Phosphorus-31 nuclear magnetic resonance studies of phosphorus—fluorine compounds

Spectrochimica Acta, 1964, Vol. 20, pp. 1835 to 1842. Pergamon Press Ltd. Printed in ~orthern Ireland

Phosphorus-31 nuclear magnetic resonance studies o~ phosphorus-fluorine compounds JOHN F . NIXON* a n d ~ E I N t I A R D SCHMUTZLER University Chemical Laboratory, Lensfield Road, Cambridge (Received 13 April 1964)

Abstract--:P8~ NMR chemical shifts and phosphorus-fluorine coupling constants are reported for a wide variety of phosphorus fluorides, in which the coordination number of phosphorus can have the values three, four, five, and six. The chemical shift,s extend over a wide range (~350 ppm) but are relatively constant for any particular class of compounds, and become more positive as the coordination number of phosphorus increases. The pa~ NMR spectra of the fluorophosphoranes, R,PFs_~, confirm the previous conclusions regarding their stereochemistry. THE l i t e r a t u r e c o n t a i n s p h o s p h o r u s c h e m i c a l shift m e a s u r e m e n t s for a large n u m b e r of i n o r g a n i c a n d o r g a n i c p h o s p h o r u s c o m p o u n d s [1-9], a n d s e v e r a l a t t e m p t s h a v e b e e n m a d e to c o r r e l a t e t h e d a t a w i t h such b o n d p r o p e r t i e s as ionic c h a r a c t e r , t h e d e g r e e of h y b r i d i z a t i o n a n d t h e e x t e n t of m u l t i p l e b o n d i n g [2-4, 6, 10, 11]. V e r y , l i t t l e d a t a a r e a v a i l a b l e , h o w e v e r , on p h o s p h o r u s - f l u o r i n e c o m p o u n d s , w h e r e a d d i t i o n a l i n f o r m a t i o n can be o b t a i n e d f r o m m e a s u r e m e n t s of t h e p h o s p h o r u s fluorine, F a l - - F 19 s p i n - s p i n c o u p l i n g c o n s t a n t . W e h a v e t h e r e f o r e m e a s u r e d t h e p a l N M R s p e c t r a of a n u m b e r of w i d e l y differing p h o s p h o r u s fluorides in o r d e r to see h o w t h e c h e m i c a l shifts a n d c o u p l i n g c o n s t a n t s are r e l a t e d to t h e c o o r d i n a t i o n n u m b e r of t h e p h o s p h o r u s a t o m . * Present address: Department of Chemistry, St. SMvators College, University of St. Andrews, Fife, Scotland. [1] H. S. GuTowsKY, D. W. McCALL, and C. P. SLICHrER, J. Chem. Phys. 21, 279 (1953). [2] I-I. S. GUTOWSKYand D. W. McCALL,J. Chem. Phys. 22, 162 (1954). [3] N. MULLER, P. C. LAUTERBUR, and J. GOLDENSON,J. Am. Chem. Soc. 78, 3557 (1956). [4] J . ]:~. VAN WAZER, C. F. CALLIS, J-. N. SHOOLERY, a n d R. C. JONES, J . A m . Chem. Soc.

78, 5715 (1956). [5] H . FINEGOLD, Ann. N . Y . Aead. Sci. 70, 875 (1958).

[6] C. F. CALLIS,J. R. VAN WAZER, J. N. SHOOLERY,and W. A. ANDERSON,J. Am. Chem. Soc. 79, 2719 (1957). [7] K. MOEDRITZER, L. MAIER, and L. C. D. GROENWEGHE, J. Chem. Eng. Data 7, 307 (1962). [8] R. A. ~ff. JONES and A. R. KATRITZK¥, J. lnorg. Nuclear Chem. 15, 193 (1961). [9] M. L. NIELSEN and J. V. PUSTI~GER, JR., J. Phys. Chem. 68, 152 (1964). [10] J. R. :PARKS,J. Am. Chem. Soc. 79, 757 (1957). I l l ] L. C. D. GROENWEGItE, L. MAIER, and K. MOEDRITZER, J. Phys. Chem. 66, 901 (1962). 1835

1836

JOHN F. N I x o N and REINHARD SCHMUTZLER EXPERIMEI~TAL

Preparations. Synthetic details for nearly all the compounds reported in this study have already appeared in the literature [12], those for the heptafluoropropyl compounds will be published separately elsewhere [12p]. The volatile derivatives were purified by distillation or by fractional condensation in a high-vacuum system. In m a n y cases a small amount of sodium fluoride was placed in the bottom of the NMR tubes to remove possible traces of hydrogen fluoride. pal NMI~ spectra. The phosphorus-31 NMR spectra were obtained at 16.2 Mc/s using a Varian Associates V4300 B nuclear magnetic resonance spectrometer and 12" electromagnet with flux stabilization and a field homogeneity control unit. The chemical shifts reported are in parts per million (ppm) of the applied field, and are relative to 85 per cent phosphoric acid as the standard. Upfield shifts are denoted by a positive sign, downfield shifts by a negative sign. Samples were sealed or stoppered in Pyrex tubes of l0 mm outside diameter. The 85 per cent phosphoric acid was contained in sealed capillaries of ~ 2 mm diameter which were placed inside the 10 mm sample tubes. The calibration of the spectra was carried out using the normal sideband technique of either the standard or the compound. In some cases, in order to obtain more accurate values for the p31 chemical shifts, the phosphorus-fluorine spin-spin coupling constant JP-F, measured from the F 19 NMI% spectrum, was used for calibration. The reported chemical shift values are the average of several determinations. RESULTS AND DlSCUSSlOX The tricoordinate phosphorus-fluorine compounds employed in this study, viz. fluorophosphines, dialkylaminofluorophosphines and fluorophosphites, all exhibit large negative p~1 chemical shifts.

(a)

Dialkylaminofluorophosphines (R2N)nPF~_ ~

1)31 chemical shifts for three dialkylaminodifluorophosphines, R~NPF~, were found between -- 139.0 and -- 144.0 ppm, P - - F coupling constants were ranging [12] a. b. c. d. e. f. g. h. i. j. k. 1. m. n. o. p.

R. SCHMUTZLER, .4dvances in Chemistry Series (American Chemical Society) 87, 150 (1963). L . M . YAGUPOL'SKII and ZH. M. IVA~OVA, Zhur. Obsheh. Khim. 29, 3766 (1959). W . C . S•[TH, J. A m . Chem. Soc. 82, 6176 (1960). I. P. KOMKOV, S. Z. IvIl~, K. ~,~,r. KARAWANOV and L. JE. SMIR~-OV, Zhur. Obshch. Khim. 32, 301 (1962). R. SCHMUTZLER, Chem. and 1rid. 1868 (1962). A . B . BURG and G. BRENDEL, J. A m . Chem. Soc. 80, 3198 (1958). V. N. KULAKOW~, YU. M. ZI~OV'EV and L. Z. SOBOR0VSKI~, Zhur. Obshch. Khim. 29, 3957 (1959). J . F . N l x o N , Chem. and Ind. 1555 (1963). R. SCHMTJTZLER,Inorg. Chem. 3, 410 (1964). It. SCH~UTZLER, Inorg. Chem. 3, 416 (1964). R. SCH=~UTZLER, Inorg. Chem. 3, 421 (1964). R. SCHr~UTZLER, Chem. Ber. 96, 2435 (1963). R. SCHMUTZLER, J. Inorg. Nuclear Chem. 25, 335 (1963). ~V. MAHLER, Inorg. Chem. 2, 230 (1963). R. SCH~TZLER, to be published. J . F . N I x o ~ , J. Chem. Soc., in press.

Phosphorus-31 nuclear magnetic resonance studies of phosphorus-fluorine compounds

1837

b e t w e e n 1194 t o 1197 c/s. T h e s p e c t r a c o n s i s t e d o f basic 1 - 2 - 1 triplets [13b]. F o r [(CH3)~N]~PF, a 1-1 d o u b l e t w a s o b s e r v e d ; c h e m . shift - - 1 5 0 . 8 p p m ; J P - F =1042 c/s. (b) Fluorophosphites (I~O),PF3_ . A n u m b e r o f a l i p h a t i c a n d a r o m a t i c d i f l u o r o p h o s p h i t e s , R O P F 2, a n d bisd i f l u o r o p h o s p h i t e s , F 2 P O - - R ' - - O P F ~, s h o w e d p31 c h e m i c a l shifts in t h e n a r r o w r a n g e - - 1 0 9 . 8 to - - 1 1 2 . 0 p p m , t h e P - - - F c o u p l i n g c o n s t a n t s v a r i e d b e t w e e n 1278 t o 1328 e/s. [13b]. T h e p31 c h e m i c a l shift w a s a t l o w e r field in t h e h e t e r o c y c l i c c o m p o u n d s , 1 . 3 . 2 - d i o x a - 2 - f l u o r o p h o s p h o l e , C ~ H 4 0 2 P F ( - - 1 2 3 . 8 p p m ) , a n d 1.3.2dioxa-4.5-benzo-2-fluorophosphole, C6H402PF (--123.1 ppm). The P--F coupling Table 1. Tricoordinate phosphorus-fluorine compounds (c) Fluorophosphines, RnPFa_ n

(~(ppm)

,lp_F (e/s)

Remarks

CICH2PF2

Compound

--201-8

1203

CaF~PF~

--167.8

1250

CFaPF2

-- 158.3

1245

(CaF~)~]?F CClaPF,., (CFa)2PF [14]

-- 138.8 -- 130-9 -- 123.9

1020 1290 1013

--97"0

1410

1-2 1 triplet; P--H coupling apparent 1-2-1 triplet, each line split into broad 1-2-1 triplet; JP-CFa ~ 90'5 e/s 1-2-1 triplet, each line split into a 1-3-3-1 quartet; JP-CFS ~ 87'2 c/s very broad 1-1 doublet 1-2-1 triplet 1-1 doublet, each line split into a septet; JP-eF3 ~ 89.6 c/s 1-3 3-1 quartet

PF a [2]

c o n s t a n t s were 1226 a n d 1307 c/s, r e s p e c t i v e l y [13b]. T h e r e p o r t e d m u ] t i p l c t s t r u c t u r e s arise f r o m s p i n - s p i n c o u p l i n g b e t w e e n p h o s p h o r u s a n d fluorine nuclei. T h e c o u p l i n g c o n s t a n t s o n t h e w h o l e t e n d t o be l a r g e r t h a n t h o s e o b s e r v e d in c o m p o u n d s w h e r e p h o s p h o r u s h a s a h i g h e r c o o r d i n a t i o n n u m b e r . I t is n o t e w o r t h y t h a t t h e p h o s p h o r u s n u c l e u s is s o m e w h a t m o r e shielded in t h e f l u o r o p h o s p h i t e s t h a n in t h e o t h e r t r i c o o r d i n a t e species, a n d t h e c h e m i c a l shifts o f t h e difluorop h o s p h i t e s are r e m a r k a b l y c o n s t a n t . O n l y a l i m i t e d n u m b e r o f well e s t a b l i s h e d f l u o r o p h o s p h i n e s h a v e b e e n r e p o r t e d in t h e l i t e r a t u r e [] 2 e - h i , all o f w h i c h h a v e a n e l e c t r o n e g a t i v e s u b s t i t u e n t a t t a c h e d t o p h o s p h o r u s (e.g. p e r h a l o a l k y l ) . I t h a s b e e n f o u n d t h a t f l u o r i n a t i o n o f a l k y l or a r y l c h l o r o p h o s p h i n e s a l w a y s leads t o f l u o r o p h o s p h o r a n e s , RnPFs_~, w h i c h c o n t a i n p e n t a v a l e n t p h o s p h o r u s [12b,d,e,i]. T h e c o m p o u n d s p r e v i o u s l y b e l i e v e d to be

C~HsPF 2 and C~HsPCIF have recently been shown to be in fact C~HsPF 4 and C2HsPOF2, respectively [13a], and their reported pal chemical shift values do fall well within the ranges observed for other related members of their class (vide infra). The nonavailability of alky] or aryl fluorophosphines precludes a direct comparison of their chemical shifts with those of the perhaloalkyl derivatives, but it can he [13] a. R. SCHMUTZLER,J. Chem. Soc., in press. b. G. S. ]=~EDDYand R. SCHMUTZLER,tO be published.

1838

JOHN F. NIxoN and I:~EINHARDSCHI~IUTZLER

seen t h a t chloromethyldifluorophosphine which is the nearest approach to an alkyldifluorophosphine has the lowest field shift of all the compounds studied, and that replacement of both hydrogen atoms by chlorine does in fact lead to a shift to high field of about 70 ppm. This trend is borne out in other systems described in this paper where a direct comparison can be made. Phosphorus trifiuoride is included for comparative purposes, and clearly has an anomalously high pal chemical shift, compared with the other fluorophosphines. An empirical equation for the chemical shift 5p in PX a molecules has been described [3, 10], which is based on the observation by SAIKA and SLICHTER t h a t fluorine chemical shifts and chemical shifts of nuclei having other than s electrons associated with them are determined largely by the paramagnetic shielding term. In order to find the number of "unbalanced p-electrons" in the trivalent phosphorus halides, both the ionic character of the P - - X bond and an accurate knowledge of the orbital hybridization of the phosphorus atom are required. The bond angles for the fluorophosphines listed in Table 1 are as yet unknown, but the variation of chemical shifts within the R P F 2 series (~-- 70 ppm) suggests t h a t other factors may be important. In the dialkylaminofluorophosphines and fluorophosphites there is an additional factor to consider, namely the extent of supplementary p,--d~ bonding arising from the interaction of the lone electron pairs on nitrogen or oxygen with vacant orbitals on the phosphorus atom. This effect will be enhanced by the electronegative fluorine atoms which will lower the energy of the 3d orbitals, and will thus reduce the basic character of nitrogen or oxygen and increase the electron density around the phosphorus atom. Supporting evidence is provided by infrared [12j] and X-ray crystallographic studies [15] on adducts of dialkylamino-difluorophosphines with transition metals, which confirm that coordination occurs via phosphorus and not via nitrogen. The p31 chemical shifts of the tetracoordinate phosphorus-fluorine compounds shown in Table 2 occur at much higher fields than those of the tricoordinate compounds. The values for the alkylphosphonic difluorides, RPOF2, which are more positive than for the phosphinic fluorides, R~POF, are clustered in the narrow range from --27 to --29 ppm, and show little variation in their P - - F coupling constants. The chloromethyl and phenylphosphonic difluorides have higher field shifts, as do the dialkylamino- and alkoxy derivatives. The observed order of decreasing chemical shift, POF3 > ROPOF~ > RPOF 2 parallels the order observed for the tricoordinate series, PFa > R O P F 2 > R P F 2. The alkyl and aryl fluorophosphoranes, RnPFs_n, all have positive or slightly negative chemical shifts, and the phosphorus-fluorine coupling constants are less than 1000 c/s. The rather low chemical shift of the heterocyclic C4HsPF a may be a consequence of the phosphorus being part of a strained ring system. Successive replacement of a methyl group by phenyl in the R2PF a system leads to a steady increase in the shielding of phosphorus : (C~Hs)~PF a > (C~Hs)CH3PFa > (CHa)2PF a, and the same trend occurs in the RPF4 system. I t is interesting to note t h a t the [14] K. J. PACKER,J. Chem. Soc. 960 (1963). [15] B. GREENBERG,A. AMENDOLA,and R. SCHMUTZLER,Naturwissenschaften 50, 593 (1963).

Phosphorus-31 nuclear magnetic resonance studies of phosphorus-fluorine compounds

1839

T a b l e 2. Tetracoordinate phosphorus-fluorine c o m p o u n d s

Compound

($p (ppm)

Remarks

JP F (c/s)

CH3POF a [5] C2HsPOF 2 n C4HgPOF~ i-CsHHPOF 2 CICH2POF 2 C~IIaPOF e P O F a [7] POF~C1 [2] POFC12 [2] (CHa)2POF (C6Hs)2POF

- - 27.4

--29.2 --29.3 -- 27.2 -- 12.0 -- 11.4 @ 35"5 .-z 15.0 0.0 --67.3 --40.5

-1130 ll40 1126 1142 1 115 1080 1138 1180 990 1020

-1-2.-1 triplet 1-2-1 triplet 1-2-1 triplet 1 2-1 triplet 1 -2-1 triplet 1-3-3-1 quartet 1-2-1 triplet 1 1 doublet 1-1 doublet 1-1 doublet

C2H~P. II\CI-I(CHa)z O

--30"7 --30.4

1040 1050

1-1 doublets,

observed on two independent preparations

[5 ] / /F CHAP\ li \OCH(CI-[3)2 O

1 1 doublet

- 16. I /F

[C6HsNH3] \ IC~H5 * p/

~

--9.1

(C~Ha)2NPOF 2 (CeI-IsO)POF ~ (C~HsO)POF ~

@ 3.6 @20.9 @27.1

955

1-1 doublet

(compound dissolved in dimethylsulfoxid e) 1-2-1 triplet 1-2-1 triplet 1-2-1 triplet

1004 1015 1030

chemical shift for PF 5 is not particularly high, and in fact lies between the values observed for CH3PF a and C6HsPF 4. Once again the perhaloalkyl derivatives have much more positive chemical shifts than their alkyl counterparts, and there is also a marked increase in their P - - F coupling constants, viz. CF3PF 4 Jp_v~ = 1103 c/s t CH3PF 4 Jp_~ 965 c/s J

(CFa)2PF3 JP-F = 1260 C/S ] (CH3)2PF3 JP--F (eq.)= 975 c/s JP-F

(CF3)3PF~ J P - F

=

(ax.)--

7 8 7 c/s

9 8 8 C/S 1

(n-C4Hg)3PF2 JP-F = 553 C/S J The pattern of two overlapping 1-2-1 triplets, at room temperature, is shown by all the I~2PF a systems studied, except cyclotetramethylenetrifluorophosphorane, (CH2)4PF 3, and (CFa)2PF 3, and indicates that two of the fluorine atoms are in an environment different from the third. This confirms the result of a previous F la NMR study [17] which established that the R groups in fluorophosphoranes, R~PF5_., occupy equatorial positions of a trigonal bipyramid, and that the apparent equivalence of fluorine atoms in (CH2)4PF a arises from an intramolecular exchange process. The same type of exchange would also account for the apparent equivalence of fluorine atoms observed in all the R P F 4 systems investigated. [16] E . L . MUE~YrERTIES a n d W . D . PHILLIPS, J. Am. Chem. Soc. 81, 1084 (1959). [17] E . L . MUETTERTIES, %V. MAHLER, a n d R . SCHMUTZLER, Inorg. Chem. 2, 613 (1963).

J o ~ N F. N I x o N and ]:~EINIIARD SCHMUTZLER

1840

Table 3. Pentacoordinate phosphorus-fluorine compounds (a)

Fluoropho~phoranes, R n P F s _ ~

Compound

61" (ppm)

(CI-I3)~PF a

--8.0

(C2Hs)2pF 3

--6'3

CH z

JP-F (e/s) 975 787 975 827

(equatorial) (axial) (equ.) (ax.)

Remarks two overlapping 1-2-1 t r i p l e t s two overlapping 1-2-1 t r i p l e t s

~pF a C6I-I5/

-~- 13-4

955 (equ.) 813 (ax.)

two overlapping 1 2-1 t r i p l e t s

(C6Hs)2PF 3

-~ 34'8

968 (eq.) 834 (ax.)

two overlapping 1-2 1 triplets

--29.8

915

1-3-3-1 q u a r t e t

CH3PF a

-~- 29.9

965

C1CI-IupF 4

-~- 43" 7

1008

C~H~PF a

Jr 30.2

995

C6HsPF 4 CI-IaC6H4pF 4 (n.C4Hg)sPF~

-}- 51.7 ~- 49.7 ~ 15.4

973 950 553

P F 5 [7]

+ 35-1

1010 916

1-4-6 4-1 q u i n t e t ; each c o m p o n e n t split further into 1-3-3 1 quartet; JP-H == 19 c/s 1-4-6-4-1 quintet ( P - - H coupling apparent) 1-4-6-4-1 quintet ( P - - H coupling apparent) 1-4-6-4-1 quintet 1 4-6-4-1 quintet 1 2-1 t r i p l e t (broadened b y P - - H coupling) sextet (value from F 19 N M R s p e c t r u m [16])

pF a

(b) Dihalodifluorophosphoranes, R P F 2 X 2 CClaPF~C1 ~ -~- 7.4 CClaPF2Br 2 -~ 27" 0 CaF7PF2C1 ~ ~- 14.2

1106 1109 1095

(c) Perhaloalkyl Fluorophosphoranes, (Rx)nPF5_n CFaPF 4 -~ 66.4 1103

CClaPF 4 (CFa)~pF s

-~-66-9 ~-50.9

1120 1260

(CFs)apF 2

-~ 59' 8

988

1-2-1 t r i p l e t 1 - 2 - 1 triplet 1-2-1 triplet; further split into 1 2-1 triplet; J r cFa = 129 c/s 1-4-6 4-1 quintet, each c o m p o n e n t further split into 1 - 3 - 3 - 1 q u a r t e t ; J P - c r a = 172 c/s 1-4-6-4-1 quintet 1-3-3-1 q u a r t e t , further split into septets; JP-CFa -- 174 c/s 1-2-1 triplet, further split into overlapping deeets (not c o m p l e t e l y resolved); J p _ c F a ~ 1 6 7 c/s

The hexacoordinate phosphorus-fluorine compounds show the largest 1)31 chemical shifts to high field, and the value observed for the compound [p-C1CsH4N2] + 1)F 6- ( + 143-7 ppm) agrees more closely with the higher value ( + 148 ppm [1]) of the two previous reports for H P F 6 mentioned in the literature [1, 2]. Removal of the perfect octahedral symmetry of the 1)F 6- anion by replacement of one fluorine atom by a phenyl group causes a downfield shift of ~ 8 ppm, and the

Phosphorus-31 nuclear magnetic resonance studies of phosphorus-fluorine compounds

1841

Table 4. H e x a c o o r d i n a t e phosphorus-fluorine c o m p o u n d s Compound H P F 6 [1, 2] [p-C1C6H~N2]+PF6(in acetonitrile solution) [C6HsPF[N(CHa)~]2] + / * [CsHsPFs]t (in acetonitrile solution)

6p (ppm)

J?-F (c/s)

Remarks

÷ 148 [1] ÷ 118 [2] ÷ 143-7

708

septet

707

÷ 136-0

septet

J a = 697~ J , = 816

two overlapping 1 - 4 - 6 - 4 - 1 quintets

* The cation p31 s p e c t r u m consisted of a 1-1 doublet; Jp_r = 1037 c/s; 6 = - 5 6 p p m . t See text.

pa~ NMR spectrum shows two partially overlapping quintets with the recorded coupling constants JP-F,~ and JP-Fe, consistent with the structure shown in Fig. 1 : -(-)

Fa

Fg

Fe

/ _/

//

Fe

C~H5 CONCLUSIONS

(1) It can be seen from the data presented in the Tables that the phosphorus chemical shift values in phosphorus-fluorine compounds extend over a range of about 350 ppm, and are progressing towards high field as the coordination number of phosphorus increases, reaching a maximum at the spherically symmetrical hexafluorophosphate anion, RnPF3_ n ~. (R2N)nPF3_ n

<

(RO)nPF3_~ ~ RnPOF~_n

<

R2PF a

<

RPF 4

<

RaPF2 RPF~X2 <

(Rx)~PFs_~

<

P F 6-

(2) The phosphorus-fluorine spin-spin coupling constants vary in magnitude b y almost a factor of three and no clear relationship between JP-F and the coordination number of phosphorus is apparent. There is a general decrease in JP-F in going from left to right in the above series, but the lowest value is observed in RaPF2, and the perhaloalkyl derivatives always have much larger coupling constants than the other members of the respective class. (3) Alkyl derivatives of any particular class of compounds always have very similar chemical shifts, but substitution of a phenyl or polyhaloalkyl group leads to a high field shift, the effect being particularly marked for the latter.

1842

JOHN F. NIXON and REINHARD SCHMUTZLER

(4) T h e p31 NMI~ s p e c t r a of I~3PF 2 a n d R 2 P F a s y s t e m s confirm the results p r e v i o u s l y o b t a i n e d f r o m an F 19 N M R s t u d y , t h a t the l~ g r o u p s o c c u p y equatorial sites in a trigonal b i p y r a m i d , while in R P F 4 s y s t e m s there is an a p p a r e n t equivalence of t h e fluorine a t o m s , which p r o b a b l y arises f r o m an i n t r a m o l e c u l a r e x c h a n g e process. T h e chemical shift trends agree fairly well with the theoretical c o n c e p t t h a t t h e shielding increases w i t h the electrostatic p o t e n t i a l a t the nucleus which is reduced b y a second order p a r a m a g n e t i c t e r m arising f r o m the lack of spherical s y m m e t r y i n t r o d u c e d b y the bonding electrons. I t is clear, however, t h a t a fuller knowledge of b o n d angles a n d of t h e e x t e n t of 7r b o n d i n g is needed before an e x a c t i n t e r p r e t a t i o n of t h e d a t a can be expected.

Acknowledgements--The authors wish to thank Dr. N. SKEPrAnD for helpful discussions, and Mr. E. LIDDELL for recording the spectra. We are indebted to I.C.I. for the award of a fellowship (to J.F.N.), and to Deutscher Akademischer Austausehdienst for awarding a N.A.T.O. Fellowship (to R.S.).