Spectroscopic studies of Lewis acid-base complexes Part 7. Vibrational spectra and normal coordinate calculations for trimethylphosphine-gallium halide complexes

SPECTROCHIMICA ACTA PART A

ELSEVIER

Spectrochimica Acta Part A 52 (1996) 305-314

Spectroscopic studies of Lewis acid-base complexes Part 7. Vibrational spectra and normal coordinate calculations for trimethylphosphine-gallium halide complexes Robert

Cooper

Taylor*,

David

L.W. Kwoh

Department qf ('hemistrv, UniversiO' o1' Michigan, Ann Arbor, MI 48109. USA

Received 20 May 1995; accepted 26 August 1995

Abstract

Raman and infrared spectra of normal and deuterated species of trimethylphosphine-gallium tribromide and trimethylphosphine gallium triiodide complexes have been obtained from microcrystalline solids, mulls and KBr discs. Assignments are made and confirmed by normal coordinate analyses. For completeness, the calculations are extended to include minor revisions to the results from the chloride complex previously published. Keywords: Assignments; Force constants; Frequencies; Infrared; Raman

I. Introduction

Although the gallium halides are known to be strong Lewis acids, previous spectroscopic investigations of their complexes have largely been confined to those of the chloride. Specifically, in the case of Lewis complexes involving the phosphorus bases, phosphine, triphenylphosphine and trimethylphosphine, only the GaC13 complexes have been studied [1 4]. Moreover, with the exception of Ref. [4] which is the predecessor to the present work, frequencies for deuterium compounds have not been reported. In investigations where symmetry data cannot be obtained, deu-

* Corresponding author.

terium isotope shifts are invaluable in arriving at satisfactory assignments. It is generally accepted that the acid strength of third group halides increases in a regular progression from chloride to iodide. However, although molecular properties such as the dative bond force constant parallel this trend in the case of boron acids [5], the differences down the series are not pronounced and the picture is complicated by the effects of intramolecular steric crowding, In the trimethylphosphine-gallium chloride complex such internal steric interactions are reduced to the point that intermolecular forces in the crystal are sufficient to force the molecule into an eclipsed configuration rather than the staggered structure found in the boron complexes [6 8]. It is likely that the bromide and iodide compounds have

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306

R.C. Taylor, D.L. 14/. Kwoh / Spectrochh~ffca Acta Part A 52 (1996) 305 314

structures similar to the chloride, and the extension of the studies to the lower halides is desirable to provide comparative data.

2. Experimental Gallium halides were prepared by direct reaction of the elements under anhydrous conditions. Trimethylphosphine was made by methylating phosphine [13]. The complexes were prepared by direct combination of the acid and base in a standard high vacuum line equipped with Teflon stopcocks, as described in the preparation of the chlorides [4]. Unfortunately, the use of solvents in the spectroscopic work was precluded either by the reactivity of the compounds or their insolubility. Consequently, spectroscopic samples were prepared as mulls or KBr discs in a controlled nitrogen atmosphere box. In addition to being extremely moisture sensitive, trimethylphosphine gallium halide complexes readily take up oxygen from the air [4]. Even with careful precautions, minor contamination from this source occasionally occurred during transfer to the spectrometer. However, when it occurred it could be detected by the presence of weak bands in the vicinity of 1 0 2 0 - 1 0 4 0 c m ~ attributed to the R - O P linkage. The spectroscopic equipment used has been described in the previous investigation [4].

3. Results and assignments Experimentally observed values for the vibrational frequencies of the bromide and iodide complexes are listed in Tables 1 and 2, respectively. Because the observed frequencies varied slightly depending on the technique and state of the sample, Table 3 presents a set of best values for the fundamentals arrived at from a consideration of all the data obtained and their relative reliabilities. Table 4 gives the numbering scheme for the fundamental modes and a qualitative description of the motions involved. Inasmuch as m a n y of the lower frequency modes associated with the heavy atoms are strongly mixed, the descriptions do not necessarily depict the actual motions of the atoms

involved. They are still useful for discussions. Far-infrared spectra of the normal bromide and iodide complexes are shown in Figs. 1 and 2, respectively, and the R a m a n spectrum of the iodide complex is shown in Fig. 3. Although the actual symmetry of Me3P:GaC13 in the crystal is not C3v, it is sufficiently close to be assumed for the molecular analysis. The vibrational structure is thus 10A~ + 5 A 2 q- 15E. The Aj and E frequencies are R a m a n and infrared active, but those in the A 2 class are inactive. No experimental frequencies were observed which could be attributed to the A 2 class. Although all the compounds were in the solid state, single crystals were not available and direct symmetry information therefore could not be obtained. Accordingly, assignments were based on internal consistency, deuterium shifts, trends within the spectra of the three complexes, and comparisons with the spectra and assignments for the free base [9,10]. Tentative assignments were confirmed by the normal coordinate analysis, although in some cases unequivocal symmetry assignments were not possible. For discussion, the fundamentals can be grouped into three categories, the C - H stretching motions near 3000 cm t, the methyl group motions in the vicinity of 1000 1500cm ~, and the molecular skeletal frequencies below about 1000 cm ~. The torsional modes are considered with the methyl group motions. Assignments of the C - H stretching frequencies were of minor importance in the study, and little effort was devoted to them. The possibility of Fermi resonances with methyl deformation overtones also contributes to frequency uncertainties in these modes. Two high bands near 3000 cm were assigned to the a~ and e fundamentals in which the internal symmetry of the methyl group is not preserved during the vibration, and three lower bands in the vicinity of 2900cm ~ were assigned to the one a~ and two e modes in which it is. More specific identification of the C - H motions did not appear justifiable; the listings given in Table 4 were the most compatible with the results of the normal coordinate calculations. The midrange frequency group between about 900 and 1500 cm-~ includes five modes involving deformation of the C H 3 group, two of a~ and

R.C. Taylor, D.L.W. Kwoh / Spectrochimica Acta Part A 52 (1996) 305 314 Table 1 O b s e r v e d v i b r a t i o n a l f r e q u e n c i e s (in c m

~) f o r n o r m a l a n d d e u t e r a t e d ( C H 0 s P : G a B r x

( C H •)s P : G a B r s

( C D 3)s P : G a B r ~

Infrared K B r disc 2995 2986.3 2982.4 2972.4 2913.6 2902 1427.5 w 1416.5 s 1410 s 1405 w 1397.3 w 1383.3 w 1341 w 1308,6m 1298,1 w 1289,8 s 1020 w 973 s 961 s 954 s 922.5 w 850.7 m 844.2 m

781.3 w 761.4m 759m 668.2 w 376 w 364.6 w

275 s

307

Nujol

1419.6 s 1412.5 1407.3 w

Raman

Infrared

Solid

K B r disc

Nujol

2249.8 2241 2233.7 2125.5 2133

2249.8 2240.2 2232.7 2124.8 2132

1029 m 1015 s 1020.5 sh 1018 sh

1029 m 1014.4 s 1021 sh 1018.2 sh

1427.9 1416.6 1408.1 1398.6

w w w w

Raman Solid

Assignment

VI6 VI 1'17 1'18 I% 1'6 ~- V24

1031.4 w 1016.6 w

l'19 V20

1400.3 w 1344.4 1310.6 1300.7 1294.2

w m w s

975.3 s 969.4 s 957.3 s 926.4 w 853.2m 847.3m

784.3 w 764.1 m 761.4m 724m 672 w 379 w 366 m 359.5 w 285 s 278.1 s 258 w 250 w 242 s 216 w 200 w 197 sh 176m 154 w 121 w 112.5 s 92 m 85 s 81.5m 52.5 m 42.4 s

965 w

965.6 w

955.3 w 1040.8 w 802.1 s

956 w 1045.8 w 801.7 s 634.4 w

638.6 w

l'22

687.5

689 w

690.6 s

t'23

802. I s

851.1 w 801.7 s

792.3 w 764 s 762.5 s

789.5 s 785.3 s

790 s 785.4 s

790,7 w 787.9 w

V24

671.6 s

606 w

605.9 w 567 w 373.5 w 337 m

605.5 s

l' 6

333.9 w

I' 7

285 s

290.1 w 279.3 s

1290.2 w

964.2 w

852 w 847.4 w

363.3 w 289.5 w 278.8 w 254.7 w 236.4 s

369.8 w 334.5 w 282 s 276.1 s

248 w 217 w 233 s

107.3 s 99.8 m 88.8 m

188 w 141.5 w 176 w 161.5 w 146 w l18w 108.5 s 90 m 81m

77.7 m 52.6 w

74 w 51m

202.7 w 192.2 s 174.3m

K e y : m , M e d i u m ; s, s t r o n g ; w, w e a k intensity.

V21 I'4

R-O V5

217.3 w 229.9 s

1'26

141.7 w 177.6 s 163.0 m

1'27

117.7 105.8 95.3 85.7

w s m w

76 m 51.8 w

V28

VIO 1'29

1"4{I

P impurity

R.C. Tco,Ior, D.L. 14". Kn'oh / Spectrochhnica Acta Part A 52 (1996) 305 314

308

Table 2 O b s e r v e d v i b r a t i o n a l frequencies (in c m - i )

for n o r m a l a n d d e u t e r a t e d ( C H 3 ) 3 P : G a l ~

( C H 3)3 P:Gal~

(CD~ h P:Gal~

Infrared K B r disc

Nujol

Raman

Infrared

Solid

K B r disc

Nujol

2242.8 2237.7 2233 2227.7 2126.4 2118.5

2242.8 2238.7 2233 2226.7 2126.4 2118.5

1029 1017 1025 1013

1029 sh 1(t18.3 s 1(126.5 s 1023.5 s

2987.3 2982.3 2973.4 2909 2896 1425 w 1414 s 1408 s 1399.5 w 1396.3 sh 1335.5 w 1305.6 m 1289.2 s 1287 s 1015 w

1417.4 s 1411.5 s 1402.3 w 1398.2 w 1338.4 w 1308.6m 1290.8 s 1288.6 s

1412 w 1402.5 w

m s m s

949.4 w 962.3 w

1289 w

1039 w 987.2 w 976.4 sh 967.3 s 960.4 s 956.6 s 952.8 s 917.4 w 874 w 847.8 m 842.2 m

780.9 w 760m 756 m 718 w

668.3 w

238 s

987.3 w 969.4 961.5 957.4 953.4 919.4

s s s s w

849.5 m 844.2 s

761.3 m 756.5 m

668.2 w 356.7 m 277.8 274.6 252.8 242.7 234.6

w sh m s m

218.6 m 176.2 m 158.4 m 90 s 82.1 m 70.5 m 63 m

Raman Solid

Assignment

|'16 1' I /'17 1'1g I'2

1019 w 1027.2 w 1016.1 w

952 w 963.8 w

v,9 v~ v2o

L'2I I' 4

R-O R-O

1044.2 w 1038.1 w

983 w

959.0 w 953.7 w

685.6 w

686.6 w

688.1 s

v2~

795 s

795.6 s

793 w

v5

634.4 w

636.2 w

1'23

V24

847 w 843.7 w 784.8 w 780.9 w 761.5 m 757.4m 715.5 w 688.1 m 672.8m 668.7 s 354.7 w 279.7 w 272.2 w 251.4m 242.4m

212.8 203.7 168.7 156.9 147.4 89.9 84.6 72.3 61.6

m w s s w s s s s

Key: m, M e d i u m ; s, strong; sh, shoulder; w, w e a k .

845.2 w

845.8 w

785 s 781.3 s

785.9 s 781.9 s

787 w 784.2 sh

322.2 m

603.4 w 327.5 s

603.7 s 324.2 w 305 w

210 sh

212.9 w 242.5 w 234.2 w

I'26 L'25

195.7 m 147.7 sh 161.7 w 152.3 135.9 w 87 m 80 m 67.7 m 60.9 m

V9

239 217 196 142 159 153

s sh s w sh m

86 s 66 m 60.2 sh

V27 L'28 V8 VIO V29 V30

P impurity P impurity

R.C. Taylor, D.L.W. Kwoh / Spectrochimica Acta Part A 52 (1996) 305 314

309

Table 3 Best e x p e r i m e n t a l values for f u n d a m e n t a l frequencies o f ( C H 3 ) 3 P : G a X 3 c o m p l e x e s and differences between observed and calculated values Fundamental

Me~ P:GaC13 H Species Exptl.

A ~ Class vt C H v2 C H v3 CH~ va CH 3 v5 CH 3 % P C v7 Ga P v~ Ga X v9 PC3 vm GaX~ E Class v l6 C -H vt7 C H via C-H vK9 CH3 v2o CH3 v21 CH 3 v22 CH 3 v2~ CH~ v24 P C v2~ Ga X v26 PC~ v27 CH 3 v2x PC 3 v29 GaX 3 v~o GaX 3

str str def def

rock str str

str def def

str str str def def def

rock wag

str str def tors rock def rock

M e 3 P:GaBr~ D Species

Diff.

Exptl.

Me~ P : G a l 3 D Species

H Species Diff.

Exptl.

Diff.

Exptl.

Diff.

2241 2133 1030 a 956 802 a 606 334 230 177 106

11 1 0 - 5 - 2 - 3 1 0 0 1

2982 2896 1411 1289 951 669 355 157 169 90

2987 2973 2909 1414 1402 1307 968 847 761 252 ~ 239 203 169 ~ 71 62

2993 2910 1423 1293 973 673 370 ~ 338 ~ 208 143

- 8 -2 1 1 0 1 -1 -1 2 -2

2246 2130 1034 956 805 607 323 361 188 142

8 0 --1 --1 -1 -2 1 I -2 2

2982 2902 1417 1290 974 670 364 237 ~ 193 '~ 108

-- 8 0 1 3 1 3 1 1 0 -1

2999 2981 2918 1414 1403 1311 957 ~ 851 a 763 380 261 213 183 133 94

- 9 -9 - 13 - 8 2 1 - 1 -6 2 1 5 - 1 0 2 I

2252 2244 2134 1021 1017 974 691 640 789 378 224 159 170 128 90

ll 17 27 -9 7 -1 5 11 3 -1 -4 11 3

2986 2972 2914 1410 1400 1309 958 ~ 852 ~ 762 287 253 201 175

-ll -9 -14 -1 0 -1 -1 --1 --1 -1 0 1 1

2250 2233 2125 1015 1019 965 690 636 788 285 217 142 163

16 12 20 -1 0 0 1 1 1 1 0 -1 -- 1

- 2

99

2

95

- 2

0

78

0

76

1

" C a l c u l a t i o n s indicate strongly m i x e d m o d e s

D Species

H Species Exptl.

Diff.

Exptl.

-4 1 0 3 1 3 1 I - 2 0

2233 2118 1027 ~1 963 795 ~' 603 326 162 152 87

9 I 0 I -1 -3 1 I 1 2

-6 -6 16 2 2 3 1 -1 1 1

2239 2227 2126 1018 1016 952 687 635 786 238 "

8 8 23 3 -2 4 -1 0 - 1 - 3

-l -1

Diff.

212 147 0 2 0

161 " 67 60

0 I 2 -2 1

designation is s o m e w h a t arbitrary.

three of e species, plus a methyl group rocking mode and a wag, both e species. The methyl deformation motions were easy to identify and, following the precedent used for the C - H stretching modes, the higher frequencies were assigned to the motions in which the internal symmetry of the CH3 group is absent and the lower frequencies around 1300cm ~ to those in which it is preserved. However, assignments in terms of the overall symmetry of the molecule were more arbitrary and must be regarded as having an element of uncertainty. Proper designation of the rocking and wagging modes was more difficult and the normal coordinate calculations showed that these two modes were mixed. The higher frequency of

the 957/851 cm-~ pair was assigned as the rocking motion in the hydrogen/chloride complex, based on the results of the normal coordinate analysis. However, the normal coordinate calculations indicated that the wagging motion predominated at the higher frequency of the pairs 958/852 and 968/847 cm ~ of the corresponding bromide and iodide complexes, respectively. The remaining methyl group motion, the CH3 torsion, was well separated from the other methyl group frequencies and was located close to 200 cm i, dropping to around 150cm ~ in the deuterated compounds. As usual, this mode was low in intensity and did not change significantly in frequency in the three complexes.

310

R.C. Taylor, D.L.W. Kwoh

Spectrochimica Acta Part A 52 (1996) 305-314

Table 4 Numbering and description of fundamental modes of trimethylphosphine gallium halide complexes Mode

Description ~

Mode

A~ Class v~ P2 v3 v4 % v6 v7 v8 v9 rio

Asymetric C H stretch Symmetric C-H stretch Asymmetric CH 3 deformation Symmetric CH 3 deformation Methyl group rock P C stretch Ga P stretch Ga X stretch PC3 deformation GaX 3 deformation

E Class v~6 vw L,~ v~j v20 v2~ v~2 v23 v24 v2~ /~26

A2

v~ v~2 v~3 v~4 v~5

Class

v27 Asymmetric C-H stretch

v2s

Asymmetric CH 3 deformation

l'29 v3~

Methyl group wag Methyl group torsion Torsion around Ga P bond

" Asymmetric and symmetric refer to the internal symmetry of the O f m o r e interest, o f course, were the skeletal m o t i o n s falling b e l o w a b o u t 8 0 0 c m 1. In the h y d r o g e n c o n t a i n i n g complexes, the s y m m e t r i c a n d a s y m m e t r i c P C stretching m o t i o n s were readily identified close to 670 a n d 760 c m - J , respectively, b a s e d on a s s i g n m e n t s for the free base a n d o t h e r t r i m e t h y l p h o s p h i n e c o m p l e x e s [4,9,10]. It is o f interest to n o t e t h a t the frequency o f the e species P C m o d e in all three c o m p l e x e s actually increased a b o u t 25 c m - l u p o n d e u t e r i u m substitution. This reverse shift is a t t r i b u t e d to the fact that, with d e u t e r i u m , b o t h the a s y m m e t r i c CD3 r o c k a n d the CD3 w a g shift b e l o w the p o s i t i o n expected for the a s y m m e t r i c P C stretch with the result t h a t the repelling effect between f u n d a m e n t a l s o f the same s y m m e t r y has m o v e d the P C m o d e u p w a r d s . This observation and argument provide corroborative evidence for the s y m m e t r y a s s i g n m e n t s o f the frequencies involved. B o t h the s y m m e t r i c a n d a s y m m e t r i c PC3 d e f o r m a t i o n s showed a systematic b u t g r a d u a l t r e n d to lower frequency in the series f r o m c h l o r i d e to iodide. T h e b a n d s were easy to identify due to their relative insensitivity to c h a n g e in h a l o g e n a n d to their a p p r e c i a b l e shift with d e u t e r i u m substitution. H o w e v e r , the a s s i g n m e n t o f the s y m m e t r i c PC3 d e f o r m a t i o n at 193 cm ~ in the b r o m i d e c o m p l e x is r a t h e r a r b i t r a r y due to the

Description " Asymmetric C H stretch Asymmetric C H stretch Symmetric C H stretch A s y m m e t r i c C H 3 deformation A s y m m e t r i c CI-I 3 deformation Symmetric CH 3 deformation Methyl group rock Methyl group wag P-C stretch Ga X stretch PC3 rock PC3 deformation Methyl group torsion GaX 3 deformation GaX~ rock

CH 3

groups.

close p r o x i m i t y of, a n d resultant s t r o n g mixing with, the G a - B r stretch. A g a i n , i n f o r m a t i o n on v i b r a t i o n s o f the free base a n d its c o m p l e x e s with b o r o n halides [9,10] was useful in the identification o f these modes. T h e r e m a i n i n g f u n d a m e n t a l s include the a~ and e GaX3 stretches a n d d e f o r m a t i o n s , the GaX3 r o c k i n g m o t i o n a n d the G a - P valence vibration. T h e stretching a n d d e f o r m a t i o n m o t i o n s involving the halide atoms, as expected, s h o w e d high intensity in b o t h the infrared a n d R a m a n spectra, a s u b s t a n t i a l shift with the n a t u r e o f the halide a t o m , a n d an insensitivity to d e u t e r i u m substitution. T h e two e class m o d e s d e s i g n a t e d as the GaX3 d e f o r m a tion a n d r o c k were very low in frequency a n d their a s s i g n m e n t was b a s e d largely on the results o f the n o r m a l c o o r d i n a t e calculations. T h e m o d e called the G a X 3 r o c k m o r e a c c u r a t e l y s h o u l d be described as a flexing o f the m o l e c u l a r b a c k b o n e . T h e G a - P valence vibration, o f course, was o f c o n s i d e r a b l e interest because o f its a s s o c i a t i o n with the dative b o n d . A l t h o u g h the b a n d was relatively s h a r p in the spectra o f all o f the complexes, it exhibited unexpectedly low intensity in b o t h the infrared a n d R a m a n spectra. Also, the b a n d p o s i t i o n did n o t wiry significantly with h a l o g e n substitution, the wdues for the CI, Br a n d I c o m p l e x e s being

R.C. Taylor, D.L.W. Kwoh / Spectrochimica Acta Part A 52 (1996) 305 314

311

z

z

400

I

I

I

300

200

100

FREQUENCY

(1/¢m)

Fig. l. Far-infrared spectrum of trimethylphosphine gallium tribromide.

1

z .el (/1 z

400

I

I

I

300

200

100

F'REQUENCY

(1/era)

Fig. 2. Far-infrared spectrum of trimethylphosphine gallium triiodide.

370 c m - 1 , 364 cm ~ and 355 c m - 1, respectively, for the hydrogen species. Product rule comparisons for the three complexes are shown in Table 5 and support the proposed assignments. It should be noted that, although they were included when calculating the ratios in Table 5, the frequency values for the C - H and C - D frequencies are subject to considerable uncertainty and consequently contribute a disproportionate amount to what differences exist between the observed and experimental ratios.

Their removal via substitution of the calculated for experimental values significantly improves the comparison based on the remaining frequencies.

4. Normal coordinate calculations

N o r m a l coordinate calculations were carried out using a standard least squares program based on symmetry force constants which has been de-

R.C. Taylor, D.L.W. Kwoh / Spectrochimica Acta Part A 52 (1996) 305 314

312

IO.

I 1400

l

I 12100

i.

~\

II~

900

BOO

Frequency

700

k

300

200

I00 cm-'

(l/crn)

Fig. 3. Raman spectrum of solid trimethylphosphine gallium tribromide.

scribed previously [4,11]. For the chloride compound, the G matrix elements were calculated using the X-ray parameters [6]. In the case of the bromide and iodide compounds, the geometry was assumed to be similar to that of the chloride with the bond lengths involving bromine and iodine being increased by the respective increments in the covalent atom radii relative to chlorine. In the X-ray study of the chloride complex, the hydrogen atom positions were subject to a large uncertainty and the exact orientation of the methyl groups was unclear. Two configurations are possible for the methyl hydrogens compatible with the C3v symmetry assumed for the molecule. In one, the hydrogen atoms which lie in the symmetry planes are cis to the gallium atom while in the other they are trans to gallium. In the present work, the cis configuration was assumed. Small differences in the force constants associated with the methyl groups were found, depending on whether the in-plane hydrogens were placed cis or trans, but the cis position resulted in smaller overall interatomic contacts and was selected on Table 5 Hydrogen/deuterium product rule comparisons H / D ratio A1 class

(CH3)3P:GaCI 3 (CH3)3P:GaBr 3 (CH3)3P:GaI ~

E class

Theor. a

Exptl.

Theor. ~

Exptl.

5.547 5.581 5.598

5.427 5.630 5.405

21.186 21.488 21.706

20.311 21.523 22.182

Theoretical ratios calculated from G matrix determinants.

that basis; the assumed location of the hydrogen atoms had essentially no effect on skeletal force constants. The symmetry force constants given in Table 6 were obtained by simultaneous fitting of the hydrogen and deuterium frequencies. Derived valence stretching force constants are listed in Table 7. Due to the redundancies present, valence force constants involving angles, of course, could not be extracted from the data. Comparison of the force constants in Table 7 with the corresponding data for boron halide complexes with trimethyamine [5] leads to several interesting observations. First, although the magnitudes of the dative bond force constants in the gallium complexes are appreciably less than those found in the t r i m e t h y l a m i n e - b o r o n complexes, the trend and relative change in magnitude are almost the same in the two series. The difference in magnitude between the B N and G a - P constants thus appears to be a logical consequence of the greater bond distances in the latter compounds plus the fact that trimethylphosphine is a "softer" base than trimethylamine. The similarity in the trend of constants suggests that the boron and gallium acids are similar in acid strength, i.e. acceptor ability. Secondly, the marked decrease in the galliumhalogen force constant from chlorine to iodine parallels that found in GaX~- ions [12]. Moreover, the magnitudes of the respective G a X constants are very similar in the two series indicating that trimethylphosphine and the X - ion are equally effective as electron donors. Finally, the p h o s p h o r u s - c a r b o n force constant increases slightly but not markedly from chloride to iodide suggesting that the specific acid has little

R.C. Taylor, D.L.W. Kwoh / Spectrochimica Acta Part A 52 (1996) 305 314 Table 6 Symmetry force constants (mdyn A

Index

313

~) for trimethylphosphine-gallium trihalide complexes

(CH0~P:GaC13

(CH3)3P:GaBr 3

(CH3)3P:Gal 3

Symmetry force constants - A~ class l, 2, 3, 4, 5, 6, 7, 8, 9, 10,

l 2 3 4 5 6 7 8 9 10

4.7899 4.5809 0.2584 0.4423 0.5088 3.4221 2.0201 2.3495 0.9542 0.6765

4.7548 4.4667 0.2645 0.4481 0.4866 3.7083 2.2447 1.7668 0.8884 0.9133

4.7461 4.5385 0.2789 0.4212 0.4182 3.7251 2.5087 1.5116 0.4795 1.1316

2, 3, 4, 4, 5, 5, 6, 7, 7, 7, 8,

4 5 6 7 7 9 7 8 9 10 9

--0.4645 0.0955 0.4117 --0.2662 0.0574 0.3216 0.1382 0.5762

--0.5303 0.0907 -0.3342 0.2230 0.1087 0.3383 0.9586 0.7469

-0.4465 0.0880 -0.3343 0.2230 0.3888 0.2389 0.9478 0.0453 0.1819

-0.3876 -0.3216

Symmetry force constants - E class 16, 17, 18, 19, 20, 21, 22, 23, 24, 25 26, 27, 28, 29, 30,

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

4.7567 4.8077 4.9536 0.3438 0.3370 0.3847 0.4140 0.3661 3.3100 1.6636 0.6330 0.0649 0.6225 0.5358 0.5275

4.7290 4.7786 4.9435 0.3481 0.3204 0.3923 0.4076 0.3746 3.3713 1.1686 0.5643 0.0667 0.6333 0.7178 0.6936

4.7206 4.7643 4.9339 0.3501 0.3152 0.3957 0.3809 0.4097 3.3285 0.7080 0.5833 0.0612 0.7856 0.6706 0.5708

20, 21, 22, 23 26, 26, 27,

21 24 23 24 27 28 28

-0.0089 -0.4219 -0.0426

-0.0157 -0.4725 -0.0472 0.0234 0.0294 --0.0113 0.0339

-0.0225 -0.4725 -0.0475 0.0257 0.0357 0.017

-0.0058 --0.0751

effect on the electronic structure of the base, once it has been incorporated into the complex. This is in contrast to the trimethylamine boron halide

series where the N C constant decreases systematically by about 20% from the chloride to the iodide complex. The difference can be rationalized

314

R.C. Taylor, D.L.W. Kwoh Spectrochimica Acta Part A 52 (1996) 305 314

Table 7 Valence bond stretching force constants (mdyn A ~) for trimethylphosphine gallium trihalide complexes Bond

Chloride

Bromide

Iodide

Ga P Ga X P C C H (ip) C H (op)

2.020 1.892 3.347 4.788 4.757

2.245 1.368 3.484 4.753 4.811

2.509 0.976 3.461 4.753 4.727

Interaction constants Ga P/Ga X Ga P / P - C Ga-X/Ga X P C/P-C

0.333 0.080 0.229 0.037

0.431 0.553 0.199 0.112

0.026 0.547 0.268 0.132

with the argument that the amine is a "hard" base and the degree o f donation of its electron pair is thus sensitive to the attracting power of the acid whereas the phosphine, being a "soft" base, donates its electrons completely with little dependence on acid strength. Comparison of the force constants of the complexed phosphine with those of the free base is complicated by the fact that the constants reported by McKean et al. [10] for Me3P are harmonic constants whereas those presented in the present work have been obtained from anharmonic frequencies. Nevertheless, the values for the P C constant in the complexes, which range from 3.4 to 3.7 mdyn A ~, are significantly higher than the value o f 3.1 m d y n A ~ found for the free base. This is in accord with the shortening of the P - C bond from 1.84 A in the free base to 1.79 in the chloride complex [6] and the corresponding increase o f the C - P - C angle from 98.8 ° to about 107.6 ° . The geometrical data thus show that trimethylphosphine already has a nearly tetrahedral configuration in the chloride complex which is consistent with the force constant data.

References [1] M.J. Taylor and S. Riethmiller, J. Raman Spectrosc., 15 (1984) 370. [2] M.J. Taylor, D.S. Bohle and S. Riethmiller, J. Raman Spectrosc., 15 (1984) 393. [3] J.R. Durig and K.K. Chanerjee, J. Mol. Struct., 81 (1982) 167. [4] D.L.W. Kwoh and R.C. Taylor, Spectrochim. Acta Part A, 47 (1991) 409. [5] P.H. Laswick and R.C. Taylor, J. Mol. Struct., 34 (1976) 197. [6] J.C. Carter, G. Jugie and J. Galy, lnorg. Chem., 17 (1978) 1248. [7] P.H. Clippard, J.C. Hanson and R.C. Taylor, J. Cryst. Mol. Struct., 1 (1971) 363. [8] D,L. Black and R.C. Taylor, Acta Crystallogr. Sect. B, 31 (1975) 1116. [9] D.L. Black, Dissertation, The University of Michigan, 1971. [10] D.C. McKean, G.P. McQuillan, W.F. Murphy and F. Zerbetto, J. Phys. Chem., 94 (1990) 4820. [11] R.C. Taylor, R.W. Rudolph, R.J. Wyma and V.D. Dunning, J. Raman Spectrosc., 2 (1974) 175. [12] L.J. Basile, J.R. Ferraro, P. LaBonville and M.C. Wall, Coord. Chem. Rev., 11 (1973) 21. [13] W.L. Jolly (Ed.) Inorg. Syn., Vol. XI. McGraw-Hill New York, 1968, p. 126.