An electron diffraction determination of the molecular structure of cyanodifluorophosphine in the gas phase

Journal of Molecular Sfructure Elsevier Publishing Company, Amsterdam.

11

Printed in the Netherlands

AN ELECTRON DIFFRACTION DETERMINATION OF THE MOLECULAR STRUCTURE OF CYANODIFLUOROPHOSPHINE GAS PHASE

G.

C. HOLYWELL

Department Ian&) (Received

AND

of Chemistry, September

D. W.

IN THE

H. RANKIN

University of Edinburgh,

West Mains Road, Minburgh,

EH9 3JJ (Scot-

lst, 1970)

ABSTRACT’

The sector-microphotometer method of electron diffraction has been used to determine the molecular structure of cyanodifluorophosphine, PF,CN, in the gas phase. The structural parameters are found to be: r(P-F) = 1.568 +_0.003 A; r(P-C) = 1.792-t_O.O09 A; r(C=N) = 1.165+0.005 A; +(F-P-F) = 97.9f0.3”; Q(F-P-C) = 98.310.3”. These results provide no evidence for any x-interaction between the cyanide group and the phosphorus d orbitals.

INTRODUCTION

Silyl compunds in which the silyl group is attached to a nitrogen or oxygen atom have been shownre4 to have silicon-nitrogen or -oxygen bonds that are appreciably shorter than predicted by the Schomaker-Stevenson rule”, and these results have usually been interpreted in terms of z-bonding between silicon and the first row atom. However, although similar z-interaction between a silicon atom and a cyanide or acetylene group should be possible, the observed silicon-carbon bond lengths6*’ are no shorter than would be expected, after making due allowance for the effect of the sp hybridisation of the carbon atoms. The structures of a number of diflaorophosphine derivatives of nitrogen have been found’ to resemble closely those of the corresponding silyl derivatives. In the present study, of cyanodifluorophosphine, the phosphorus-carbon bond length has been found to be 1.792+0.009 A, which, though shorter than those in trimethylphosphineg and the other methyl phosphines”, is close to the length predicted by the Schomaker-Stevenson rule, with correction for the sp hybridised carbon atom. J. Mol.

Structure.

9 (1971)

11-16

G. C. HOLYWELL,

12

D. W. H. RANKLN

EXPERIMENTAL

Samples of cyanodifluorophosphine were prepared by streaming bromodifluorophosphine over dry silver cyanide’l, and were purified by fractional condensation in a vacuum system. The purity was checked by IR spectroscopy, but on standing at room temperature slow disproportionation took place, giving tri-

fluorophosphine

as a more volatile product.

Electron diffraction

intensity data were

recorded photographically on the Balz&s’ KD.G2 gas diffraction apparatus at the University of Manchester Institute of Science and Technology’2. The sample and the nozzle were both maintained at room temperature, 296” K. The electron wavelength of 0.057 913 +_O.OOO030 A was determined by direct measurement of the accelerating vo!tage. Data were transferred to punched paper tapes using an automated Joyce-Loebl microdensitometer, and data reduction to uphill curves was performed on the Cambridge University “Titan” computer, using the Cambridge data reduction programme 13*. All other calculations were carried out using the Edinburgh Regional Computing Centre’s IBM 360/50 computer, and data reduction and least squares programmes that are described elsewhere8*14. Data from 1000,500 and 350 mm nozzle-to-plate distances were used, giving an overall range of O-7-29.2 A-’ in the scattering variable s. The weighting points, used in setting up the weight matrix14, correlation parameters and scale factors are given in Table 1. In all refinements, the complex scattering factors of Cox and Bonham” were used. TABLE 1 WEIGHTING

FUNCTIONS,

CORRELATION

Height dels (mm)

&nm

250 1000

5.200 0.700

0.400 0.100

PARAMIXERS

AND SCALE

52

7.000 1.750

23.000 5.200

29.200 7.300

FACTORS

Ah

Scalefacror

0.4791 0.4954

1.128&0.026 0.886f0.033

REFINEMENT

In the molecular model it was assumed that the two fluorine atoms were equivalent, and that the P-C-N group was linear. With these assumptions the structure may be defined by five geometrical parameters. These were chosen to be three different bonded distances, and the angles F-P-F and F-P-C. The possible independent variables in the refinement were these five geometrical parameters, the amplitudes of vibration of the three independent and four dependent distances, l

For detailed experimental

1. Mol. Structure,

9 (1971)

data, apply to the authors.

11-16

STRUCTURE

OF

13

CYANODIFLUOROPHOSPHINE

and the scale factors for the three data sets. It was soon found that refinement of all of these parameters simultaneously was possible, once reasonable values for

the bond lengths had been found. ft was also apparent

that the 500 mm plate (the

first to be exposed) included scattering by an impurity, probably trifluorophosphine, This data set was therefore discarded, and the cut-off and weighting points for the 250 and 1000 mm data were adjusted so that the whole s range was covered with a total weight of at least 1.

PM/r

‘3

-

PF2CN

‘7 R

Fig. 1. Observed and difference radial distribution curve, P(r)/r. were multiplied by s - exp[-0.0025 s2/(zP-_IP)(zN--fN)].

Before Fourier inversion the data

The radial distribution curve, Fig. 1, shows five distinct peaks. Of these, the one at about 1.65 A includes both the P-F and the P-C bonded distances, and that at about 2.45 13, includes F + - - F and F * * * C non-bonded distances. It has been observedf6 that when two or more peaks overlap in this way, small amounts of extraneous scattering may lead to values for refined amplitudes that seem unreasonable. In this case it seems probable that the amplitudes for the P-C and P-F bonds are too large, and that those for the F - - - F and F . * * * C atom pairs are too small. The estimated errors for these amplitudes (Table 2) have been increased to allow for this effect, The possibiiity that shrinkage corrections might be necessary to allow for large amplitude vibrations in the molecule was investigated. Various corrections to distances were applied, but invariably the R factors obtained in the refinements were higher than those for refinement without corrections. No corrections have been applied, therefore, to the parameters listed in Table 2. The refinement converged to R factors which were Ro = (U’ WU/Z'WZ)* = = 0.09, where Z is the vector of intensities, U 0.14 and R, = (~WjiUj’/~WjjIjqf

the vector of residuals,

and W the weight matrix with elements

'ojk-

14

G. C. HOLYWELL,

D. W. H. RANKIN

TABLE 2 MOLECULAR

(A)

PARAMETERS

Independent distances Distance

I S68 f0.003 1.792&O-009 1.165f0.005

r 1 (P-F) r 2 (P-C) r 3 (N-C)

Amplitude

Anharmonic constant

0.058 f0.004 0.090f0.010 0.073 &0.009

2.0 2.0 2.0

(B) Dependent distances d 1 (F.-a

F)

2.365 10.006 2.545 *,O.Ol 1 3.540*0.014 2.956fO.016

d2(F---C)

d3 (F---N) d4(P**-N)

0.058 0.073 0.201 0.101

f0.01 I rrtro.009 f0.012 f0.007

(C) Independetzr m&es 9: : (F-P-F) 32

97.9f0.3 98.3 f0.3

(F-P-C)

Distances and amplitudes are given in hgstrom unit = 100 picometres; 1” = 0.01745 radians.

units; angles are given in degrees. I Angstrom

RESULTS AND DISCUSSION

The observed and difference molecufar intensity data are shown in Fig. 2. The molecular parameters are given in Table 2, and the final least-squares correlation matrix in Table 3. All distances quoted are rB(f)17. The errors quoted in Table 2 are estimated standard deviations obtained from the least-squares refinement, increased to allow for systematic errors and the effects of correlation of parameters. The phosphorus-carbon bond length in cyanodifiuorophosphine is found to be 1.792+0.009 A. The sum of the covalent radii of phosphorus and carbon” is about 1.87 A, and the Schomaker-Stevenson electronegativity correction’ is -0.036 A. A further correction of -Oo.04 A should be made to allow for the sp hybridisation of the carbon atom ‘*, leaving 1.794 A, in excellent agreement with the experimental value. Phosphorus-carbon bond Iengths reported for methyl phosphines range from 1.841 A for trimethylphosphine’g (microwave) to 1.858 A in methylphosphine”. The phosphorus-fluorine bond length is essentially the same as that in tri~uorophosphine 20, 1.569+0.001 _ A, and those in a number of the other ffuorophosphine derivatives 8. Similarly, the angles at phosphorus are very close to those in trifluorophosphine. These latter results would suggest either that the cyanide group has similar stereochemical effects to those of a third fluorine atom, and hence J. Mol. Smtcme,

9

(1971) II-16

STRUCTURE

OF CYANODIFLUOROPHOSPHINE

15

250mm

I

lNTENSliY

OAlA

FOR PF$N

Fig. 2. Observed and weighted difference molecular intensity data.

TABLE

3

LEAST SQUARES

CORRELATION

MATRIX

MULTIPLIED

BY

1000 --

41

r3

r2

rl

ul

42

II2

II 3

115

II 6

11

7

N8

kl

1000

607

-6

-349

-583

266

14

139

204

193

62

28

335

607

1000

-268

-120

-899

573

131

170

374

363

99

77

683

-6

-268

-349

-120

-583

-899

loo0 -260

-260

156

-172

7

-128

1000 7

156

1000

-463

-94

-44

19 -104

-56 -140

156 -364

-175

481 -185

-282

69

-30

-1114

-176

-99

-60

-99

-546

-449

388

158

110

801

334

2

25

17

36

148

97

1000

104

97

39

15

184

64

2

104

1000

747

115

-15

515

214

388

25

97

747

1000

100

-50

502

204

115

100

1000

-95

196

122

-95

loo0

131

34

573

-172

- 128

-463

1000

393

14

131

-94

19

-104

393

1000

-11

139

170

-44

-140

137

-11

204

374

20

-403

388

193

363

-25

156

-364

62

99

-282

28

77

335

683

293

481

1

20 -403

-25

293

388

266

-56

1

k3

137

-30

-60

158

17

39

-114

-99

110

36

15

-175

-176

-546

801

148

184

515

502

196

131

1000

367

-185

-99

-449

334

97

64

214

204

122

34

367

1000

69

-15

-50

J. Mol. Srfucfltre, 9 (1971) 11-16

16

G. C. HOLYWELL,

D. W.

H. RANKIN

that it has similar electron-withdrawing power, or that the third phosphorus substituent has little effect on the stereochemistry of the PF, group. The normality of the PCN group suggests that the latter is the case.

ACKNOWLEDGEMENTS

We thank Professor D. W. J. Cruickshank and Dr. B. Beagley for the provision of experimental facilities. G. C. H. thanks the Science Research Council for a maintenance grant, and D. W. H. R. thanks Imperial Chemical Industries for a Research Fellowship.

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9 (1971)

1 l-l 6