H(sinecos<i)i o  x  n  +  sin6sinc}>I + cosQI ) y  z  The Hamiltonian matrix i s  h h (AII-2)  -h -hg 3 HsinGe" * o n  -Jig o3nHcosG  1  <! m |H |laiii> = ,  S  MAG  -hg 3 H s i n G e * o n +i  and the eigenvalues are  (AII-3)  El =  (AII-4)  E2  =  -hg 3 H o n  +hg ^o n  3H  hg 3 HcosG o n  174  with eigenvectors  (AII-5)  |B1>  =  -sin |  e~ \h,h> +  (AII-6)  |B2>  =  cos |  \h,h> +  U  cos |  \h,-h>  Je  \h,~h>  +±<i>  sin  S i m i l a r l y , the Hamiltonian operator for the excited state 3 with 1 =  is  (AII-7)  H ^  =^  {3I  Z  - 1(1+1) + £ (l  +  + I_ )> 2  2  -g.3 H (sin6cosd)I + sinOsinctl + cos6l ) . In x y z  2 I f we define A =  and  XI  = -g,3 H 1 n  then the excited state Hamiltonian matrix i s  (AII-8)  <|-|H  3 2 3A + J a cos6  1 2  Ja sinSe *  1 2  3r)A  1  |  0  ||m >  1 2  3 2  3  M + Q  J a sin9e * - i  -3A + J a cos9  a sinOe ^ 1  3nA  1  2  2  "2  3nA  a sinSe * - 1  -3A - J a cos8  J a sinSe * 1  3nA  J a sinGe  3A - J a cos9  175  Solutions to this matrix eigenvalue problem may be found 2 for any case of e qQ, n and H by computer diagonalization of the matrix for appropriate values of 8 and <f). The energy eigenvalues may be represented as E^', E^', E^', E^' and the corresponding eigenvectors i n terms of the basis ket vectors as —.  —  a  21  a  31  (AII-9)  *  a  12  a  22  a  32  _ 42. a  "*13  9  a  14  a  23  a  24  a  33  a  34  *A3__  >  _ 44_ a  S i m i l a r l y , the energy eigenvectors f o r the ground state are designated  (AII-10)  11  12 and  21  22  in terms of the basis kets.  Now, the exact solutions f o r the energies of the t r a n s i t i o n s would involve integrations over a l l values of 8 and cf).  I t i s convenient  to replace these i n t e g r a l s by sums over a f i n i t e number of values of 8 and cf>. The r e a l energies may be simulated to any degree of exactness required by i n c l u s i o n of s u f f i c i e n t terms i n these sums.  I f there are n increments  i n <j>, then f o r each of these increments, there w i l l be n increments i n cos8  i n order to approximate the elemental area -d(cos8)dc)> over the unit  sphere.  In f a c t , since the e.f.g. axes have been chosen as the coordinate  176  axes and since the e.f.g. has mirror symmetry about i t s p r i n c i p a l planes, only values of 8 and $ l y i n g i n one octant need be chosen i n approximating the solutions  33  Having found the energy eigenvalues for each increment i n 8 and <j>, one must have the t r a n s i t i o n p r o b a b i l i t i e s as w e l l i n order to simulate a spectrum.  To do t h i s , i t i s necessary to know the eigenvectors  for both the ground and excited states i n terms of the basis kets ( i . e . AII-9 and AII-10), the Clebsch-Gordan c o e f f i c i e n t s connecting the various substates, and then sum over the r e l a t i v e amounts of y-quanta a v a i l a b l e with appropriate p o l a r i z a t i o n s .  Usually, there are two choices f o r the d i r e c t i o n of a p p l i c a t i o n of the  y-beam;  e i t h e r p a r a l l e l to or perpendicular to the d i r e c t i o n of 15 33 3A  a p p l i c a t i o n of the magnetic f i e l d , H_  '  '  Our experimental  situation  i s the former so only the r e s u l t s f o r that case w i l l be considered. Since H and the Y-beam are p a r a l l e l , they w i l l be at an angle 8, 4> r e l a t i v e to the e.f.g. coordinate system.  Thus the amounts of r a d i a t i o n  available with r i g h t - and left-hand c i r c u l a r p o l a r i z a t i o n s and l o n g i t u d i n a l p o l a r i z a t i o n (|l,l>, | l , - l > , (AII-11)  |l,l>'  (AII-12)  |l,-l>»  =  =  |l,0> r e s p e c t i v e l y ) are given by  A ( l ) | l , l > + A ( - l ) | l , - l > + A (0) |l,0> +  +  +  A _ ( l ) | l , l > + A_(-l) | l , - l > + A_(0) |l,0>  which are e n t i r e l y equivalent to equations AI-8 and AI-9 since the Y-transitions f o r "* Fe and 7  1 1 9  S n are also pure Ml.  177  Thus, at each increment of 9 and <J> the t r a n s i t i o n p r o b a b i l i t y connecting any two of the energy states obtained on diagonalization of the Hamiltonian matrices, say the states E l and E l ' with eigenvectors ll  l  11  and  respectively, 12  l  12  J  l  13  *14 i s the square of a term f o r | l , l > ' quanta l i k e (AII-13)  Br i J b  2  S-^^  0  c (,-1, )1  C ( ,2)  2  +  +  2  C  + 2 » 2? (  -1-1  c( ,)  2  +  2  ll  l  N  2  12  l  •1-3,  c( ,) +  2  *13  2  14  l  where, (AII-14)  C (m ,m ) +  g  e  = <- 1 m (m -m ) \j g  = <| 1 m m , m g e the  g  e  m^A^-m^)  g  M|| m >  A (M)  e  +  1 are the values of I„ i n the basis representation, <•=• m Z ' 2' g 31  Clebsch-Gordan c o e f f i c i e n t s  and A (M)  3 Mm > are 2 e  1  are the c o e f f i c i e n t s f o r  +  r e l a t i v e amounts of radiation with appropriate polarizations o r i g i n a t i n g from source quanta of the type | l , l > ' (eqn. AII-11). The square of the above term (AII-13) f o r | l , l > ' quanta i s added to the square of a term f o r | l , - l > ' quanta (AII-15)  b  ll'  b  12  c  -<2»2*  ^^'^  Q  -i2 (  ,  )  0  "11 12  l  C_( "J*"?)  C_( 2>T)  '-'_( ~2t~y)  '13 14  4  178  where  (AII-16)  C (m ,m ) - g e  = <^ 1 m M|J- m >A (M) 2 g ' 2 e -  and the terms are as i n (AII-14) except A (M) replaces A (M) +  since  they are the appropriate c o e f f i c i e n t s f o r the r e l a t i v e amounts of r a d i a t i o n with appropriate p o l a r i z a t i o n s originating from the source quanta | l , - l > '  (AII-12).  

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