Review of Metal-Halogen Vibrational Frequencies
R . J . H . C L A R K
William Ramsay and Ralph Forster Laboratories, University College, London, England 1. Introduction . .
2. Metal-Fluorine Vibrational Frequencies A . Octahedral Molecules . .
B . Tetrahedral Molecules . . C. Heptafluorides . . D . Other Stereochemistries E . A s s i g n m e n t s in Metal Complexes
3. Metal-Chlorine, M e t a l - B r o m i n e , a n d M e t a l - I o d i n e Vibrational Frequ A . Octahedral H e x a h a l o a n i o n s
B . Tetrahedral A n i o n s C. Square Planar I o n s D . Five-coordinate H a l i d e s E . Linear Species . .
F . A s s i g n m e n t s in Metal Complexes 4. Conclusion
References
85 86 86 91 92 93 93 94 94 97 98 99 101 102 116 116
1. Introduction
W i t h t h e increasing use of infrared spectroscopy, p a r t i c u l a r l y in t h e region below 600 cm-^, t h e c h a r a c t e r i z a t i o n of low frequency s t r e t c h i n g a n d b e n d i n g m o d e s is b e c o m i n g a m a t t e r of some i m p o r t a n c e . M a n y v i b r a t i o n a l m o d e s m a y occur in t h i s region of t h e s p e c t r u m , e.g. m e t a l - halogen, m e t a l - s u l p h u r , m e t a l - n i t r o g e n , m e t a l - o x y g e n , a n d m e t a l - p h o s p h o r u s s t r e t c h i n g v i b r a t i o n s , as well as t h e 8(MC0), δ(ΜΟΑ) a n d δ(ΟΜΟ) b e n d i n g m o d e s in m e t a l carbonyls a n d m e t a l cyanides. M e t a l - halogen b o n d s occur frequently in inorganic complexes, a n d therefore t h e c h a r a c t e r i z a t i o n of t h e i r v i b r a t i o n a l frequencies is of p a r t i c u l a r im
p o r t a n c e . R e c e n t studies h a v e shown t h a t m e t a l - h a l o g e n a b s o r p t i o n b a n d s v(MX) are often intense, a n d therefore readily identifiable, a n d t h a t t h e frequencies of t h e s e v i b r a t i o n s are r e l a t e d t o t h e o x i d a t i o n s t a t e a n d t h e coordination n u m b e r of t h e m e t a l a n d also t o t h e stereochemistry of t h e complex. <2)
I t would clearly b e of v a l u e t o h a v e a m e a s u r e of t h e s t r e n g t h of b o n d s
8 5
86 R. J. H. CLARK
a s t h e y a c t u a l l y exist in molecules. T h e force c o n s t a n t of a m e t a l - l i g a n d b o n d should provide one such m e a s u r e , albeit d e p e n d e n t on t h e physical model used. H o w e v e r , frequently t h e r e is insufficient information on t h e vibrational s p e c t r a of inorganic complexes (particularly R a m a n d a t a ) for t h e calculation of even a p p r o x i m a t e force c o n s t a n t s . H e n c e t h e necessity for gleaning as m u c h information as possible a b o u t complexes from their v i b r a t i o n a l s p e c t r a alone.
Most emphasis will b e placed on t h e v i b r a t i o n a l frequencies of octa
hedral complexes, b u t corresponding d a t a for t e t r a h e d r a l , s q u a r e p l a n a r , trigonal b i p y r a m i d a l , p e n t a g o n a l b i p y r a m i d a l a n d linear species will also b e discussed. T o some e x t e n t t h e properties of fluoro complexes s t a n d a p a r t from those of o t h e r halo complexes, a n d accordingly ν (MF) v i b r a t i o n s are discussed s e p a r a t e l y from t h e o t h e r v(MX) v i b r a t i o n s . F i n a l l y t h e factors affecting t h e frequencies of i/(MX) v i b r a t i o n s will b e s u m m a r i z e d . While t h e v(MX) v i b r a t i o n a l frequencies discussed in t h i s review centre on those of t h e t r a n s i t i o n m e t a l s a n d i m m e d i a t e post- t r a n s i t i o n elements, t h e t e r m ' ' m e t a l " will b e used r a t h e r loosely, a n d w h e r e r e l e v a n t t h e v i b r a t i o n a l frequencies of pre-transition elements will also be discussed.
2. Metal-Fluorine Vibrational Frequencies A. Octahedral molecules
(i) Neutral Species
T h e best characterized v(MF) v i b r a t i o n s are t h o s e of t h e o c t a h e d r a l m e t a l hexafluorides<^-^^) in t h e gaseous s t a t e . These c o m p o u n d s h a v e been studied b y infrared a n d frequently also b y R a m a n t e c h n i q u e s a n d a l m o s t complete a s s i g n m e n t s h a v e b e e n m a d e for t h e i r six v i b r a tional frequencies. T h e s t r e t c h i n g m o d e s v-^ a n d lie in t h e r a n g e s 628-771 a n d 616-748 cm~^ respectively. T h e m o d e V g , being inactive in b o t h t h e infrared a n d t h e R a m a n , m u s t be o b t a i n e d from c o m b i n a t i o n b a n d s (Table I ) .
T h e corresponding s p e c t r a for t h e n o n - m e t a l hexafluorides are also i n c l u d e d i n T a b l e I . T h a t t h e b o n d i n g in b o t h t h e m e t a l (of which t h e r e are 14) a n d t h e n o n - m e t a l (of which t h e r e a r e four) hexafluorides is similar, is e v i d e n t from t h e close similarity b e t w e e n t h e v i b r a t i o n a l s p e c t r a of t h e s e t w o ''classes" of c o m p o u n d . I t is also interesting t h a t for t h e t r a n s i t i o n m e t a l hexafluorides of t h e t h i r d r o w lies 31-62 cm~^
a b o v e t h e m o d e of t h e corresponding second-row hexafluorides, suggesting t h a t t h e effects of t h e l a n t h a n i d e contraction in t h e t h i r d t r a n s i t i o n series a r e being reflected in t h e i/(MF) m o d e s .
T h e b o n d stretching force c o n s t a n t s for t h e m e t a l hexafluorides a r e also higher for t h i r d - r o w t r a n s i t i o n elements t h a n for t h e corresponding
METAL-HALOGEN VIBRATIONAL FREQUENCIES 87 second-row elements, a n d decrease w i t h i n each series w i t h increasing TT-electron o c c u p a t i o n of t h e n o n - b o n d i n g shell of t h e m e t a l (Table I) . ( i ^ )
T A B L E I . V i b r a t i o n a l s p e c t r a ( c m - ^ ) o f m e t a l h e x a f l u o r i d e s / ( m d / Â ) Réf.
R R I . R . I . R . R I Réf. 10
Metal hexafluorides (gaseous state) f
MoFe 741 643 741 262 312 122 4-84 3
W F e 771 673 711 258 315 134 5-20 4
TcFg 712 639a 748 265 2 9 7 a 174 4-79 3
R e F e 755 6 7 l a 715 257 2 9 5 a 193 5-17 5
R u F e 675 624 735 275 283a 186 4-61 6
O s F e 733 668a 720 272 276a 205 5 1 4 7
R h F e 634 592 724 283 269 189 4-27 6
IrFe 701 646 719 276 258 206 4-94 8 , 9
P t F e 655 600 705 273 242 211 4-52 7
U F e 667 535 623 184 201 140 3-81 10
N p F e 648 528 624 198 205 165 3-73 10
P u F e 628 523 616 203 211 173 3-62 10
Non-metal hexafluorides (gaseous s t a t e ) t
SFg 773 641 938 613 519 350 5-53 11
SeFe 708 661 780 437 403 262 5-04 11
TeFe 701 674 752 325 313 195 5 1 1 11
Anionic hexafluorides (condensed phases)
613 — 560 — 275 — — 17a
581 — — — 228 — — 17a
W F g ^ - 589 — — — 230 — — 17a
i > S i F 6 2 - 655 474 740 485 395 — — 17a, 18a
cGeFe^- 627 454 600 350 318 — — 18b
to.dSnFe^ - 585 470 556 — 241 — — 17a
PbFg^- 543 — 502 — — — — 17a
ePtFg^- 600 576 571 281 210 143 — 20
f A s F e - 682 583 706 402, 389 372 — — 21
eNbFe" 683 562 585, 619sh 256, 232 280 — — 17b, 26b
«TaFg- 692 581 560 240 272 — — 17c, 26b
HUF^- 506 — 503 150 145 100 — 22
"j- T h e frequencies q u o t e d are t h e revised v a l u e s of W e i n s t o c k a n d Goodmani^^^); t h e reference n u m b e r s a p p l y t o t h e m o s t recent p u b l i s h e d literature for e a c h molecule.
a T h e v a l u e s of ν2 and v g for t h e d^- a n d iZ^-hexafluorides h a v e b e e n o b t a i n e d b y inter
polation of t h e corresponding v a l u e s for t h e non-Jahn-Teller-active hexafluorides.(^°^)
^ R a m a n spectrum of a q u e o u s solutions of t h e a m m o n i u m salt.
c Fluorogermanic acid solutions.
d Infrared spectrum of t h e salt [ ( n - C 3 H 7 ) 2 N H 2 ] 2 S n F 6 in MeCN solution.
e R a m a n s p e c t r u m refers t o a solution of t h e s o d i u m salt; infrared spectra refer t o nujol mulls and K B r pressed discs of t h e c a e s i u m salt.
' Infrared s p e c t r u m refers t o t h e A S C I 4 + salt.
? R a m a n a n d infrared spectra of t h e crystalline c a e s i u m salt.
^ D e d u c e d from a n analysis of t h e vibronic s p e c t r u m of t h e caesium salt.
R = R a m a n - a c t i v e ; I R = infrared-active; I = inactive. T h e force c o n s t a n t referred t o is t h e metal-fluorine stretching force constant.
88 R. J. H. CLARK
Because of t h e e x t r e m e difficulties associated w i t h t h e r a d i o a c t i v i t y of t h e m a t e r i a l , t h e s p e c t r u m of polonium hexafluoride h a s n o t y e t been reported. <^^>
X e n o n hexafluoride i^^) h a s six b o n d i n g a n d one n o n - b o n d i n g electron pairs a b o u t t h e central a t o m , a n d t h e r e was some speculation therefore, as t o w h e t h e r t h e molecule would be completely s y m m e t r i c a l (cf. t h e T e C y - ion). H o w e v e r , b o t h infrared a n d electron diffraction d a t a clearly indicate t h a t t h e molecule is signiflcantly d i s t o r t e d from octahedral.<i^^)
A t t e m p t s t o p r e p a r e t h e m o s t likely of t h e o t h e r possible hexafluorides (AmFg a n d PdFg) h a v e so far been unsuccessful. H o w e v e r , t h e v e r y u n s t a b l e c h r o m i u m hexafluoride was recently synthesized b y Glemser et αΖ.,<^^) w h o found t h a t in t h e solid s t a t e t h e v i b r a t i o n is split i n t o t h r e e b a n d s a t 730, 785 a n d 800 cm-^.
T h e v i b r a t i o n a l s p e c t r a of certain m e t a l hexafluorides h a v e been cited as providing evidence for t h e d y n a m i c J a h n - T e l l e r eff*ect. T h e t h e o r e m , e n u n c i a t e d in 1937,^^^»^^) s t a t e s t h a t s y m m e t r i c a l s t r u c t u r e s in non-linear p o l y a t o m i c molecules are u n s t a b l e w i t h respect t o certain nuclear dis
p l a c e m e n t s if t h e electronic s t a t e of t h e molecule is degenerate. If such is t h e case, t h e molecule c a n lower its energy b y a distortion of t h e nuclear configuration t o one of lower s y m m e t r y , t h e r e b y destroying t h e electronic degeneracy. W h e n t h e gain in stability on distortion is com
p a r a b l e w i t h t h e energy of t h e r e l e v a n t v i b r a t i o n a l m o d e , no s t a t i c distortion will be observed, b u t r a t h e r certain abnormalities in t h e v i b r a t i o n a l spectra are t o be e x p e c t e d as a result of vibrational-electronic coupling.
T h e i i i (TcFfi, ReFg), (RuFg, OsFg), a n d (RhFg, IrFg) m e t a l hexafiuorides h a v e d e g e n e r a t e electronic g r o u n d s t a t e s , a n d it m a y be shown t h a t a J a h n - T e l l e r effect could be e x p e c t e d for b o t h t h e Cg {v<^) a n d t^g (vg) v i b r a t i o n a l m o d e s of t h e s e molecules. T h e e x p e r i m e n t a l observations of Weinstock, Claassen a n d Chernick<^) were as follows:
for m e t a l hexafiuorides in which a J a h n - T e l l e r distortion is n o t possible (e.g. d^ molecules) v-^ -\- Vg a n d + occur w i t h similar frequencies, intensities a n d s h a p e ; however, for t h e d^ a n d hexafiuorides, t h e
^2 + c o m b i n a t i o n b a n d is v e r y m u c h b r o a d e r t h a n t h e v-^ + b a n d . H o w e v e r , in a r e c e n t comprehensive s u r v e y of t h e spectra of all k n o w n hexafiuorides, W e i n s t o c k a n d G o o d m a n h a v e concluded t h a t t h e Vg a n d V5 m o d e s for t h e d^ a n d d'^ hexafiuorides c a n n o t b e o b t a i n e d simply from t h e spectra, b u t m u s t be e v a l u a t e d from a detailed analysis of t h e d y n a m i c coupling of t h e electronic a n d v i b r a t i o n a l m o t i o n s of t h e mole
cules. A static distortion is ruled o u t b y t h e lack of splitting of t h e t r i p l y degenerate f u n d a m e n t a l s . Corresponding abnormalities were n o t ob
served for t h e d^ fiuorides, a n d reasons for this h a v e been given.
METAL-HALOGEN VIBRATIONAL FREQUENCIES 89 (ii) Anionic Species
These v e r y complete studies d o n o t , of course, e x t e n d t o condensed phases, e x c e p t in a few cases, i^^-^^)
T h e d a t a on GeFg^- in T a b l e I o b t a i n e d b y Griffiths a n d Irish(i8a)^
refer t o fluorogermanic acid s o l u t i o n s . T h e y also s t u d i e d Nujol mulls a n d caesium iodide pellets of t h e c o m p o u n d s CsgGeFe, (NH4)2 GeFg a n d B a GeFg, in o r d e r t o ascertain t h e effect of lowering t h e site s y m m e t r y of t h e g e r m a n i u m a t o m from Oh (as in t h e caesium salt) t o D^^ (^s in t h e b a r i u m salt). T h e s p e c t r a , r e p r o d u c e d in F i g . 1, indicate t h a t b o t h a n d
r e m a i n t r i p l y d e g e n e r a t e in t h e caesium salt, b u t t h a t splits b y ^ 3 0 0 c m- i in t h e b a r i u m salt, in a g r e e m e n t w i t h t h e selection rules for v i b r a tional s p e c t r a of crystals w h e n t h e space g r o u p a n d n u m b e r of molecules p e r u n i t cell are k n o w n , i^^) (v^ is b r o a d e r in t h e b a r i u m salt, b u t does
Cs2GeF6
Space group: 0/
Site group: O^,
(NH4)2GeF6
Space group: ^ 3 ^ ; (9/
Site group: ^ 3 ^ ;
6 0 0 5 0 0 4 0 0 Frequency (cm"')
F I G . L Infrared spectra of CsaGeFe, (NH4)2GeF6 and BaGeFg in caesium iodide discs.ii»)
90 R. J. H. CLARK
not apparently split. ) T h e a m m o n i u m salt crystallizes i n b o t h a hexagonal f o r m (site group D^^) a n d a cubic f o r m (site group Oh) a n d the scan i n F i g . 1 is more consistent w i t h the latter f o r m .
I n their R a m a n study of the SnF^^- ion, as present in the salt KaSnFg.HgO, K r i e g s m a n n a n d Kessleri^^^) claimed t h a t > i n contrast to the general behaviour of m e t a l hexafluorides. V e r y recently, however, E v a n s a n d Dean<i^^) have shown t h a t the shoulder a t 624cm assigned as b y the flrst group of workers, arises f r o m the solid state splitting of the v i b r a t i o n , a n d t h a t < v-^ as expected.
T h e spectra of the VfF^^-^^^) P t C ^ - a n d PtBrg^- ions^^s) are interest
ing because t h e y appear to provide the first experimental evidence for TT-bonding in the latter t w o species or indeed in a n y diamagnetic species of t h e t y p e MXg. T h e R a m a n spectrum of the PtFg^- ion shows the n o r m a l intensity p a t t e r n for octahedral MXg molecules, w i t h being more intense t h a n v^. H o w e v e r , t h e reverse is t r u e for the chloride a n d bromide, a n d i t is suggested b y W o o d w a r d a n d Ware^^^) t h a t t h e six -electrons on the p l a t i n u m are involved i n 7r-bonding to t h e v a c a n t , low-energy cZ-orbitals of the chlorine a n d bromine atoms.
Fluorine does n o t possess corresponding low-energy acceptor orbitals for electron delocalization f r o m the p l a t i n u m , a n d hence t h e R a m a n intensities of the bond stretching modes r e m a i n n o r m a l . This con
clusion is supported b y the observation t h a t the metal-fluorine force constants decrease progressively i n the series WFg (d^), R e F ^ [άλ), OsFg (d!^), IrFg {d^) a n d P t F g [d"^) (no 77-bonding possible) b u t increase i n t h e series ReCle^- {d^, OSCIQ^- {d^) a n d PtClg^- [d^), consistent w i t h t h e presence of increasing 7r-bonding i n the hexachloro species.
T h e close similarity of t h e R a m a n spectrum of t h e crystalline solid CsNbFg to those of k n o w n hexafluoro species has established for the first t i m e t h a t the N b F g - ion exists i n this salt.^^"^^) I t is also present i n aqueous H F solutions containing a t least 2 5 % H F . I n the same study, i t was demonstrated t h a t the complex KaNbOFg.HgO contains the N b O F g ^ - ion (and w a t e r of h y d r a t i o n ) i n the solid state, a n d i n aqueous solution, rather t h a n the alternative Nb(OH)2F52- ion. I n K F / L i F melts, t h e predominant species is, however, t h e NbF7^~ ion.^^'^'^)
T h e d a t a for the UFg" ion were derived f r o m a n analysis of the vibronic spectrum of the caesium salt i n the near-infrared a n d visible regions. <22^) Using these d a t a , t h e U — F bond stretching force con
stant <22i>) was calculated to be 2-44 m d / Â , assuming a U — F bond distance of 1-98Â.
T h e m a j o r i t y of d a t a currently available on M—^F vibrational fre
quencies, however, refer only to the infrared-active stretching mode.
These d a t a , initially obtained b y Peacock and Sharp, i^^^) are summarized
METAL-HALOGEN VIBRATIONAL FREQUENCIES 91
MFe2- Ti V Cr Mn F e Co N i Ge
560 583 556 622 — — 654 600
Zra N b Mo Tc R u R h P d Sn
< 4 0 0 — — — 581 589 602 552
H f T a W R e Os Ir P t P b
— — — 541 548 568 583 502
M F r V A s
715 700
N b Mo Tc R u Sb
580 623 — 640 660
T a W R e Os Ir
580 594 627 616 667
* T h e ZrFg^" ion is a c t u a l l y eight-coordinate (dodecahedral) in t h e solid s t a t e , w i t h four bridging a n d four terminal fluorine a t o m s b o u n d t o each zirconium a t o m .
in T a b l e I I . T h a t v(MP) frequencies characteristically increase w i t h in
creasing o x i d a t i o n s t a t e of t h e m e t a l is clear from a comparison of t h e d a t a in T a b l e s I a n d I I , e.g. increases in t h e series RuFg^-, 581 ; RuFg", 640; RuFg, 735; IrFg^-, 568; IrFg-, 667; IrFg, 718 cm-i. T h e c o r r e s p o n d ing m o d e s in t h e t e r v a l e n t hexafluorides (cryolites) a n d b i v a l e n t h e x a fluorides (perovskites) occur t o lower frequencies again, in t h e ranges 446-617 c m- i (M = Al, Sc, T i , V , Cr, M n , F e , Co, Ga, R u , R h , I n ) a n d 407-489 c m- i (M - Mg, Cr, M n , F e , Co, N i , Cu, Zn) respectively. T h e
^ S i u ) b e n d i n g m o d e h a s been observed a t 292 cm-^ in KgVFg a n d a t 308 c m- i in K^CrF^A^^^) I t also occurs in t h e r a n g e s 234-289 c m- i for t h e salts CsgMFg (M R e , R u , Os, R h , I r , P d , a n d P t ) a n d 220-315 c m- i for t h e salts CsMFg (M = V , N b , T a , S b , M o , W, R e , R u , Os a n d I r ) ; is little d e p e n d e n t o n t h e o x i d a t i o n s t a t e , a n d is frequently split into t w o b a n d s in solid-state infrared spectra, i^e^)
B. Tetrahedral molecules
F o r t e t r a h e d r a l m e t a l fluorides, even fewer d a t a a r e c u r r e n t l y avail- able(22-30) (Table I I I ) , b u t it is e v i d e n t t h a t v(MF) m o d e s occur a t higher frequencies t h e lower t h e coordination n u m b e r of t h e m e t a l , e.g. 1^3(^2)
for GeF^ = 800 c m- i , b u t ^3(^1^ for GeFg^- = 600 c m- i . I t should b e n o t e d t h a t t h e d a t a in T a b l e I I I refer t o t h e gaseous molecule; in t h e solid s t a t e , t h e coordination p o l y h e d r o n a b o u t t h e m e t a l in zirconium a n d hafnium tetrafluorides is eight-coordinate s q u a r e a n t i p r i s m a t i c , while t h a t in t h o r i u m tetrafluoride is eight-coordinate dodecahedral. (^i) T h e i^(MF) m o d e s of such fluorine-bridged s t r u c t u r e s occur a t lower frequencies t h a n in t h e gaseous s t a t e .
T A B L E I L I n f r a r e d a c t i v e v ( M F ) m o d e s (v3,tiu, c m - ^ ) i n c o m p l e x h e x a i i u o r o a n i o n s ( p o t a s s i u m s a l t s )
92 Β . J. Η. CLARK
V2(e) ^4(^2) References
SiF^ 800 268 1010 390 28
GeF4 738 205 800 260 29
ZrF4 6 0 0 - 7 2 5 * 1 5 0 - 2 0 0 * 668 190 ± 2 0 30
HfF4
— —
645—
30ThF^
— —
520—
30a Calculated value.
As in t h e case of t h e o c t a h e d r a l fluorides, t h e r e is n o gross difference b e t w e e n t h e v i b r a t i o n a l frequencies of essentially metallic gaseous t e t r a fluorides, a n d non-metallic ones such as
CF4
a n d SiF4.<2^'^2)I n a d d i t i o n , S t a m m r e i c h , Sala a n d Bassi^^^) h a v e r e c e n t l y p u b l i s h e d a full v i b r a t i o n a l analysis of t h e t e t r a h e d r a l ions C r O g X - (X = F , CI);
t h e v i b r a t i o n s which a r e essentially C r — X stretching m o d e s occur a t 637 c m- i ( X - F ) a n d 438 c m- i ( X = CI).
C. Heptafluorides
T h e infrared spectra (vapour state) a n d R a m a n s p e c t r a (liquid state) of t h e t w o k n o w n n e u t r a l heptafluorides,
IF7<^^^)
a n d ReF7<^^^) a r e b o t h satisfactorily a c c o u n t e d for o n t h e basis of t h e p e n t a g o n a l b i p y r a m i d a l model (Z>5ii) (although t h e s a m p l e of t h e former m a y h a v e been cont a m i n a t e d w i t h
IOF5).
A l t h o u g h t h e s y m m e t r y of iodine heptafluoride in t h e solid s t a t e h a s b e e n t h e subject of considerable d i s p u t e , d u e t o i n a d e q u a t e X - r a y d a t a on a possibly i m p u r e sample, t h e m o s t r e c e n t analysis favours t h e D^^^ model in this s t a t e also.(^^^) F o r this model, t h e r e a r e flve e x p e c t e d R a m a n - a c t i v e v i b r a t i o n s (2αι' -f e^" -\- 2e^) a n d also five expected infrared-active v i b r a t i o n s {2α.^' + 3βι') w i t h o n e m o d e inactive [e^"). Only t h r e e of t h e infrared-active m o d e s were located for t h e r h e n i u m c o m p o u n d . T h e highest frequency f u n d a m e n t a l s , which a r e essentially b o n d - s t r e t c h i n g v i b r a t i o n s , occur a t 678 (R) a n d 670 (i.r.) for IF7, a n d a t 737 (R) a n d 703 c m- i (i.r.) for R e F ^ .Only o n e infrared-active v(MF) m o d e h a s been r e p o r t e d for t h e seven- coordinate anions<^4*^) N b F 7 2 - a n d T a F 7 2 - a t 5 2 4 a n d 518 cm-^ respec
tively (potassium salts), (^β) T h e R a m a n s p e c t r u m of t h e former (also a s t h e crystalline p o t a s s i u m salt) contains t h r e e b a n d s , a t 782 (w), 630 (vs) a n d 388 (m).<^^^> H o w e v e r , in a q u e o u s H F solutions t h e ion is n o t seven-coordinate; t h e n i o b i u m is p r e s e n t a s t h e N b F g - ion (in concen
t r a t e d H F solutions) a n d a s t h e NbOFg^- ion (in dilute H F solutions).
This conclusion agrees w i t h t h a t d r a w n from n.m.r. measurements.<^^^)
T A B L E I I I . V i b r a t i o n a l s p e c t r a (cm"^) o f t e t r a h e d r a l m e t a l fluorides i n t h e v a p o u r s t a t e
METAL-HALOGEN VIBRATIONAL FREQUENCIES 93 I n a similar study of t h e corresponding t a n t a l u m s y s t e m , t h e
R a m a n spectra o f t h e crystalline complexes NagTaFg, (NH4)3TaOF6, K2TaF7 a n d CsTaFg as well as K e N b O F g a n d (NH4)3 N b O F g have been recorded. F r o m R a m a n spectra o f 2 4 M H F solutions o f t a n t a l u m ( ^ ^ I M i n t a n t a l u m ) t h e predominant species was identified as t h e T a F ^ - i o n , b u t i n more dilute H F solutions, t h e TSLF^^- ion is present i n appreciable amounts. I n melts o f TaFg i n t h e L i F / K F solvent, t h e p r e d o m i n a n t anionic species is t h e TaF^^- i o n , whereas w i t h t h e L i F / N a F solvent, t h e T a F e " ion is also present.
D. Other stereochemistries
T h e infrared spectrum o f VF5 v a p o u r has been studied, (^^*) b u t t h e d a t a alone were insufiicient t o determine whether t h e molecule has a trigonal b i p y r a m i d a l or a square p y r a m i d a l structure. M o r e refined infrared spectra, together w i t h R a m a n spectra, however, have demon
strated t h a t t h e molecule has t h e t r i g o n a l - b i p y r a m i d a l structure i n the vapour phase, b u t t h a t i t is highly associated i n t h e l i q u i d phase.
Monomeric molecules are evidently present i n l o w concentration i n t h e room-temperature l i q u i d , b u t constitute t h e m a i n component i n t h e l i q u i d a t temperatures above 100°.
B o t h VF3 a n d VF4 show a broad b a n d a t 5 3 0 - 5 4 0 c m - \ p r o b a b l y associated w i t h t h e bridging i/ ( V F ) modes o f t h e octahedral polymeric units. H o w e v e r , t h e tetrafiuoride has additional bands a t 7 8 0 - 8 3 7 cm~^
which, i f t h e y are fundamentals rather t h a n combination or overtone bands, m a y be associated w i t h t h e t e r m i n a l V — F bonds o f the molecule.
A useful compilation o f M— X stretching frequencies i n n o n - m e t a l halides a n d oxyhalides o f t h e types
MX3,
OMXg,O3MX,
a n dMX5
( M = B r , I ) has been presented b y Nakamoto,<^^) a n d w i l l n o t be re
peated here. T h e spectra o f t h e halides a n d m i x e d halides o f phosphorus a n d a n t i m o n y are discussed later. T h e spectra o f t h e linear difluorides, K r F g a n d X e F g have also been reported recently.
E, Assignments in metal complexes
F e w assignments o f v ( M F ) modes i n m e t a l complexes have y e t been made. P a r t o f t h e difficulty arises because such vibrations w o u l d be expected t o occur i n t h e 4 0 0 - 7 0 0 cm^^ region i n which several vibrations (notably aromatic ring vibrations) o f t h e more common ligands are k n o w n t o occur. H e n c e unambiguous identification is difficult. E a r l y assignments o f v ( T i F ) modes were m a d e b y R a o o n adducts o f t i t a n i u m tetrafluoride w i t h pyridine a n d acetonitrile a n d more recent d a t a o n these vibrations<^^'^i) are summarized i n T a b l e I V . Corresponding v i b r a tions i n adducts of silicon tetrafluoride w i t h pyridine, triphenylphosphine
94 R. J. H. CLARK
v(MF) 8(FMF) References
TiF^bipy 634, 562, 450? 311, 272, 254 40
TiF^.phen 645, 570, 462? 308, 276, 256 40
Ti(acac)2F2 629 306? 41
Sn(acac)2F2 581
—
41b i p y = 2,2'-bipyridyl; p h e n = 1,1 O-phenanthroline; H a c a c = acetylacetone.
oxide a n d o-phenanthroline occur^^^) in t h e 759-813 cm-^ region for i/(SiP), a n d a t 399-490 cm-^ for 8(FSiP); cf. a n d of SiFg^- occur a t 726 a n d 480 cm-^ respectively. (^6) ( F r o m t h e n u m b e r of r(SiF) m o d e s active in t h e infrared, t h e first t w o complexes are believed t o h a v e t h e trans configuration.) F o r t h e v e r y limited n u m b e r of six-coordinate complexes of Si, Ti a n d Sn so far s t u d i e d , it a p p e a r s t h a t v(MF) m o d e s occur a t slightly higher frequencies in m e t a l fluoride complexes t h a n in t h e p a r e n t MF^^- ions.
K r i e g s m a n n a n d Kessleri^^^) a n d K o l d i t z a n d Nussbucher(^^) h a v e s t u d i e d t h e infrared a n d R a m a n s p e c t r a of t h e complex anions [SnFsOH]^-, [ A s F g O H ] - a n d [ S b P g O H ] - a n d assigned b a n d s occurring in t h e 547-620, 670-675, a n d 615-640 cm-^ regions t o v{SnF), v(AsF) a n d i/(SbF) v i b r a t i o n s respectively. T h e oxygen-bridged dimeric anions [MgFgOa]^- (M = As, Sb) also h a v e v(MF) b a n d s in t h e r e l e v a n t regions a b o v e . (^^)
B a n d s occurring a t 444-562 a n d 367-372 cm-^ in t h e infrared s p e c t r a of t h e c o m p o u n d s SnClF, SnFg a n d ( N H 4 ) S n F 3 h a v e b e e n assigned as v(SnF) a n d 3(FSnF) m o d e s respectively, t h e d a t a being i n t e r p r e t e d t o i m p l y t h e presence of t h e [ S n F 3 ] - ion in each compound.i^^^) F o r c e c o n s t a n t calculations b y D o n a l d s o n et alA^^^^ suggest t h a t some revision t o these a s s i g n m e n t s m a y b e necessary.
I t h a s r e c e n t l y b e e n shown(^^^»^) t h a t , c o n t r a r y t o p r e v i o u s r e p o r t s , alkyl a n d aryl t i n fluorides of t h e t y p e s R 3 S n F a n d RgSnFg a b s o r b in t h e 328-350 c m - ^ r e g i o n a n d t h a t t h i s b a n d m u s t arise from a v(SnF) m o d e . T h e c o m p a r a t i v e l y low frequency for t h i s v i b r a t i o n m u s t i m p l y t h a t t h e fluorine a t o m s a r e bridging in t h e c o m p o u n d s (cf. X - r a y d a t a ) .
T h e c o m p o u n d ( C 6 F 5 ) 2 T I F is also believed t o be a fluorine-bridged d i m e r because of t h e c o m p a r a t i v e l y low values for t h e v(TlF) v i b r a t i o n s
(320 a n d 165 c m- i ) . ( ^ 5 )
3. Metal-Chlorine, M e t a l - B r o m i n e a n d M e t a l - I o d i n e Vibrational Frequencies
I t is convenient t o deal w i t h t h e frequencies t o g e t h e r .
T A B L E I V . v ( M F ) a n d S ( F M F ) m o d e s ( c m - ^ ) i n c o m p l e x e s o f t h e G r o u p I V e l e m e n t s
METAL-HALOGEN VIBRATIONAL FREQUENCIES 9 5
A. Octahedral Hexahaloanions
T h e r e a p p e a r s t o b e n o information o n t h e v i b r a t i o n a l s p e c t r a of n e u t r a l h e x a h a l i d e s MXg ( X = CI, B r , I ) , b u t some information is n o w available o n t h e hexahaloanions.(^^'^δ,47-56) These d a t a h a v e only r e cently been o b t a i n e d because (a) t h e colours of t h e complexes often m a k e it impossible t o o b t a i n t h e i r R a m a n s p e c t r a w i t h t h e usual blue H g 4 3 5 8 Â exciting line a n d (b) infrared s p e c t r o m e t e r s h a v e only r e c e n t l y been developed commercially t o locate t h e low-lying f u n d a m e n t a l s of such molecules.
H o w e v e r , b y use of t h e green H g line ( 5 4 6 1 Â ) a n d t h e yellow a n d r e d H e lines ( 5 8 7 6 a n d 6 6 7 8 Â , respectively) for excitation, t h e R a m a n spectra of t h e ions ReCle^"", ReBrg^- a n d OsClg^- h a v e b e e n recorded b y W o o d w a r d a n d W a r e , i^^) T h e infrared-active c o m b i n a t i o n b a n d s of t h e s e molecules show some confusing aspects w h e n c o m p a r e d w i t h t h e corre
sponding b a n d s of hexafluoride molecules w i t h t h e s a m e configura
tion. B o t h ReCle^- a n d ReBrg^- (i^^) a n d OsCle^" (tig) b e h a v e like P t F ^ ^ - (tlg) b y giving a n infrared p a t t e r n i n which t h e c o m b i n a t i o n b a n d 1^2 + ^3 is observed b u t v-^^ + is n o t . This is t h e opposite result t o t h a t e x p e c t e d from a consideration of t h e J a h n - T e l l e r effect, a n d from t h a t found e x p e r i m e n t a l l y for t h e n e u t r a l hexafiuorides, b u t m a y b e a con
sequence of t h e fact t h a t t h e s e infrared results were o b t a i n e d for nujol mulls of ionic solids r a t h e r t h a n for gaseous molecules (as i n t h e case of
T A B L E V . V i b r a t i o n a l s p e c t r a ( c m - ^ ) o f h e x a h a l o a n i o n s r(MX)A(23) R e f e r e n c e s
T i C l e ^ - 4 6 3 3 4 0 330«· 2 5 2 2-34 4 7 , 4 8 a
R e C l e ^ - 3 4 6 2 7 5 3131^ 172^ 159 2-37 2 0
O s C l e ^ - 3 4 6 2 7 4 314^ 1771) 165 2-36 2 0
P d C l e ^ - 317 2 9 2 3 4 0 d 175d 164 4 8 a , 4 9
P t C l e ^ - 3 4 4 3 2 0 3 4 3 d 182d 162 2-34 4 8 a , 4 9
G e C l g ^ - 3 1 8 2 1 3 293^ 2 0 5 a 191 — 4 8 a , 5 3 a
S n C l g ^ - 3 1 1 2 2 9 2 9 4 a 1 6 5 a 158 2 - 4 3 4 8 a , 4 9 , 5 0 , 5 3 a
P b C l e ^ - 2 8 5 2 1 5 2 6 5 » 130 137 2 - 5 0 4 8 a , 4 9 , 5 1 , 6 9
SeCle^- - - 3 4 6 2 5 2 2 9 4 e 1 8 2 e 164 2-41 5 2 , 5 3 a , b
T e C l e ^ - 2 8 7 2 4 7 2 2 8 e 105e 131 2-51 5 3 a , b
p c i e - 3 6 0 2 8 3 4 4 4 a.c 285» 2 3 8 — 5 3 a , 5 4 a
A s C l f i - 337 2 8 9 3 3 3 220» 2 0 2 — 5 3 a
S b C l g - 337 2 7 7 3 4 5 180 172 — 5 3 a , 5 5
R e B r e ^ - 2 1 3 174 217^ 118^ 104 2 - 5 0 2 0
P t B r e ^ - 2 0 7 190 2 4 4 d 90d 97 — 2 5 , 4 9
S n B r e ^ - 185 138 — — 95 2-64 2 5
P t i e ^ - — — 186d 4 6 d — — 56
T l C l e ^ - 2 8 0 2 6 2 — — 155 2 - 4 9 143
» T e t r a e t h y l a m m o n i u m s a l t .
^ C a e s i u m s a l t . c A s t h e P C I 4 +s a l t .
I P o t a s s i u m s a l t .
' T e t r a m e t h y l a m m o n i u m s a l t .
96 R. J . H. CLARK
nihu) L a t t i c e m o d e ?
K g W C l e 3 2 4 165 77
R b a W C l e 3 0 6 1 6 0 6 6
C s g W C l e 3 0 8 166 71
K a M o C l e 3 4 0 174 7 4
R b g M o C l e 3 3 4 1 7 2 7 0
C s a M o C l e 3 2 5 170 7 0
K g W B r e 2 2 9 7 4
—
R b a W B r e 2 2 0 7 8
—
C s g W B r e 2 1 4 6 0
—
highest frequencies for a n anion. T h e b a n d a t 66-77 cm~^ in t h e infrared s p e c t r a of t h e chlorides (Table V I ) is p r e s u m e d t o b e either a lattice m o d e , or t h e V6(Î2m) m o d e , r e n d e r e d active b y t h e low site s y m m e t r y of t h e ion in t h e crystal lattice.
T h e infrared s p e c t r a of t h e ions MXe^- (M - Se, T e ; X = Cl, B r , I ) h a v e r e c e n t l y b e e n e x a m i n e d w i t h t h e conclusion t h a t t h e ions do n o t d e v i a t e from 0 ^ s y m m e t r y , i^^*)
t h e n e u t r a l hexafluorides). A n o r m a l coordinate analysis of t h e ReClg^", ReBrg^- a n d OsClg^- ions b y Yeranos^^^) h a s yielded force c o n s t a n t s of 1-34, 1-11 a n d 1-34 m d / Â respectively for t h e stretching of t h e m e t a l - halogen b o n d s .
I n a d d i t i o n t o t h e a b o v e results, infrared spectral m e a s u r e m e n t s h a v e located t h e v^ihu) niode for t h e following h e x a h a l o anions^^^^'^'^) as their t e t r a e t h y l a m m o n i u m salts: ZrClg^", 293; RuClg^-, 332; IrClg^-, 316, 324;
ThCle^-, 263; U C ^ - , 260; N p C ^ " , 265; ThBre^-, 177; UBrg^-, 178;
NbCle", 330; TaClg-, 330; WCle", 329 c m- i ; a n d AsCle", 352 c m- i . T h i s result for t h e t u n g s t e n d e r i v a t i v e disagrees w i t h t h a t of Β agnail. B r o w n a n d d u Preez,<^^) w h o o b t a i n t h e v a l u e 305 cm-^ for t h e m o d e of t h e complex E t 4 N [ W C l 6 ] . They(^^) also r e p o r t t h a t t h i s m o d e occurs a t 317 cm-^ in t h e caesium salt, a n d 315 cm-^ in t h e t e t r a m e t h y l a m m o n i u m salt of t h e WClg- ion. F o r t h e UClg" ion, t h e vg m o d e occurs a t 303-310 cm~^ for four salts, i.e. t h e r e is '^50 cm~^ rise in per u n i t rise in oxi
d a t i o n s t a t e of t h e u r a n i u m a t o m . I n addition, from a n analysis of t h e electronic s p e c t r u m of t h e c o m p o u n d CsaUClg, it h a s been suggested t h a t vi(aip) = 310 cm"^ for t h e anion.(^^)
T h e r e is a r a t h e r p r o n o u n c e d dependence of a n d t o a lesser e x t e n t of ï^4, on t h e size of t h e cation. As t h e l a t t e r is decreased in size, t h e c a t i o n - a n i o n interactions increase leading t o a n increase in b o t h vg a n d Ï/4 (Table VI). T h u s sodium or p o t a s s i u m salts c o m m o n l y give rise t o t h e
T A B L E V I . A b s o r p t i o n b a n d s (cm"^) o f s o m e h e x a h a l o a n i o n s a s a f u n c t i o n o f t h e c a t i o n
METAL-HALOGEN VIBRATIONAL FREQUENCIES 97
V2(e) ^4(^2)
R R I.R., R I.R., R
»AsCl4+ 422 156 500 187
GeCl4 397 132 451 171
GeBr4 234 78 328 111
Gel4 159 60 264 80
GaCl4- 346 114 386 149
GaBr4- 210 71 278 102
G a l 4 - 145 52 222 73
ZnCl42- 282 82 277 116
ZnBr42- 172 61 210 82
Znl42- 122 44 170 62
a Ref. 2 1 .
of t h e analogous species MX4-. I n fact, t h i s is a general result t h a t b o n d stretching frequencies, a n d t h e r e l a t e d force c o n s t a n t s increase as t h e oxidation s t a t e of t h e m e t a l increases. R e a s o n s for t h i s h a v e b e e n dis
cussed b y W o o d w a r d a n d b y G o d n e v a n d Alexandrovskayai^^). I t is also n o t e w o r t h y t h a t m e t a l - h a l o g e n single b o n d l e n g t h s decrease as t h e o x i d a t i o n s t a t e of t h e m e t a l increases, a n d t h a t v^^ (and t h e corresponding force constants) fall off in t h e order CI > B r > I for a given series of m e t a l t e t r a h a l i d e s .
T h e infrared s p e c t r a of t h e m e t a l t e t r a h a l o g e n species
MX42-
h a v e b e e n fairly extensively i n v e s t i g a t e d b y several a u t h o r s , a n d s o m e t i m e s V4 being located.(^s^»^^'^^) T h e results are s u m m a r i z e d in F i g . 2 for t h e vib r a t i o n . Clearly, M—Cl stretching frequencies occur a t ^^290 cm-^, M — B r stretching frequencies a t ^^220 cm-^, a n d M — I s t r e t c h i n g frequencies a t ^^180 cm-^ for t h e s e species. T h e corresponding v a l u e s for V4 are 112-133 cm-^ for t h e chloro species, a n d 71-92 cm-^ for t h e b r o m o species. T h e m e t a l - h a l o g e n s t r e t c h i n g frequencies follow t h e order M n < F e < Co > N i > Z n w h i c h is t h e order of t e t r a h e d r a l ligand-field stabilization energies. T h e copper complexes a r e excluded from t h e a b o v e generalization, because t h e y d e p a r t a p p r e c i a b l y from t e t r a h e d r a l s y m m e t r y . This s a m e t r e n d w i t h c h a n g e in t h e central m e t a l h a s since b e e n observed(^^) for t h e corresponding s t r e t c h i n g m o d e of t h e series M ( N C 0) 4 2 - (M = Mn, F e , Co, N i , Zn).
B. Tetrahedral Anions
A comprehensive collection of t h e extensive d a t a relating t o t h e v i b r a t i o n a l frequencies of t e t r a h a l o g e n molecules h a s been published b y Nakamoto.^^^) Typical results only are given in T a b l e V I I . E v e r y v i b r a tional m o d e of a given species
MX42-
lies below t h e corresponding m o d eT A B L E V I I . V i b r a t i o n a l f r e q u e n c i e s ( c m - ^ ) for a n i s o e l e c t r o n i c s e r i e s o f t e t r a h e d r a l M X 4 units(^'>
98 Β . J. Η. CLARK
Μη Fe Co Ni Cu Zn
F I G . 2. ^3(^2) m o d e s o f ( E t 4 N ) 2 M C I 4 , ( E t 4 N ) 2 M B r 4 , a n d ( n - P r 4 N ) 2 M I 4 a s n u j o l mulls.ie^.es)
T h e s p e c t r a of v a n a d i u m tetrachloride/^^'^^) a n o d d electron molecule (d^), do n o t suggest t h a t it is d i s t o r t e d from t e t r a h e d r a l . H o w e v e r , it is n o t certain t h a t v^ie) h a s been located a n d indeed t h i s is t h e m o d e which m i g h t b e e x p e c t e d t o suffer b r o a d e n i n g a t r o o m t e m p e r a t u r e as a result of J a h n - T e l l e r effects.
T h e s p e c t r a of v a r i o u s m i x e d halides (e.g. TiBr2Cl2) h a v e also been s u m m a r i z e d . I n a d d i t i o n D e h n i c k e h a s r e c e n t l y r e p o r t e d t h e s p e c t r a of t h e m i x e d halide SnFaClg as well as t h o s e of several oxyhalides. ^^^^
T h e s p e c t r u m of t h e halide SnFaClg w a s i n t e r p r e t e d on t h e basis of t e t r a h e d r a l s y m m e t r y , a l t h o u g h t h e physical properties of t h e com
p o u n d (solid till 153°, insoluble in benzene a n d c a r b o n tetrachloride) a n d indeed t h e v(SnF) m o d e s (all below 570 cm-^) are m o r e consistent w i t h those e x p e c t e d for a fluorine-bridged o c t a h e d r a l polymer.
C. Square Planar Ions
S t a m m r e i c h a n d Forneris<^^) r e p o r t e d t h e R a m a n s p e c t r a of t h e s q u a r e p l a n a r ions A u C l 4 ~ , A u B r 4 ~ a n d P t C l 4 ^ ~ in 1960, a n d t h e infrared s p e c t r a of these ions were r e p o r t e d m o r e r e c e n t l y b y several sets of w o r k e r s . (^^-^1)
T h e m o s t complete of these, b y Sabatini, Sacconi a n d Schettino^^^), is s u m m a r i z e d in T a b l e V I I I , t o g e t h e r w i t h t h e force c o n s t a n t s derived b y these a u t h o r s b y use of a general valence force field. T h e conclusions of previous sections hold here also; n a m e l y that/j^ci > /ΜΒΓ /M(TII)CI >
/Μ(Π)ΟΙ c o m p a r a b l e c o m p o u n d s .
METAL-HALOGEN VIBRATIONAL FREQUENCIES 99
/ M X (md/A) References R R I . R . R L R . , R L R . , R
PtCl^^- 3 3 5 164 160 3 0 4 3 1 6 185 1-78 6a, 71
—
—
174—
3 2 6 194 — 6 9PdCl^^-
—
— 168 — 3 3 2 190 — 6 9PdBr^^- — — 106
—
2 5 4 136 — 6 9AUCI4- 3 4 7 171 143 3 2 4 3 5 6 173 2 1 0 6 8 , 71
AuBr^- 2 1 2 1 0 2 — 196 2 5 2 1 0 0 1-76 6 8 , 7 1
AUI4- —
—
— — 190 — — 6 9a Infrared d a t a refer t o t h e caesium salts, e x c e p t for t h e d a t a of Reference 6 8 , which refers t o t h e p o t a s s i u m salts. T h e vibrations are n u m b e r e d according t o t h e procedure of Nakamoto.t^^)
L a t t i c e m o d e s for VtCl^'^- salts were identified<^^) for t h e K + , R b + a n d Cs+ salts a t 106, 82 a n d 80 cm-^ respectively. T h e site s y m m e t r y of t h e
PtCl4^~
ion is D^h in t h e p o t a s s i u m salt, a n d accordingly t h e selection rules a r e t h e s a m e as t h e y would b e for t h e free gaseous ion. H o w e v e r , t h e site s y m m e t r i e s of t h eAUCI4-
a n d A u B r 4 - ions a r e lower t h a n D^i^, a n d as a consequence t h e modes show site s y m m e t r y splittings of u p t o 11 cm~^.D. Five Coordinate Halides
T h e k n o w n data<^^^''^^~'^^) for trigonal b i p y r a m i d a l p e n t a h a l i d e s MX5 a n d
MX3Y2
a r e s u m m a r i z e d in T a b l e I X (the s t a n d a r d v i b r a t i o n a l n u m b e r i n g scheme for molecules w i t h i^g^ s y m m e t r y is a d h e r e d t o ) .P h o s p h o r u s p e n t a c h l o r i d e is a n interesting molecule because it occurs w i t h t h e trigonal b i p y r a m i d a l s t r u c t u r e in t h e gaseous s t a t e , a n d also in t h e solid s t a t e if formed b y v a p o u r deposition o n t o a cold p l a t e a t
'-'90°K; however, a t r o o m t e m p e r a t u r e in t h e solid s t a t e it exists^^^^»^^) as t h e ionic species
PCI4+PCI6-.
T h e v i b r a t i o n a l s p e c t r a of t h e p h o s p h o r u s chlorofluorides PCl^Fg.^ also indicate t h a t these molecules exist b o t h in low t e m p e r a t u r e molecular forms a n d in r o o m t e m p e r a t u r e ionic forms.<^2^) T h e moleculePF2CI3
h a s full D^^ s y m m e t r y , a n d t h e r e fore t h e fluoride a t o m s are in axial positions. A n o r m a l coordinate analysis h a s been carried o u t on t h e moleculesPF5, PF2CI3
a n dPCI5
w i t h t h e complete G - m a t r i x a n d t h e a b o v e v i b r a t i o n a l assignments, i"^^^) T h e spectra of t h e halides SbFg a n d SbFgClg a r e included in T a b l e I X , on t h e basis of v i b r a t i o n a l work b y Dehnicke a n d Weidleinj^"^^) a l t h o u g h o t h e r physical properties of these molecules indicate t h a t t h e y a r e asso
ciated in t h e liquid s t a t e . Certainly t h e earlier formulation of t h e m i x e d halide SbF3Cl2 as t h e ionic species SbCl4+SbF6- (cf. t h e a u t h e n t i c AsCl4+AsF6'') is n o w considered t o b e incorrect.
TABLE V H I . Vibrational frequencies (cm-^) of square planar MX4^- anions^
100 R. J. H. CLARK
v i ( a / ) Me') Mel
R(p) R(p) I . R . I . R . R ( d p ) ; R{dp); R ( d p ) ; R(dp)
I.R. I.R. L R .
817 640 945 576 1026 533 301 514 72a
PF2CI3 ^ 633 387 867 328 625 404 122 357 72a
PCI5 c,d 395 282 441 301 581 281 100 261 7 3 , 7 4
SbFs e.f 667 491
— —
716 264 - - 9 0 228 7 5 , 7 6 tSbFgCla f 610 392 399
—
655 442 147 292 76tSbCls g 356 307 371 154 395 172 (72) 165 54a
VF^i^ 719 608 784 331 810 282 0 ^-200) 350 36b
f R e c e n t ^^F n.m.r. work (E. L. Muetterties, W . Mahler, K. J . Packer, a n d R. Schmutz
ler, Inorg. Chem. 3, 1302, 1964) indicates t h a t t h e c o m p o u n d s SbF5 a n d SbF3Cl2 are associated in t h e liquid state. H o w e v e r , t h e n.m.r. data were recorded at 20° whereas t h e R a m a n data were recorded at 80°.
a Infrared spectrum of gas; R a m a n spectrum of liquid at —86°.
^ Infrared spectrum of gas; R a m a n spectrum of liquid at —40°.
c R a m a n spectra in benzene a n d in carbon tetrachloride solutions.
d Infrared spectra in benzene a n d carbon disulphide solutions.
e Infrared spectra for b o t h liquid a n d vapour; R a m a n spectrum for liquid.
f Infrared a n d R a m a n spectra of liquid a n d solid.
κ Infrared a n d R a m a n spectra of t h e liquid.
^ Infrared and R a m a n spectra of t h e vapour.
T h e R a m a n s p e c t r u m (^^^) of t h e h a h d e SbCl4P in t h e soUd s t a t e is consistent w i t h t h e polar form SbCl4+F- b u t , in t h e m o l t e n s t a t e a n d in non-polar solvents, a v(SbP) m o d e a p p e a r s a t 542 cm-^ consistent w i t h t h e non-polar form for t h e molecule, a n d w i t h t h e molecular weight m e a s u r e m e n t s in non-polar solvents (Cgv s y m m e t r y assumed).(^^) H o w ever, a n X - r a y diffraction study^^^^) h a s shown t h a t in t h e solid s t a t e t h e molecule is a c t u a l l y a cis fluorine bridged t e t r a m e r .
T h e o t h e r chlorofluorides of p h o s p h o r u s also a p p e a r t o h a v e t h e trigonal b i p y r a m i d a l structure,<^2a,so) ^ j ^ ^ s p e c t r a of PCI4F being b e s t i n t e r p r e t e d in t e r m s of t h e Ggv s t r u c t u r e w i t h one fluorine a t o m occupying a n axial position, a n d t h o s e for PCI2F3 in t e r m s of t h e C^y s t r u c t u r e in which t h e fluorine a t o m s o c c u p y b o t h axial positions a n d one trigonal position. T h e molecule CF3PCI4 is also believed t o h a v e t h e trigonal b i p y r a m i d a l s t r u c t u r e on t h e basis of its v i b r a t i o n a l spectrum,(^i^> w i t h t h e CF3 g r o u p in a n axial position. N o information could b e o b t a i n e d on t h e b a r r i e r t o i n t e r n a l r o t a t i o n of t h e CF3 g r o u p . Similarly, t h e molecule (CF3)2PCl3 is trigonal b i p y r a m i d a l , w i t h t h e CF3 g r o u p s occupying t h e axial positions.
A n u m b e r of infrared a n d R a m a n m e a s u r e m e n t s h a v e also b e e n m a d e o n n i o b i u m a n d t a n t a l u m p e n t a c h l o r i d e s , p a r t i c u l a r l y b y Carlson,
T A B L E I X . V i b r a t i o n a l s p e c t r a o f t r i g o n a l b i p y r a m i d a l m o l e c u l e s (Dgh s y m m e t r y ) (cm-^)
METAL-HALOGEN VIBRATIONAL FREQUENCIES 101 b u t in n o case does sufficient care a p p e a r t o h a v e b e e n t a k e n t o exclude t r a c e s of m o i s t u r e from t h e solvents. T h u s t h e m e a s u r e m e n t s i m p l y t h e presence of oxychlorides in solution. F u r t h e r m o r e , n i o b i u m p e n t a chloride is k n o w n t o dimerize b y chlorine bridging in b o t h t h e solid s t a t e (^2) a n d in carbon t e t r a c h l o r i d e solution (^^) a n d only for t h e gaseous state<®^) h a s t h e trigonal b i p y r a m i d a l s t r u c t u r e b e e n established (electron diffraction d a t a ) . I t therefore seems u n r e a s o n a b l e t o follow Carlson's p r o c e d u r e a n d a t t e m p t a s s i g n m e n t s of t h e v i b r a t i o n a l s p e c t r a of t h i s molecule in condensed p h a s e s on t h e basis of D^^ selection rules. A d d i tional d a t a on t h e s e halides h a s b e e n o b t a i n e d b y B a d e r a n d H u a n g ;<^^^) t h e s e a u t h o r s also assigned some of t h e v i b r a t i o n a l m o d e s of t h e p e n t a chlorides of m o l y b d e n u m a n d t u n g s t e n .
E. Linear Species
T h e infrared s p e c t r a of a series of gaseous t r a n s i t i o n - m e t a l halides a t 600-1000° h a v e b e e n recorded b y Leroi et al,,^^ a n d i n t e r p r e t e d in t e r m s of t h e linear (Dooh) m o d e l (Table X ) . Significant c o n c e n t r a t i o n s of t h e d i m e r were also indicated. T h e a s y m m e t r i c stretching frequencies (vg) a n d t h e force c o n s t a n t s (1-99-2-67 m d / Â ) derived therefrom each p a s s t h r o u g h a m a x i m u m b e t w e e n m a n g a n e s e a n d zinc.
T A B L E X . V i b r a t i o n a l f r e q u e n c i e s ( c m - ^ ) o f l i n e a r m e t a l d i h a l i d e s
Molecule S t a t e v i( l l ) V 8( I . I I . )
MnClg gas 4 6 7
FeClj, gas —
—
4 9 2CoCla gas — — 4 9 3
NiCla gas — — 5 1 6
CuCla gas 3 7 0 — 4 9 6
ZnCla gas — 2 9 5 5 1 6
CoBra gas — — 3 9 6
ZnBra gas — 2 2 5 —
CuCla- solution 2 9 6 — —
solid — — 4 1 0
C u B r g - solution 1 9 0 — —
HgCl^ g a s 3 6 0 7 0 4 1 3
m e l t 3 1 3 (100) 3 7 6
solid 3 1 4 1 1 6 3 7 5
HgBr^ gas 2 2 5 4 1 2 9 3
m e l t 1 9 5 (90) 2 7 1
solid 1 8 4 2 5 1
H g i a g a s 156 3 3 —
E s t i m a t e d v a l u e s in parentheses.
Creighton a n d Lippincott^^^) h a v e recorded t h e R a m a n s p e c t r a of t h e ion CuCl2~ formed b y e x t r a c t i o n of a solution of c u p r o u s chloride in
