An NMR study of the fluxionality of group VI metal pentacarbonyl complexes of 1,3-dithiolanes

Polyhedron Vol. 9, No. 5, pp. 703-71 I, 1990 Printed in Great Britain

0

0277-5387/90 $3.00+.00 1990 Pergamon Press plc

AN NMR STUDY OF THE FLUXIONALITY OF GROUP VI METAL PENTACARBONYL COMPLEXES OF 1,3_DITHIOLANES EDWARD W. ABEL, KEITH G. ORRELL,* KHURSHID and VLADIMIR SIK

B. QURESHI

Department of Chemistry, University of Exeter, Exeter EX4 4QD, U.K. (Received

12 June 1989 ; accepted 25 September

1989)

Abstract-The complexes [M(CO),(mRR’)] (M = Cr, MO or W, R = R’ = H ; M = Cr or W, R = R’ = Me, R = H, R’ = Me, Bu’ or Ph) have been synthesized and characterized. Their solution stereodynamics were examined by ‘H NMR spectroscopy. Pyramidal inversion of the metal-coordinated sulphur atoms occurs rapidly at ambient temperatures (AGt (298 K) = 4&44 kJ mol- I). An appreciably slower 1,3-metal-sulphur shift is also evident at temperatures where sulphur inversion is rapid on the proton NMR timescale. For the ligand complexes (R = R’ = H, Me) this shift is purely intramolecular in nature (AG’ (298 K) values - 78 kJ mol- I), in contrast to the 2-mono-substituted ligand complexes (R = H, R’ = Me, Bu’, Ph) where it is accompanied by a dissociation process.

Group VI metal pentacarbonyl complexes with ligands have been chosen for identifying any 1,3chalcogen ligands are widely known and their solu- shifts that might be occurring in five-membered cyction structures have been rigorously examined by lic polythioethers of M(CO)S. These ligands have recently been shown to form NMR methods. I Much of this previous work has been centred around M(C0)5 complexes of stable palladium(I1) and platinum(I1) complexes, (R = R’ = H, cyclic polythioethers such as SCH2SCH2SCH2,2,3 viz. trans-[MCl,(mRR’)] Me ; R = H, R’ = Me, Bu’, Ph), in which 1,3-metal$CHMeSCHMeSCHMe2-4 and SCH2SCH2 sulphur shifts do indeed occur in most cases, usually mH2,’ where in addition to facile pyramidal sulphur inversion and ligand ring reversals, slower accompanying pyramidal sulphur inversion or 1,3-metal-sulphur shifts occur. Because of the alter- ligand dissociation/recombination. 8*9This present nating -C-S-C-Sligand backbone, the work therefore serves both to examine the influence of the metal on the fluxional nature of the M + S M(CO), moiety is able to commute freely around these six-membered rings. In five-membered rings, bond in five-membered ligand ring complexes and however, this is not possible, as for example in also to compare the effect of ligand ring size on the case of the ligand complexes [M(CO), 1,3-metallotropic shifts. To this end the following (Me,CCH,EECH,)] (E = S, Se) where the com- complexes have been synthesized, viz. [M(CO), mutation is restricted to the two adjacent E atoms.6 (-RR’)] (M = Cr, MO, W, R = R’ = H ; M = Cr, W, R = R’= Me, R = H, R’= Me, Fluxional 1,3-metal shifts restricted to three bonds Bu’ or Ph), and we report herein their solution might be expected to occur in M(CO)s complexes structures as evidenced by dynamic ‘H NMR of the five-membered rings -Ph, SC,H,SP (0)Ph and m(S)Ph, but NMR inves- spectroscopy. tigations up to 150°C produced no evidence for such fluxionality, possibly due to the donor in these ligands being the phosphorus, oxygen or exocyclic EXPERIMENTAL sulphur atoms rather than the endocyclic sulphur. 7 The 1,3_dithiolanes, -‘RR’ (R = H, Materials and preparations R’ = H, alkyl, aryl), however, can only coordinate Ligands. The preparations of the ligands 1,3via their endocyclic sulphur atoms and so these dithiolane, 2-alkyl-1,3_dithiolanes, 2-phenyl-1,3*Author to whom correspondence should be addressed. dithiolane and 2,2-dimethyl-1,3-dithiolane were 703

E. W. ABEL et al.

704

based on the original method of Gibson. lo Full details are given in our earlier papers. 8*9 Metal pentacarbonyl complexes. These were all prepared by reacting the appropriate ligand with [M(CO),THF] (M = Cr, MO, W; THF = tetrahydrofuran). The latter was prepared by UV irradiation of a solution of [M(CO),] (0.5 g) in THF (130 cm3) in an immersion reaction vessel and a 450 W Hanovia medium pressure Hg lamp. The solution was deoxygenated and purged with nitrogen before being irradiated for 12 h to produce a yellow solution of [W(CO),THF], or for 40 h to produce a deep orange solution of [Cr(CO),THF], or for 88 h to produce a yellow solution of wo(CO), THF]. The irradiated solution was slowly filtered into a Schlenk tube containing a stirred, ice-cold solution of the ligand (1 molar equivalent) dissolved in dichloromethane (20 cm’). The reaction mixture was stirred for 3 h whilst being allowed to warm up to room temperature. The solvent was removed and the residue dissolved in dichloromethane-petroleum spirit (1: 1) (20 cm3) and chromatographed with grade II alumina. The chromatogram was developed using dichloromethane-petroleum spirit (4@-6O”C)(1: 1) at room temperature, the product being eluted as a yellow band. The solvents were removed from the yellow fraction under reduced pressure and the yellow complex dried in uacuo. Analytical data for the complexes are given in Table The complexes [M(CO),(~HPh)] &I = Cr, W) remained as oils and were not isolated as analytically pure crystalline products.

NMR spectra

All spectra were obtained using a Bruker AM250 FT spectrometer operating at 250.1 MHz (‘H). All complexes were examined as solutions in either dichloromethane-dz (low temperatures) or 1,1,2,2tetrachloroethane-nitrobenzene-d5 mixtures (high temperatures). Proton shifts were measured relative to Me,$i (internal). Sample temperature variation was achieved by a standard variable-temperature unit, calibrated to read temperatures accurate to at least + 1°C. Band shape analyses employed the authors’ version of the DNMR program of Kleier and Binsch. 1’ IR spectra

The IR-active carbonyl stretching modes of these complexes were measured using a Perkin-Elmer Model 398 spectrophotometer. The complexes were examined as solutions in hexane and data are given in Table 2, where the assignments of the three fundamental stretching modes (2A 1+ E) are based on an assumed Cc point group. In most cases an additional band in the region 1975-1990 cm-’ was also observed due to the presence of a trace of M(C0)6 in the hexane solutions. RESULTS [M(CO),(mH,)]

(M = Cr, MO, W)

Room temperature ‘H NMR spectra of these complexes consisted of a singlet for the -SCH *S-

Table 1. Physical data for the complexes [M(CO),(aRR’)]

Complex”

IWCO) SC-H

Al

PWW&~HJl ]Mo(CO) 5(-H lW(C0) d~WPf-41

211

[WW4~W)Me)1 [WWM~MeJl

P-(C~M~Me,)I [W(COM=i%=U-Wu’)l D(~~M~U-Wu?1 [w(CO) d~@V’U ~~r(W5(~O-UPhll

Melting point (“C)

Yield (%)b

35 35 22 105 100 40 105 48 45

33.0 12.0 33.0 4.0 26.0 50.0 51.0 46.0 85.0 52.8 62.6

d d

Found C(%) H(%) 23.0 31.0 28.6 22.7 33.0 26.2 34.3 29.5 40.7 e e

1.7 1.2 1.5 1.2 1.7 2.2 2.0 2.9 4.0 e e -

a All are yellow in colour. ‘All yields are quoted relative to the M(C0)6 utilized. cDecomposed. dYellow oils. ‘Not determined.

Calculated C(%) H(%) 23.3 32.2 28.0 24.3 34.6 26.2 36.8 29.6 40.6 33.2 44.9

1.4 2.0 1.7 1.8 2.5 2.2 3.0 2.9 3.9 1.9 2.6

Group VI metal pentacarbonyl Table

2.

IR

data” for (-RR’)]

complexes

[M(CO),

Metal-carbonyl stretching wavenumbers v(C0) (cm- ‘)

Complex M

R

R

W Cr MO W Cr W Cr W Cr W Cr

H H H H H Me Me H H H H

H H H Me Me Me Me Bu’ Bu’ Ph Ph

E

‘4, 2070 2060 2080 2070 2070 2075 2060 2070 2070 2070 2070

m s s s s s s s s s s

A*

1940 m 1940 m 1950 w 1935 s 1950 w 1940m 1940 m 1935 s 1950 m 1945 w 1950 m

1935 m 1935 s 1945 m 1915 w 1940 m 1935 m 1935 m 1920 w 1940 s 1940 m 1940 m

705

complexes of 1,3-dithiolanes

the coordinated sulphur atom which renders the protons isochronous and also two -SCH$removes distinction between the geminal proton pairs of the -SCH2CH2Sgrouping. This is depicted in Fig. 1, where the two structures represent an enantiomeric pair, with the proton averaging being AB z$ BA and CDEFeDCFE. The correctness of this interpretation was confirmed by low-temperature measurements, since on cooling to ca -70°C the -SCHISsignal split into an AB quartet (Av = 129 Hz, *JAB = 10.4 Hz, for M = W and Av = 123 Hz, 2JAB = 10.4 Hz for M = MO), and the -SCH2CH2Ssignals became more complex, indicative of four different chemical shifts C,

"C

“E

)t--c &-

QJ----f “, “F

(OC),M

S

(1In n-hexane solutions. m = medium, s = strong, w = weak.

‘.

“, -

“0 “A

s

-

/

I

“8

protons and a symmetrical second order AA’BB’ type pattern for the -SCH2CH2Sprotons. Chemical shift data are given in Table 3. Such a spectral pattern is indicative of rapid inversion of

m:H, m(H)Me -Me, SC,H,SC(H)Bd

m(H)Ph

Complex

lWCW~HJ1 D-(W5(~H21 [Mo(CG) ,(-H&1 lW(CG) 4~WMell

[WW,~~WPfe~1

[WWQ5(~Me2)1 [Cr(CG)S(~Mez)l [W(CG),(~(H)Bu?l [Cr(CG)d~(H)Bu’)l

WPV,~~(Whll ICr(CG),(m(H)Ph)l

Temperature (“C)

--SCRR’S-

22 21 20 -80 21

3.9 4.60 1.80 4.51 5.63

Temperature (“C)

--SCR R’S

30 20 20 30 20 30 20 25 20 22 20

“A

Fig. 1. Enantiomerism in [M(CO),(mH,ll complexes as a result of pyramidal inversion of the coordinated sulphur. The labels A-F on the ring protons refer to the environments, not the identities of the protons.

Table 3. ‘H NMR chemical shifts (Sy for [(-RR’)]

Ligand

s

y?

(OC)$l

4.10 3.92 4.00 4.38 4.07 1.80 1.78 4.51 4.27 5.48 5.33

and [M(CO),(SmRR’)]

-SCRR’S3.9 1.59 1.80 1.01 7.42

-SCRR’S4.10 3.92 4.00 1.78 1.75 1.80 1.78 1.09 1.06 7.40 7.44

-SC,H,S3.22 3.30, 3.20’ 3.42 3.16, 3.15* 3.49, 3.33*

MSCH2-* 3.43 3.30 3.43 3.83 3.66 3.68 3.73 3.51 3.38 3.88 3.78

“Relative to Me,Si; solvent CD&&. *Chemical shifts for all AA’BB’ and ABCD spin systems have only been approximately measured.

-SCH$H,* 3.29 3.28 3.33 3.58 3.32 3.48 3.73 3.30 3.08 3.60 3.65

706

E. W. ABEL et al.

D, E and F. In the case of the chromium complex, the band shape changes were less clearly defined, probably as a result of accidental near equality of the geminal methylene shifts and the somewhat lower purity of the sample. Accurate expanded spectra of the --SCH$Ssignals of the molybdenum and tungsten complexes were recorded over the temperature range -25 to ca -90°C and the band shapes were computed as exchanging AB e BA systems. Good fits of experimental and computed spectra were obtained as

shown in Fig. 2 for the complex [w(CO), (mH1)]. Examination of the ‘H NMR spectra of [w(CO), (-Hz)] above room temperature revealed further changes in the band shapes due to the -SCH&H$protons (Fig. 3). The two methylene multiplets commenced to broaden and proteed towards coalescence at ca 115°C as the temperature was raised. However, above ca 80°C decomposition became significant but spectra for band shape analysis were acquired up to 105°C (Fig.

-60

-25

r

170 Hz

1

Fig. 2. Variable temperature 250 MHz ‘H NMR spectra of [w(CO),(mH,)])] --SCH,S-region, with the computer synthesized spectra alongside.

showing the

Group VI metal pentacarbonyl

707

complexes of 1,3-dithiolanes

I# \

I

50 Hz

61.0

I

Fig. 3. Variable temperature 250 MHz ‘H NMR spectra of [w(CO),(mH,)] showing the -SCH,CH,Sregion and the effects of the 1,3-shift. Theoretical spectra are shown alongside. 3). Spectra up to ca 80°C were fully temperature reversible and clearly indicative of an intramolecular 1,3-shift fluxion which caused averaging of the two ring methylene environments. The kinetics of this process were compu~d on Lhe bti of an exchanging AA’BB’ Z$ BB’AA’ spin system where

Attempts to repeat these experiments with the chromium complex were unsuccessfu‘uldue to the production of paramagnetic species, and in the case of the molybdenum complex, because of the very ,~a33 chemica3 shift djfference (v~ -yBj, 0f 53~ adjacent methylene protons.

the chemical shifts A, B represent inversion-averaged u&es and
Low and high temperature NMR studies were performed only on the tungsten complex of 2,2-

708

E. W. ABEL et al, Table 4. Static ‘H NMR parameters used for calculating energy barriers for the 1,3-metal shift Temperature (“0

Complex lW(CO) 5(-H lW(Co)~(~Me2)1

70 50

211

(2,

(l-z,P

‘JAN (Hz)

*JBF (Hz)

823.0 865.0

864.0 911.0

-6.1 -6.9

-6.2 -7.0

‘JAB (Hz)’

‘JAB (Hz)~

T2* (s)

12.3 13.9

N0 N0

6.0 2.0

aVA=vA% bVg= vg’. c ‘JAW = 2JA,B(transvicinal coupling). d ‘JAB = 2JAw(cis vicinal coupling).

dimethyl- 1,3-dithiolane, as the chromium complex was far less stable in solution particularly at ambient temperatures and above. Even in the case of the tungsten complex the band shape changes were less well defined than in the unsubstituted 1,3-dithiolane complexes due to close similarities of chemical shifts. For instance, the geminal C-methyls were only just resolved at < 70°C on a 400 MHz spectrum, whereas the ring methylene signals consisted of an exchange-broadened ABCD type spectrum with three of the shifts very similar. The line widths of the signals were at a maximum at ca -70°C (203 K) from which approximate values of the rate constant and AGt parameter for the pyramidal inversion process at this temperature could be deduced. At above-ambient temperatures and AA’BB’ system of the -SCH2CH2Sprotons again broadened due to the onset of the 1,3-shift process and band shape fittings were carried out in an analogous way to that used for the 1,3-dithiolane complexes (See Table 4 for the static parameters used). [M(CO),(SmHR)] Bu’, Ph)

(M = Cr,

W;

R = Me,

In 2-substituted 1,3-dithiolane complexes, the coordinated sulphur is a chiral centre and pyramidal inversion produces a distinct diastereomer (Fig. 4) rather than a mirror-image enantiomer as

w C3\_ H.

(OC),M

HC

‘-‘A

R

HO HE

S

c\J_~

HJ

in the case of the unsubstituted 1,3-dithiolanes (Fig. 1). All the ring proton environments in both species are different and thus complex absorption patterns due to these protons are expected. [M(CO),(mHMe)]

At room temperature the ‘H NMR spectra of both complexes comprise a methyl 1 : 1 doublet, a methine 1 : 3 : 3 : 1 quartet and a complex symmetrical AA’BB’ type pattern for the ring methylenes (Table 3). On cooling to ca - 100°C in CD2Cl, solvent, the methylene region changed to a very complex pattern of lines associated with the strongly coupled four-spin systems of each invertomer (Fig. 4). At this limiting low temperature the methyl proton absorption consisted of two doublets in an intensity ratio 63 : 37% for the tungsten complex. The more intense signal was attributed to the tram invertomer, i.e. the structure with M(C0)5 tram to the ring methyl. The cis-tram distinction became less clear on raising the temperature somewhat and the coalescence temperature of the methyl signals was at ca -80°C from which an approximate value of AGt (193 K) was calculated from the Eyring equation. Warming the complexes to ambient temperatures was expected to change the two ABCD type patterns of the -SC$H$protons to a single ABCD spectrum, due to rapid inversion of the coordinated sulphur. However, the observed outcome of this temperature elevation was an AA’BB’ pattern of lines. Such a change can only be accommodated by assuming the onset of pyramidal sulphur inversion and/or 1,3-metal-sulphur commutation. Such an interpretation implies a considerable lowering of the energy barrier for the 1,3-metal shift process compared to the unsubstituted and 2,2-disubstituted 1,3-dithiolane complexes. An analogous observation was recorded for the 1,3-shifts of the [M(CO),(SCHMeSCHMeSCHMe)] complexes (M = Cr, W)2-4 compared to the unsubstituted trithian ligand complexes [M(CO),

* (oc)5Jsy~,s HK R

Fig. 4. Diastereomerism in [M(CO),(mHR)] complexes as a result of pyramidal inversion of the coordinated sulphur. Structure 1 is the thermodynamically favoured species.

(M = Cr, W)

Group VI metal pentacarbonyl

(SCH,SCH,SCH,)]. *s3 In that work it was suggested that steric factors confined the ring methyls to equatorial positions and the M(CO)s to an axial position with respect to the six-membered ring, with the other possible conformational structures unfavoured and undetected by NMR. For the fixed axial M(CO)s moiety the other sulphur lone pairs were ideally positioned to facilitate a 1,3-metal shift. In contrast, in the unsubstituted M(CO)rtrithian complexes, rapid sulphur inversion and ring reversal alters the relative positioning of M(C0)5 and the sulphur lone pairs and reduces the probability of the 1,3-shift. In the present [M(CO), (=HMe)] complexes similar arguments appear valid. In other words, the M(C0)5 moiety adopts a tram relationship with respect to the ring methyl and comparable amounts of energy are required to invert the configuration of the coordinated sulphur and to produce a 1,3-metal-sulphur shift without inversion of configuration. The two dynamic processes therefore cannot be separated. The pyramidal inversion process is assumed to proceed via the trigonal planar intermediate,” whereas the 1,3-shift must involve a weakly bound pseudoseven-coordinate metal intermediate which can be most readily envisaged if the five-membered ring adopts a half-chair puckered configuration’ 3-1’ (Fig. 5) although the classical envelope structure or some intermediate between these two extreme forms is almost equally probable.16 This dual fluxional process is rapid at room temperature for both the chromium and tungsten complexes. Both complexes tended to dissociate at temperatures above ca 30°C as evidenced by a colour change from yellow to green and the appearance of free ligand signals in the spectra. In contrast, in the complexes of -Hz and S-Me*, where the pyramidal inversion process does not produce distinct diastereomers but simply chemical enantiomers, the inversion process proceeds readily, and the 1,3-shifts only occur with

709

complexes of 1,3-dithiolanes

relative difficulty at temperatures where the inversion process is rapid, causing the other sulphur lone pair to be less favourably positioned than in the mono-substituted ligand complexes [M(CO),(mHR)] Ph)

(M = Cr, W; R = Bu’,

These complexes exhibited similar variable-temperature NMR spectral features to the 2-methyl 1,3-dithiolane complexes. The significant difference was that because of the greater steric factors associated with Bu’ and Ph groups compared to Me, only a single solution species was detected at low temperatures. This is confidently attributed to the structure with the M(C0)5 group tram to the ring substituent. Since only one invertomer species is thermodynamically favoured, the sulphur inversion process cannot be monitored. The variable-temperature spectra imply that any tendency of the coordinated sulphur atom to invert its configuration leads to a 1,3-shift (via the pseudo-sevencoordinate intermediate, Fig. 5) and/or ligand dissociation. DISCUSSION The results of the NMR band shape and coalescence temperature measurements are collected in Table 5. Most of the data refer to tungsten complexes which were most thermally stable in solution. Despite the somewhat limited amount of data certain clear points of discussion arise. Firstly, for the unsubstituted and symmetrical 2,2_disubstituted 1,3-dithiolanes, sulphur inversion and 1,3-metal shifts are quite separate, uncorrelated processes, AGf (298 K) values being in the region of 40-44 and 78 kJ mol- ‘, respectively. Secondly, in the case of 2-substituted 1,3-dithiolane complexes, the steric constraints of the 2-substituent favour the 1,3-shift mechanism and cause

Fig. 5. The proposed mechanism of the 1,3-metal shift fluxion in [M(CO),(mHR)] complexes via the pseudo-seven-coordinate metal intermediate. The carbonyls are labelled to indicate the stereochemical rigidity of the M(CO), moiety.

A

z)l

[w(CO) 5(-H

4~MeAl [w(CO) 5(~Me211

1,3-shift

1,3-shift

[W(CO),($CH,SCH,SCH,S~H,)l

[W(CO),(SCHMeSCHMeSCHMe)]

[W(CO),(MeSeCH,SeMe)]

rlCoalescence temperature value. bValue for W + Se shift. No data available for W c S.

39.0f 1.7 84.7k3.1

S inversion 1,3-shift

Iw(co),(SCH,SCH,S~.H31

70.7 + 3.7

52.5 +0.7 78.4f 1.6

S inversion 1,3-shift

[WPM~HMdl

82.6 f 3.4

42.5 +0.9

S inversion 1,3_shift/dissociation

1,3-shift

S inversion

[w(CO)

S inversion

54.8k1.3 69.8* 1.7

1,3-shift

AHf (kJ mol- ‘)

S inversion

Process

[Mo(CQ ,(-%)I

PWO)

4-H

Complex 36k6

7fl2

-8+9 3f9

1.8k2.8 0.8k4.6

16f 10

4+4

-23+5

85.8&o.5b

68.47 &-0.02

41.3* 1.0 83.5kO.6

53.OkO.l 78.3 f0.3

- 40.5” (193 K)

77.8f0.5

- 40.2“ (203 K)

41.2f0.4

78.OkO.3

44.1 f0.4

AGf(298 K) (kJ mol- ‘)

complexes

AS$ (J K-’ mol- ‘)

Table 5. Eyring activation parameters for fluxional processes in [M(CO),(mRR’)]

17

2

5 5

2 3

This work

This work

This work

This work

This work

This work

Reference

$ F b %

3

.m

Group VI metal pentacarbonyl complexes of 1,3-dithiolanes

this fluxion to be indistinguishable in NMR terms from the sulphur inversion process. An analogous lowering of the activation energies of 1,3-metal shifts was observed in the six-membered trithian complexes [w(CO)&CH,SCH,SCH,)] and [W(CO),(SCHMeSCHMeSCHMe)] (Table 5). Thirdly, the significantly lower sulphur inversion energy for the molybdenum complex compared to the tungsten complex is in line with more extensive data’ which reflect combined metal electronegativity and (pd)s ligand-metal interactions. In this present work we were unable to measure a sulphur inversion energy for [Cr(CO),(S~H,)] due to solution decomposition, but a AGf value of 44.5 f 1.5 kJ mall ’ has recently been reported elsewhere. ” This value, however, is of questionable accuracy as it is based on a simplified DNMR analysis of a solution which, the authors claim, consists of both mono- and bis-Cr(CO), complexes of 1,3dithiolane plus free ligand. Nevertheless, the lower limit of this value would bring it in line with the expected trend in inversion energies, namely MO < Cr < W. Fourthly, no clear dependence of ligand ring size on the inversion energy is apparent in this series of W(CO)s complexes. This is in contrast to the series [MX,(@?H,),},] (M = Pd”, Pt”, II = 3-5)19 where a decrease in ring size constrains access to the planar transition state and thus increases the activation energy of sulphur inversion. In the present work the AGf (298 K) value for is anomalously low, but this [w(CO) 5-H 211 may simply indicate that comparisons between ligands with different numbers of heterocyclic atoms are not valid. In contrast, the 1,3-shift energies do appear to show a clear dependence on ring size, the order being, five-membered < sixmembered c eight-membered < open chain. This reflects the previously noted3*5 dependence on ligand flexibility, although it should be noted that the difference in shift energies between five- and sixmembered rings is very slight (Table 5). A further point of interest in these fluxional complexes concerns the nature of the W(CO), moiety, particularly during the 1,3-shifts. This has already been investigated in the cases of the complexes /W(CO)sL] (L = MeSCH#Me, SCH MeSCHMeSCHMe and mMe2) by twodimensional exchange NMR spectroscopy (2DEXSY).” No carbonyl scrambling was noted either in the absence or presence of 1,3-shifts, implying a

711

remarkable stability of the square pyramidal geometry of W(CO)s. Clearly this moiety remains bonded to the ligand in some formal sense throughout the metal shift process (Fig. 5). No evidence for a highly fluxional “naked” W(CO)s moiety2’ is available and the 1,3-shift can be envisaged as simply a lateral movement of a rigid W(CO)5 moiety.

REFERENCES 1. E. W. Abel, S. K. Bhargava and K. G. Orrell, Prog. Inorg. Chem. 1984,32, 1. 2. E. W. Abel, M. Booth, K. G. Orrell and G. M. Pring, J. Chem. Sot., Dalton Trans. 1981, 1944. 3. E. W. Abel, G. D. King, K. G. Orrell, G. M. Pring and V. Sik, Polyhedron 1983, 2, 1117. 4. W. A. Schenk and M. Schmidt, 2. Anorg. Allg. Chem. 1975,416,311. 5. E. W. Abel, G. D. King, K. G. Orrell and V. Sik, Polyhedron 1983, 2, 1363. 6. E. W. Abel, P. K. Mittal, K. G. Orrell and V. Sik, J. Chem. Sot., Dalton Trans. 1985, 1569. 7. K. M. Higgins, PhD Thesis, University of Exeter (1984). 8. E. W. Abel, K. G. Orrell, K. B. Qureshi and V. Sik, Polyhedron 1988, 7, 1321. 9. E. W. Abel, K. G. Qrrell, K. B. Qureshi and V. Sik, Polyhedron 1988, 7, 1329. 10. D. T. Gibson, J. Chem. Sot. 1930, 13. 11. D. A. Kleier and G. Binsch, DNMR3 Program 165. Quantum Chemistry Program Exchange. Indiana University (1970). 12. J. B. Lambert, Top. Stereochem. 1971,6, 19. 13. K. S. Pitzer and W. E. Donath, J. Am. Chem. Sot. 1959,81,3213. 14. F. V. Brutcher, Jr., T. Roberts, S. J. Barr and N. Pearson, J. Am. Chem. Sot. 1959,81,4915. 15. F. V. Brutcher Jr and W. Bauer, J. Am. Chem. Sot. 1962,84,2233. 16. L. A. Stemson, D. A. Coviello and R. S. Egan, J. Am. Chem. Sot. 1971,93,6529. 17. E. W. Abel, T. E. MacKenzie, K. G. Orrell and V. Sik, Polyhedron 1987,6, 1785. 18. A. Gryff-Keller and P. Szczecinski, Spectroscopy (Ottawa) 1988,6,41. 19. E. W. Abel, M. Booth and K. G. Orrell, J. Chem. SOL, Dalton Trans. 1979, 1994. 20. E. W. Abel, I. Moss, K. G. Orrell, K. B. Qureshi, V. Sik and D. Stephenson, J. Chem. Sot., Dalton Trans. 1988, 1489. 21. J. P. Jesson and P. Meakin, J. Am. Chem. Sot. 1973, 95, 1344.