The gas-phase structure of PF2OCH3, determined by combined analysis of electron diffraction and rotational data

Journal of Molecular Structure, 238 (1990) 213-287 Elsevler Science Publishers B V , Amsterdam

273

THE GAS-PHASE STRUCTURE OF PF20CH3, DETERMINED BY COMBINED ANALYSIS OF ELECTRON DIFFRACTION AND ROTATIONAL DATA

MARTIN J DAVIS, DAVID W H RANKIN and STEPHEN

CRADOCK

Department of Chemastry, UnwersLty of Edmburgh, West Mauzs Road, Edinburgh, EH9 355 (Gt BrLtaLn) (Recewed 2 January 1990 )

ABSTRACT Electron diffraction data for PF,0CH3 have been collected and used m combmatlon with exlstmg rotation constants to determine the gas-phase structure of this compound For this purpose a normal coordinate analysis has also been performed The pnnclpal parameters ( ra.,,)found are r(P-F) 159 53(38), r(P-0) 157 42(38), r(C-0) 14460(20) pm, and LFPF 94 82(10), LFPO 10159 (13) and L COP 123 91(14) ’ The structural parameters for the PF,O- group are compared with those found previously for many other compounds contammg the group PF,Y-, and correlations between the P-F and P-Y bond lengths m compounds where Y IS oxygen, nitrogen, sulphur and carbon are studied The correlation between P-F bond length and FPF angle 1s also investigated

I

INTRODUCTION

The structures of several PF20- compounds have been determined by electron dlffractlon [l-4] In these compounds the P-F bonds are shorter than the P-O bonds, as expected from the smaller size of the fluorme atom. However, we recently determined the structures of two compounds, ( PF2)0CH2C=CCH20 (PF,) and PF20CH2CH,CN, m which the P-F bonds appeared to be longer than the P-O bonds [5] The structure of PF20CH3, determined from microwave spectroscopy data [ 61, shows slmllarly long P-F and short P-O bonds Structures of PF20- compounds determined from electron dlffractlon data suffer from strong correlations between the P-F and P-O bond lengths, since these are of similar magnitude This results m large random errors for these distances, and possibly also slgmfkant systematic errors Structures obtained from microwave spectroscopic data are also somewhat unsatisfactory since no lsotoplc substltutlon can be made for phosphorus or fluorine Combined analyses of electron lffractlon and rotational data would give much more accurate

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214

and precise structures than could either techmque alone, and m particular the correlations between the P-O and P-F bond lengths should be reduced Therefore we have collected electron diffraction data for PF,OCH, and used this, with the rotational data, m a combined structural analysis. EXPERIMENTAL

Preparation of a pure sample of PFzOCH3 by reaction of S ( PF2)z with methanol was not possible since, due to their similar volatihties, the product and PFPHS could not be fully separated. Therefore the reaction of PBrF2 with methanol, in the presence of a base, was used since here the unwanted product, HBr, could be removed as an mvolatile salt. CH, OH + PBrF, + Base-

PF, OCH, + BaseH+Br-

The base used was 2,6-dimethylpyridme because it is mvolatile and thus any excess should be easily separated from the product However, IR and NMR spectroscopy showed ca. 10% methyl bromide m the desired product and this could not be removed. Since this could be allowed for m the structure determmation, this sample was used. The electron diffraction data were collected at room temperature on Kodak electron image plates using the Edinburgh diffraction apparatus [ 71 Two plates were recorded at each camera distance using an accelerating potential of ca. 45 kV. The plates were traced using a Joyce-Loebl microdensitometer [ 81 at SERC Daresbury Laboratory, and the data were transferred to a computer on which the rest of the analysis was performed using standard data-reduction [ 81 and least-squares refinement [ 91 programs and scattering factors [lo]. Relevant experimental mformation, together with weighting functions, correlation parameters and scale factors, are given m Table 1 Refinement of the ra structure The model used assumed local symmetries of C, for the PFzO group and Csv for the methyl group The methyl bromide impurity was compensated for by a set of fixed distances [ 111 with multiphcities modified by a parameter TABLE 1 Welghtmg functions, correlation parameters and scale factors for PF,OCH, Camera height

As (nm-‘)

S,,” (nm-‘)

SW1 (nm-‘)

SW2 (nm-‘)

%I, (nm-‘)

Correlation parameter

Scale factor

Wavelength

4 2

60 20

80 40

300 122

352 144

0 4736 0 4983

0679(12) 0756(11)

5667 5667

(pm)

(mm) 1282 286 0

275

(xCH,Br) allowing the proportion of impurity to be varied. The structure can be defined by 11 parameters, the distances P-F, P-O, C-O and C-H, the angles FPF, FPO, COP, OCH and Me-tilt and the P-O and O-C torsion angles The parameter xCH,Br is the number of moles of methyl bromide per mole of PF,OCH, The P-O torsion angle is defined as zero when the FPF angle blsector is syn with respect to the O-C bond and for the O-C torsion angle when one hydrogen atom ISantz with respect to the P-O bond. Both angles are positive for anticlockwise rotations of the nearer group when viewed along the appropriate bond The Me-tilt angle allowed the methyl group to tilt away from (positive) or towards (negative) the P-O bond only The largest peak in the radial distribution curve (Fig. 1) contains the P-F, P-O and C-O distances, while to the left of this the C-H peak can be dutingmshed. The other mam peak contains F* * *F and F* *-0 distances at ca 240 pm with, to the right of these, the contributions from the P***C and F***C distances clearly visible. The small peak at ca. 200 pm is partly due to 0. - *H distances but probably also contams the C-Br peak of the CH3Br impurity Initial values for the parameters were obtained by reference to similar compounds, particularly (PF,)OCH,C=CCH,O (PFz) and PF,OCH,CH,CN [ 51, and the structure determined from microwave data [ 61 Both P-O torsion and O-C torsion angles were mitially set at zero First, the prmcipal bond lengths and angles, P-F, P-O, C-O, FPF, FPO and COP were refined Then more accurate starting values for the P-O and O-C torsion angles were determined by performing a series of refinements in which the values of the two torsion angles were varied over their entire possible ranges The mmimum value of RG was 0 084, obtained with a zero P-O torsion angle and an O-C torsion angle of 15”. The value of RG rose to 0.105 as the O-C torsion angle was increased to 60” and to 0 143 as the P-O torsion angle was raised to 45”. The two torsion

c

1

il

z

I

I 200

100

I 300 rmll)

I 400

1 500

”

Fig 1 Observed and final difference radml-dlstnbutlon curves, P(r)/r, for PF20CH, Fourier mverslon, the data were multlphed by s.exp( -0 00002s2)/(Zp-fp) (Z,-fF)

Before

276

angles were then refined from the values which gave the mmlmum RG value For both torsion angles this resulted m large uncertamtles, greater than the values themselves, so both were fixed at zero The C-H length could not be satisfactorily refined, as the value increased to more than 115 pm and with a large uncertainty. A series of refinements for varying values of xCH,Br showed a mmlmum RG of 0.080 for an rCHsBr value of 0.085, rising evenly at lesser and greater values to 0.082 with the parameter at zero The amount of impurity was thus set at 0.085. Since the P-F and P-O bonds were of very similar length, the parameters were changed to a mean value and the difference P-O minus P-F The prmclpal amplitudes of vibration were then refined, though some had to be constrained m groups. Attempts to refine u (Pm **C) resulted m this amplitude of vlbratlon and u (F* - *F ) , becoming too small (less than 5 pm ) The R-factor at this stage was 0.079 Since further analysis of the structure of PF,OCH, has been performed, full tables for the r, structure are not given, but the parameter values are included m Table 4 for comparison with the raVstructure TABLE 2 Defmitlon of symmetry co-ordmates for PF,OCH,

*t

TlS

A’ S1= W&‘) [rz+r31 S,=r,

s,= WJ2) &=ff34

[%+%I

s,=(l/J2)(y,,-1/2[cu,,+a,,l S,=r,

s10= %3 sII=%

A”

P-O

[%+%I

bond

S,=(l/,/B)[r,-r,] Sz= u/J21 [%-%I s3= WJ2) [%-%I Sq= (1/J2)lr6-r71

&= u/J2)[%-%1 Se= ‘518

ST= (1/J2)Ir6+r,l Ss=r, &I= WJ2)

= Twist about

Force fteld calculatwm An harmomc force field was calculated for PFzOCHB using the computer program GAMP [ 121. The atomic coordmates used initially were those from the r, structure determined above. However, the calculations were later repeated usmg the more accurate coordmates obtained near the end of the refinement of the r,, structure. The molecule was assumed to have C, symmetry and thus the symmetry coordinates (Table 2) were of either A’ or A” symmetry species. The mitral force constants used were general values [ 131 appropriate to the type of symmetry coordinate The observed vlbratlonal frequencies and assignments were those of Durlg and Streusand [ 141 and frequencies for both the normal and deuterlated species were used m the calculations Once a force field which reproduced the observed frequencies with chemltally reahstlc force constants (Table 3) had been obtained this was used to calculate vibrational amplitudes (u) and perpenlcular amplitude correctron coefficients (K) for each mteratomlc &stance. Vlbratlonal correction terms were also calculated for each rotation constant of the normal, deutenated, 13C and “0 lsotoplc species These corrections were subtracted from the literature B, values [ 61 to grve the B, values used m the r,, structure refinement Unfortunately it 1s not possible to calculate e s d.s for the vibrational correction terms since the force field 1s under-determined and therefore not unique. TABLE

3

Calculated force field for PF,OCH,

A’

477 5 18

4633

0 0

0

107 0

0

0

0

37 5 0

0

0

0

0 0 0

47 4

0 0

0 0

52 8 0 0 0 0

89 8 0 0 0

0 0 0 0

A”

(values m N m-l)

-572 62 6 0

79 2 0 -517

90 2 0

440 8

29 0 44 8

267

0

262

0

0 0

0

0

-280

0 0

0 0

0 0

0

4815 0 0 0 0 0

410 0 0 0

126 4 0

54

459 1 138 4

609 0 0 -426 0

245 6 0

249 9

0

0

69 5

278

However, the probable errors m these correctrons are hkely to be srgmficant and therefore it 1s important to make some allowance for them The followmg procedure was used to obtain an estimate of these uncertamtres One of the elements of the force field was changed to a slightly different value, the force field refined back to a best fit and the values of the vlbrational corrections noted This was then repeated for other elements of the original force field, glvmg a set of values for each vibrational correction from which the standard deviations were then calculated. To allow for the simultaneous variatron of all elements of the force field these standard deviations were multiplied by the square root of the number of elements, J87, to grve the values taken as the e.s.d s of the vibrational correction terms. Refinement of the r,, structure All the amplitudes of vibration were changed to the calculated values, although those whrch had been refinmg were still allowed to do so The perpendrcular amphtude correction coefficients (K) and the small corrections for bonded distances dependent on anharmomcltles and temperature varlatlon of amplitudes were entered allowmg the structure to be refined usmg r”, paramTABLE 4 Molecular parameters for PF,OCH, (distance m pm, angles m degrees) r a”

structure

Pl P2 P3

P4 P5 ~6 P7 ~8 P9 PI0 Pll P12 P13

Mean PF, PO APO-PF c-o C-H L FPF L FPO L COP LOCH P-O twist 0-ctwwt

Me-tilt xCH,Br Sh CD r(P-F)” r(P-0) r(O-C) r(C-H)

158 47(l) -2 17(37) 144 60(20) 108 85(30) 94 82(10) 10159(13) 123 91(14) 108 49(13) 0 O(flxed) 0 O(flxed) -0 2 (fixed) 0 070 (fixed) 0 4(flxed) 159 4(l)

157 Z(3) 145 7(2) 109 6(3)

ra

Microwave” structure

1585(l) 31(21) 1414(8) 110O(flxed)

158 lb -3 lb 144 6(5) 109 O(10) 94 8(6) 102 2(10) 123 l(5) 108 4(10)

structure

95 l(6) 100 2(3) 122 4(6) 110 O(fixed) 0 0 (fixed) 0 O(flxed) 0 O(flxed) 0 085 (fixed)

157 5(12) 1606(17) 1414(8) 110 O(flxed)

“Mixed rJro structure, see ref 6 bCalculated from r(P-F) and r(P-0) are r, distances

159 l(6)

1560(20) 1446(5) 109O(10)

“For r,, and ra structures

279

TABLE 5 Interatomic distances (r, m pm), amplitudes (u m pm) and perpendicular vlbratlon correction coefficients (Km pm) for PF,0CH3

d 1 d 2 d 3 d 4 d5 d 6 d 7 d 8 d 9 d10 dll d12 d13 d14 d15 d16 d17 d18 d19 d20 d21 d22 d23 d24

(P-F) (P-O) (O-C) (C-H) (P-C) (F ..F) (F e-0) (0 * H) (0 ..H) (H .H) (P ..H) (P H) (P.. H) (F .*C) (F .C) (Fe- H) (F .H) (F H) (F ..H) (F H) (F H) (Br-C) (C-H) (Br H)

Distance

u(exp )

u(calc )

K

159 4 (1) 157 2 (3) 145 7 (2) 109 6 (3) 266 3 (2) 234 7 (0) 245 1 (1) 208 3 (2) 208 1 (2) 179 4 (6) 355 l(2) 292 0 (2) 292 0 (2) 294 1 (1) 294 1 (1) 392 6 (3) 257 0 (2) 329 4 (2) 392 6 (3) 329 4 (2) 257 0 (2) 193 1 108 1 248 3

40 (2) 37(tledtoul) 4 3 (11) (fixed) (fixed) 62 (3) 66 (tledtou6) (fixed) (fixed) (fixed) (fixed) (fixed) (fixed) 133 (7) 13 3 (tied to ~14) (fixed) (fixed) (fixed) (fixed) (fixed) (fixed) 55 70 110

4 42 4 18 5 02 7 89 7 45 6 54 6 96 10 29 10 38 12 70 10 35 14 95 1495 15 11 15 11 1646 24 15 17 59 16 46 17 59 24 15

0 33 0 29 126 135 0 28 0 52 0 27 1 83 2 01 153 0 61 0 85 0 85 0 21 021 0 57 0 87 0 70 0 57 0 70 0 87

CH3Br impurity”

“All values fixed

eters The corrected observed rotation constants (B,) for the normal isotopic species were then introduced, nntially with large uncertainties (and thus low weightmgs) which were then gradually reduced towards the estimated values over several refinements The rotation constants for the deuteriated species were then also included m the refinement, again with large uncertamties at first At this point an extra parameter Sh CD was also added to allow for the slight shortenmg of the C-H bond upon deuterlation and was set at 0 4 pm. As the uncertamties for the PF,0CD3 rotation constants were reduced the refinement started to diverge instead of convergmg Small changes m some of the fixed structural parameters were made C-H was changed to 109 pm, the OCH angle to 109” and the methyl tilt to - lo Now mtroduction of the rotation constants for PF20CD3 resulted m a converging refinement. In addition the OCH angle could now be refined

280 TABLE 6 Observed and calculated rotation constants for PF,OCH, Vlb corr

B,

Calc

Dlff

Uncertainty

C

364182 3123 46

I 05 5 26 2 39

5973 08 3636 56 312107

5970 68 3636 IO 312123

2 40 -0 14 -0 16

2 03 0 11 0 08

PF,OCD,

A B C

5715 34 3252 64 2843 72

6 09 4 34 195

5709 25 3248 30 284171

5715 02 3248 35 284169

-517 -005 0 08

2 03 0 11 008

PF 2“OCH 3

A B

592479

6 63 4 96 2 38

5918 16 356145 3080 18

5917 02 356150 3080 03

1 14 -005 0 15

2 03 0 11 0 08

7 07 5 12 2 32

5955 03 3537 56 3052 55

5952 90 3537 34 3052 61

2 13 0 22 -006

2 03 0 11 0 08

PF,OCH,

A B

C PF 2O=CH 3

A B C

598013

3566 41 3082 56

596210 3542 68 3054 81

Fig 2 Observed and fmal weighted difference combined molecular-scattenng mtensltles for PF,OCH,

The rotation constants for the “0 and 13C lsotoplcally substituted species were now also introduced into the refinement and the C-H length was refined Attempts to refine the other geometrical parameters resulted m either unreallstlc values or very large uncertainties for the parameters At this stage the improved atomic coordmates were used to calculate a new force field and the resulting amplitudes of vlbratlon, K-values and vlbratlonal corrections were apphed to the refining structure Isotopic substltutlon correc-

281

Fig 3 The gas-phase

structure

of PF,OCH,

TABLE 7 Least squares correlation have been removed P3

p5

-72 86 72

p6 -84 -75

( X 100) for PF,OCH,,

matrix

p7

p8

u3

-89 -87

70

with absolute values ~50

kl

-58 -84

52

all elements

67

Pl P2 P3 P4 P5 u1

tlons (Sr) for all the bonded distances m the lsotoplcally substltuted species were also mtroduced mto the calculations of the rotation constants from the molecular model The values of these corrections were estimated using the equation [ 151 &= (3a/2)6(u2) -SK, where 6 m&cates (substltuted specles - normal species) and a 1sthe anharmomclty constant. The methyl tilt was refined but this gave a value of ca -6”, which was consldered unreasonable. A series of refinements with different values of the methyl tilt m the range - 5 to + 5’ showed no best-fit to the electron dlffractlon data and did not give a mmlmum R-factor However, the rotation constants were predicted most accurately with a methyl tilt of - 0 2’) so the parameter was fixed at this value A series of refinements with a range of xCH,Br values gave a best-fit to the ED data with a value of 0.07 and thus the parameter was fixed at this value When the Sh CD parameter was refined it became very large (about 2 pm) and refinements over a range of varymg values showed no mmlmum value of RG It 1sunlikely that the difference between the C-H and C-D bond lengths IS this large so the parameter was left fixed at 0 4 pm No attempts were made to find best fit values for the P-O and C-O torsion angles

since C, symmetry had been assumed for the force field calculations However, a model with a C-O torsion angle of 60” also has C, symmetry, but the value of RG with such a model was 0.0996, which is sqgnificantly worse for a 99% confidence interval The final value of RG obtained was 0 089. The final parameters are given m Table 4, the list of mteratomlc distances m Table 5 and the calculated rotation constants m Table 6. The experimental and final difference molecular scattermg curves are shown m Fig. 2 and views of the molecular structure are shown m Fig 3 The correlation matrix is given m Table 7 DISCUSSION

The structure obtained using the electron diffraction data alone gave a reasonable fit, the R-factor being 7 5% However, the bond lengths are poorly determined, particularly for the P-O and P-F lengths, due to the correlation of similar distances The value obtained for the P-O length is greater than that for P-F, but the e s d on the difference is sufficiently large that this difference is not significant Including the rotation constants m the refinement resulted m the P-F bond length being determined as longer than the P-O length. Moreover, the dlfference between these distances was more precisely determmed than previously and is now sqquficant. The C-O bond length is sigmficantly longer m the r,, structure than in the ra structure The R-factors for the two structures are not comparable since there are more data in the second case, but the better fit of the second structure is apparent in the uncertamtles for the geometrical parameters Furthermore, the r,, structure has two extra parameters refinmg the distance C-H and the OCH angle The uncertainties m the refining amplitudes are generally only slightly lower in the r,, structure than they were m the r, structure and u (O-C ) is shghtly larger. The values obtained for the independent distances and angles are similar to those deduced from the microwave data alone (Table 4), but the e s.d.s. are much smaller, The P-F bond length and FPF and FPO angles found m PF,OCH, are similar to those found earher for (PF2)0CH2C=CCH20 ( PF2) and PFzOCHzCHzCN [5] but the P-O bond is ca 2 3 pm longer. The C-O bond length is greater than those of 141.6 (3) and 142.8(3) pm found m domethyl ether and methanol, respectively [ 161. An increase m length might be expected due to the electronegative -PF2 group, but this 1s not observed m ( PF2 ) OCH&=CCH20 ( PF2 ) or PF,OCH&H,CN where the C-O lengths are shorter than m dlmethyl ether The COP angle found here is larger than those of 118.3(11) and 121.9(8)’ m ( PF2) OCH,C=CCH,O ( PFP) and PF,OCH,CH,CN respectively. The conformation adopted by PF,OCH, results m two short F- -*H dis-

283

tances of 257 pm. Such short F .**H contacts have previously been observed [ 2,4,17-221 m many other -PF2 compounds. Comparrson of the structures of PF2Y- groups The structure of the PF20- group found m PF,OCH, 1scompared wrth those found m other compounds contaunng the general group PF2Y- m Table 8 The P-F bond length is longer than m most other compounds whrle, m contrast, the P-O length 1s shorter than for most of the other PF,O- compounds. Plots of P-F agamst P-Y for those compounds with Y = 0, N, C and S (Frg 4) show a correlation of mcreasmg P-F bond length with decreasing P-Y bond length for the oxygen, sulphur and some of the nitrogen compounds We should consrder the posslblhty that the observed relatlonshlps between TABLE 8 Comparison of structures of F,PY-

compounds determined by electron diffraction

(distances m pm,

angles m degrees) Y

r(P-F)

2 PF,CBHS

C C

1589 1580

3 4

PF,CN PFzN(CH&

C N

1568 (3) 159 1 (1)

1792 165 7

5 6

PF,N(CH,)H PF*NH;!

N N

1593 (4) 158 1 (3)

1648 (7) 166 1 (7)

94 1 (8) 953 (11)

7 PF,NCO

N

1563

(3)

1683

(6)

979

(8)

995

(7)

8 PF,NCS 9 C(NPF,),

N N

1566 156 2

(3) (2)

1686 1680

(7) (6)

994 103 4

(9) (8)

97 7

(8)

25

944

(7)

26

996 992 1016

(2) (3) (1)

27 28 This work

Compound

1 C(CHSM’FZ

(4) (3)

L FPY

r(P-Y)

L FPF

182 2 (12) 1809 (7)

99 1 (17) 102 3 (12)

(9) (6)

979 927

(3) (4)

10 N(CH,)(PF,),

N

1583

(1)

1680

(3)

95 1 (2)

11

N

1574

(2)

171 1

(4)

969

(3)

12 PFPOCHB 13 PF,OCH,CH,CN

0 0

1594 (1) 160 1 (3)

0

1585

(7)

948 936 980’

(1) (9)

14 PF,OCBHb 15 (PF2)0CH2C=CCH20(PFJ 16 1,4X,H,(OPF,),

1572 (3) 155 1 (8) 1589 (17)

0 0

159 8 157 7

(3) (7)

946 (11) 960 (30)

17 1,3-CsHd(OPF,), 18 PO(OPF2), 19 O(PFz)z

0 0 0

158 1 (5) 158 l(10) 156 7 (4)

155 3 (8) 159 8 (13) 159 7 (10) 1576 (25) 1630 (10)

20 21 22

PF, (PF,)z PF,SCH,

F

1569

(1)

P S

1586 1590

(2) (3)

2279 2084

(3) (3)

99 1 (2) 95 4 (5)

23 24 25 26

PF,SCH,CH, (PF,)S(CH,),S(PF,) S(PF& Se(PF2)*

S s S Se

1588 1577 157 2 157 3

(2) (4) (2) (3)

2085 2112 213 2 227 3

(4) (9) (4) (5)

96 0 (5) 974 (7) 974 (5) 1006 (11)

N(PFz)s

“Flxed

970 (20) 964 (14) 992 (24) 977 (2)

99 0

Ref (6)

23

988 (11) 983 (3)

21 24

992

(4)

20

1006 (4) 1010 (11)

18 19 25

1020

(4)

5

984

(3)

4

100 7

(8)

5

978(16) 978 (8) 983 (6) 976 (12) 954 1012

2 2 3 1 29

(2) (2)

30 22

1011 (3) 1019 (12) 1002 (4) 98 7 (4)

17 17 22 22

284 161

1

160

-L 157

156 154

0

t

156

162 rlP-Yl/pm

166

170

176

/

162

I

/

166

210

I 214

Fig 4 Plot of r (P-F) against r (P-Y ) for PFzY - compounds hsted m Table 8 (Key 0, Y = C, A, Y = N, 0, Y = 0, V , Y = S ) The correckons for hybrldlsatlon state are marked * (see text )

the P-F and P-Y distances are merely the result of correlation between these distances arising from the method of structure determmatlon from electron diffraction data This could be particularly slgmficant for the PF20- compounds, where the two lengths have very similar values, and could also occur for nitrogen compounds However, such correlation should be shght for PF2Scompounds, m which the P-F and P-S bonds are of considerably different lengths Therefore the reflection of the correlations for the oxygen and mtrogen compounds m the sulphur compounds suggests that the correlations are real. Furthermore it is found that the relative positions of different classes of compounds are well defined and are the same for all Y Thus compounds with alkyl substltuents have short P-Y and long P-F bonds, compounds of the type Y ( PF2 ) n have long P-Y and short P-F bonds and compounds with aryl substltuents he somewhere between these If the correlation was an artefact of the method of determination, a random distribution of the compound classes would be expected. The fact that the distribution is not random is strong evidence that the effect is real Nevertheless it is still possible that the extent of the correlation is exaggerated, particularly for the PF20- compounds For the carbon compounds the different hybridisation of the carbon atom m each case will have a large effect on the P-C length. Comparison of the C-C single bond lengths m ethane [ 311, butene [ 321 and butyne [ 331 shows the extent of the effect of hybrldlsatlon and allows a “correction factor” to be calculated so bonds can be compared on the same basis, that of an sp3 hybridised carbon atom If these are applied to the P-C lengths m the PF,C- com-

285

pounds then a similar correlation between P-F and P-Y to that noted above is observed. The P-N bond lengths for the compounds C (NPF,)2, PF,NCO and PF,NCS will be similarly affected by the sp2 hybridised nitrogen atom. Due to a lack of structures of suitable compounds it is not easy to find a similar correction to that found for the carbon compounds If the correction for the carbon atoms is used the P-N distances are closer to correlating with those of the sp’ hybn&sed PF2N- compounds, though perhaps still shghtly too short. It may be that in these compounds the phosphorus lone pair is involved in the substituent n-system to some extent. The short P-N bond in PF,N(CH,), has been attributed [ 19,341 to the result of (p-d) n-bondmg between a filled p-orbital of the nitrogen atom and an empty d-orbital of the phosphorus atom The observed trend of lengthenmg P-N in the series of compounds PF,N(CH,),, N(CH:,) (PF,), and N(PF2)3 has then been explained as due to the x-bonding electron pair being spread over more bonds [27] Similar arguments could be advanced to explain short P-O bonds in PF,O- compounds For P-S bonds, however, the (p-d) n-mteraction should be much weaker due to poorer overlap with the sulphur 3p-orbltals Despite this, similar changes m the P-Y bond lengths for oxygen, mtrogen and sulphur compounds are observed m the PF,Ycompounds Furthermore, it appears that there may be a similar correlation m PF,C- compounds, m which there are no filled p-orbitals available to participate m (p-d) x-bondmg. Therefore, while not necessarily excluding (p-d) n-bonding, the evidence seems to suggest that it IS not an important factor m determnung the bond lengths. Ab u-utlo calculations [ 351 on slloxanes, m which the shortness of the Si-0 bond is often attributed to (p-d) n-bonding, also seem to suggest that (p+d)n-bonding is not a major factor determunng the bond length One possible explanation for the correlation mvolves the electronegativity of the substrtuent on the Y atom An electronegative substituent would remove electron density from the P-Y bond thus lengthenmg it The weaker bonding of Y to the phosphorus atom would be compensated for by stronger bonding by the fluorine atoms and thus shorter P-F bonds Therefore a very electronegative group, such as another -PF, group, would give long P-Y bonds and short P-F bonds while a comparatively electropositive group, such as -CHB, would give short P-Y bonds and long P-F bonds It appears that aryl groups give an effect mtermedlate between these two extremes The FPF and FPY angles m PF,OCH, and the other alkyl PF,O- compounds are, respectively, among the smallest and largest observed m PF,Ycompounds There is a hypothesis [ 36,371 which suggests that the BAB angle in an AB, group is related to the A-B bond length such that the B. - *B distance remains constant. Bartell [ 361 quantified this m terms of “hard sphere radu” which are summed to give distances between atoms separated by two bonds, in a similar manner to the way covalent radu are used for bonded distances. To

Fig 5 Plot of r(P-F) against angle FPF for PF2Y- compounds listed m Table 8 The contmuous lines are 10~1of constant r(F a-F)

test this hypothesis for the PF2Y- compounds, the FPF angle was plotted against P-F bond length (Fig. 5) A large number of the points do seem to he m a regron where r( F. - - F) varies by only about 5 pm, but there are also a number of points which he considerably outslde this range. Note, however, that some of the structures have very imprecise values of P-F length or FPF angle, or both, and could, with a reasonable probability, lie anywhere wlthm a fairly large area of the plot The F *.*F distance seems to be ca. 235 pm, which 1s nearly 20 pm longer than predicted using the “hard sphere radms” for fluorine atoms given by Bartell [ 361. However, these radu were calculated from atoms bonded to carbon and the data shown here demonstrate that such radii are not transferable to very different compounds

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