DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1999, 691–698 691 Oxidatively induced isomerisation of vinylidene ligands to alkynes: ESR spectra of paramagnetic vinylidene and alkyne arene metal complexes Ian M. Bartlett,a Neil G. Connelly,a Antonio J. Martín,a A. Guy Orpen,a Timothy J. Paget,a Anne L. Rieger b and Philip H. Rieger b a School of Chemistry, University of Bristol, Bristol, UK BS8 1TS. E-mail: neil.connelly@bristol.ac.uk b Department of Chemistry, Brown University, Rhode Island, RI 02912, USA Received 3rd December 1998, Accepted 27th January 1999 UV irradiation of [M(CO)3(h-arene)] and Me3SiC]] ] CSiMe3 gives [M(CO)2{C]] C(SiMe3)2}(h-arene)] (M = Cr, arene = C6H2Me4-1,2,3,5 2V, C6H3Me3-1,2,3 3V or C6H6 4V; M = Mo, arene = C6Me6 5V or C6H3Me3-1,3,5 6V).The crystal structure of [Cr(CO)2{C]] C(SiMe3)2}(h-C6H6)] 4V confirms the presence of the vinylidene ligand; the complex has approximate Cs symmetry with the C(SiMe3)2 plane orthogonal to the arenecentroid–Cr–Ca–Cb plane.Voltammetry and IR and NMR spectroscopy show that in solution [Mo(CO)2{C]] C(SiMe3)2}(h-C6H3Me3-1,3,5)] 6V thermally equilibrates with the alkyne isomer [Mo(CO)2(h-Me3SiC]] ] CSiMe3)(h-C6H3Me3-1,3,5)] 6A. The vinylidene complexes [M(CO)2{C]] C(SiMe3)2}(h-arene)] 2V–6V undergo one-electron oxidation to the alkyne cations [M(CO)2- (h-Me3SiC]] ] CSiMe3)(h-arene)]1 2A1–6A1 via fast, redox-induced vinylidene-to-alkyne isomerisation. These cations are reduced to the neutral alkyne complexes [M(CO)2(h-Me3SiC]] ] CSiMe3)(h-arene)] 2A–6A which slowly isomerise thermally to the neutral vinylidene complexes 2V–6V.Paramagnetic vinylidene and alkyne complex cations have been characterised by ESR spectroscopy; unpaired electron density is extensively delocalised from the metal centre to the C2 ligand, in agreement with the results of EHMO calculations. Introduction We have recently presented 1 the results of a detailed study of the mechanism of the redox-induced isomerisation processes linking the vinylidene complex [Cr(CO)2{C]] C(SiMe3)2}- (h-C6Me6)] 1V with the alkyne cation [Cr(CO)2(h-Me3SiC]] ] CSiMe3)(h-C6Me6)]1 1A1, quantifying the kinetics of the conversion of 1V1 into 1A1 and of 1A into 1V.We have also noted that related alkyne cations [Cr(CO)2(h-RC]] ] CR)(h-C6Me6)]1 (R = Ph, C6H4OMe-4, etc.) can be made directly from the neutral complexes [Cr(CO)2(h-RC]] ] CR)(h-C6Me6)],2,3 and have made a brief comparison of the structures of the redox-related pair [Cr(CO)2(h-PhC]] ] CPh)(h-C6Me5H)]z (z = 0 or 1).4 We now describe the characterisation of a wider series of vinylidene complexes, [M(CO)2{C]] C(SiMe3)2}(h-arene)] (M = Cr, arene = C6H2Me4-1,2,3,5 2V, C6H3Me3-1,2,3 3V or C6H6 4V; M = Mo, arene = C6Me6 5V or C6H3Me3-1,3,5 6V), including the crystal structure of [Cr(CO)2{C]] C(SiMe3)2}(h-C6H6)] 4V, and their oxidative isomerisation to the corresponding alkyne cations. Moreover, ESR spectroscopic analysis and EHMO calculations have provided insight into the electronic structure of a unique set of paramagnetic vinylidene and alkyne complex isomers.Results and discussion Synthesis of [M(CO)2{C]] C(SiMe3)2}(Á-arene)] (M 5 Cr or Mo) The chromium and molybdenum complexes [M(CO)2- {C]] C(SiMe3)2}(h-arene)] [M = Cr, arene = C6H2Me4-1,2,3,5 2V, C6H3Me3-1,2,3 3V or C6H6 4V; M = Mo, arene = C6Me6 5V or C6H3Me3-1,3,5 6V) (Chart 1) were prepared by a method analogous to that previously described 1 for [Cr(CO)2- {C]] C(SiMe3)2}(h-C6Me6)] 1V, namely by the UV irradiation of a mixture of [M(CO)3(h-arene)] and Me3SiC]] ] CSiMe3 in solution; the tungsten complexes [W(CO)3(h-arene)] (arene = C6H3Me3-1,3,5 and C6Me6) did not react with Me3SiC]] ] CSiMe3 under the same conditions.The solvent used for the photolysis reaction influences both the yield and reaction time. Thus, complex 2V was prepared in good yield by irradiation in thf whereas the use of this solvent for the less stable complexes 3V, 4V and 6V resulted in extensive decomposition and n-hexane was found to give higher yields.The molybdenum complex [Mo(CO)3(h-C6Me6)] reacted very slowly with Me3SiC]] ] CSiMe3 in n-hexane or thf (ca. 10% conversion in thf after 15 h) but the reaction rate was significantly increased in benzene or toluene (ca. 70% conversion in benzene after 15 h). During the preparation of the vinylidene complex [Mo(CO)2{C]] C(SiMe3)2}(h-C6H3Me3-1,3,5)] 6V the formation of a second product, shown below to be the alkyne isomer [Mo(CO)2(h-Me3SiC]] ] CSiMe3)(h-C6H3Me3-1,3,5)] 6A, was observed; pure samples of 6V were, however, isolated by slow crystallisation from n-hexane at 220 8C.M OC C C O C SiMe3 SiMe3 M OC C O CSiMe C SiMe3 Rn Rn V A Complex M Rn 1V/1A Cr Me6 2V/2A Cr 3V/3A Cr 4V/4A Cr 5V/5A 6V/6A Mo Mo Me6 H2Me4–1,2,3,5 H3Me3–1,2,3 H6 H3Me3–1,3,5 Chart 1 Numbering of vinylidene and alkyne complexes.692 J. Chem.Soc., Dalton Trans., 1999, 691–698 Table 1 Analytical, electrochemical and IR spectroscopic data for chromium and molybdenum arene vinylidene complexes Yield Analysis a (%) IRb/cm21 Complex [Cr(CO)2{C]] C(SiMe3)2}(h-C6Me6)] e 1V [Cr(CO)2{C]] C(SiMe3)2}(h-C6H2Me4-1,2,3,5)] 2V [Cr(CO)2{C]] C(SiMe3)2}(h-C6H3Me3-1,2,3)] 3V [Cr(CO)2{C]] C(SiMe3)2}(h-C6H6)] 4V [Mo(CO)2{C]] C(SiMe3)2}(h-C6Me6)] 5V [Mo(CO)2{C]] C(SiMe3)2}(h-C6H3Me3-1,3,5)] 6V Colour Orange Orange Orange Red Orange Orangered (%) — 39 38 22 27 19 C — 58.3 (58.2) 57.4 (57.3) 54.0 (53.9) 54.5 (54.5) 51.3 (51.6) H — 7.7 (7.8) 7.8 (7.6) 7.2 (6.8) 7.5 (7.5) 6.9 (6.9) n(CO) 1925, 1872ms 1933, 1881ms 1913, 1849ms d 1937, 1886ms 1918, 1855ms d 1949, 1900ms 1929, 1869ms d 1928, 1871ms 1905, 1834ms d 1940, 1884ms 1917, 1848ms d 1915w, 1851w f 1898w, 1818w f,d n(C]] C) 1567m 1576m 1575 d 1576m 1576 d 1586m 1585 d 1567m 1566 d 1578m 1578 d —— Ec,d/V 0.18 (20.24) 0.25 (20.13) 0.28 (20.13) 0.34 (20.03) 0.35 (20.01) 0.44 (0.13) a Calculated values in parentheses.b Strong (s) absorptions in n-hexane unless otherwise stated, m = medium, w = weak. c Peak potential, Epk, of the irreversible oxidation wave; potential for the reversible product wave in parentheses. d In CH2Cl2. e From ref. 1. f Bands for the isomeric alkyne complex [Mo(CO)2(h-Me3SiC]] ] CSiMe3)(h-C6H3Me3-1,3,5)] 6A. Table 2 Proton and 13C-{1H} NMR spectroscopic data for arene chromium and molybdenum complexes a Compound 2V 1H 0.35 (18H, s, SiMe3), 1.60 (3H, s, Me2 or Me5), 1.75 (3H, s, Me2 or Me5), 1.78 (6H, s, Me1,3), 4.55 (2H, s, H4,6) 13C-{1H} 2.7 (SiMe3), 14.7 (Me5), 19.6 (Me1,3), 19.8 (Me2), 98.5 (CH4,6), 105.7 (Cb), 107.7 (CMe5), 110.8 (CMe2), 112.4 (CMe1,3), 239.5 (CO), 349.2 (Ca) 3V 0.38 (18H, s, SiMe3), 1.63 (3H, s, Me2), 1.77 (6H, s, Me1,3), 4.65 (2H, d, JH4H5 and JH5H6 6, H4,6), 4.75 (1H, t, JH4H5 and JH5H6 6, H5) 2.6 (SiMe3), 14.9 (Me2), 19.6 (Me1,3), 96.6 (CH5), 97.2 (CH4,6), 106.4 (Cb), 110.1 (CMe2), 112.0 (CMe1,3), 239.3 (CO), 348.3 (Ca) 4V 0.36 (18H, s, SiMe3), 4.66 (6H, s, C6H6) 2.2 (SiMe3), 96.0 (C6H6), 107.7 (Cb), 238.7 (CO), 347.5 (Ca) 5V 0.41 (18H, s, SiMe3), 1.89 (18H, s, C6Me6) 2.4 (SiMe3), 17.8 (CMe), 101.8 (Cb), 114.2 (CMe), 231.4 (CO), 331.8 (Ca) b 6V 0.39 (18H, s, SiMe3), 1.79 (9H, s, CMe), 4.79 (3H, s, CH) 2.5 (SiMe3), 20.5 (CMe), 98.8 (CH), 100.2 (Cb), 116.4 (CMe), 229.4 (CO), 333.4 (Ca) 6A 0.41 (18H, s, SiMe3), 1.71 (9H, s, CMe), 4.68 (3H, s, CH) 1.7 (SiMe3), 20.2 (CMe), 93.5 (CH), 107.1 (CSiMe3), 110.1 (CMe), 237.4 (CO) a Chemical shift (d) in ppm, J values in Hz, spectra in C6D6 unless stated otherwise.b In CDCl3. There is an extensive array of complexes of the type [Cr(CO)2L(h-arene)] (e.g. L = PR3, RC]] ] CR, R2C]] CR2, RCN, py, CS, N2, thf, etc.),5 but 5V and 6V are rare examples of the molybdenum analogues [Mo(CO)2L(h-arene)]. Generally, carbonyl substitution does not occur when [Mo(CO)3(h-arene)] is treated with L, either thermally or photochemically, mainly because the Mo–Carene bonds are easily cleaved.Indeed, replacement of the arene in [Mo(CO)3(h-arene)] by L provides a convenient and general route to fac-[Mo(CO)3L3].6 Characterisation of the vinylidene complexes 2V–6V Complexes 2V–6V are orange to red crystalline solids the air stability of which decreases from chromium to molybdenum and with decreasing substitution at the arene; solutions of 4V and 6V decompose in air over several minutes.The complexes were characterised by elemental analysis and by IR (Table 1) and NMR spectroscopy (Table 2). The IR spectra show two strong carbonyl bands (between 1834 and 1949 cm21) and a third, weaker, band at lower energy (1566 to 1586 cm21); the last is assigned to n(C]] C) and is typical of a vinylidene complex.7 Both n(CO) and n(C]] C) for 1V–6V move to lower energy as methylation at the arene is increased although changing the metal from Cr to Mo has little eVect.Increased methylation causes the arene to become a better electron donor so that back donation from M to p* CO and C]] CR2 orbitals increases. The vinylidene ligand is generally considered to be a good, essentially single-faced p-acceptor 8 and the vinylidene ligand of [Mn(CO)2(C]] CHPh)(h-C5H5)] has been classified as a better p acceptor than CO.9 The carbonyl bands of 1V–6V are considerably lower in energy than those of [Cr(CO)2(C]] CR2)(h-arene)] (R = Me or Ph) 10 (e.g. 1980 and 1930 cm21 for [Cr(CO)2(C]] CPh2)(h-C6H6)]). Thus the p-accepting ability of the vinylidene ligands follows the order: C]] CPh2 > C]] CMe2 @ C]] C(SiMe3)2. Since b-silyl groups can stabilise carbocations,11 this stabilising eVect may also decrease the electrophilicity of Ca of the vinylidene, so reducing the acceptor ability of the ligand. Uniquely, the IR spectrum of pure 6V in CH2Cl2 changed with time, two extra bands appearing at 1898 and 1818 cm21.These bands continued to grow for ca. 1 h at which point equilibrium appeared to be reached (see below). The new bands are lower in energy than those of 6V (1917, 1848 cm21), consistent with the formation of the alkyne isomer [Mo(CO)2(h-Me3SiC]] ] CSiMe3)(h-C6H3Me3)] 6A. A similar diVerence was noted 1 between the bands of [Cr(CO)2{C]] C(SiMe3)2}(h-C6Me6)] 1V (1901, 1834 cm21) and those of [Cr(CO)2(h-Me3SiC]] ] CSiMe3)- (h-C6Me6)] (1A) (1874 and 1799 cm21).NMR spectroscopy The 1H NMR spectra of 2V–6V (Table 2) do not allow a distinction to be made between the presence of vinylidene or alkyne ligands. Each complex shows a single resonance for the methyl protons of the SiMe3 groups and the expected signals for the arene protons and methyl substituents. However, the diVerent C2 ligands can be distinguished by the characteristic 13C-{1H} NMR resonances of the vinylidene fragment. Thus Ca, the highly deshielded carbon atom bonded directly to the metal, is typically observed between d 258 and 382 7 whereas Cb, much less deshielded, is typically observed between d 87 and 143.For the chromium complexes 2V–4V Ca appears as a weakJ. Chem. Soc., Dalton Trans., 1999, 691–698 693 resonance between d 347 and 350; in the molybdenum complexes 5V and 6V, Ca is slightly less deshielded, at d 332 and 334 respectively. The signal for Cb is also very weak but is observed between d 105 and 108 in 2V–4V and at d 102 and 100 in 5V and 6V respectively.Similar shifts for Ca and Cb were reported for [Cr(CO)2(C]] CR2)(h-C6H6)]; Ca is at d 313 (R = Me) and 328 (R = Ph), and Cb is at d 134 (R = Me) and 132 (R = Ph).10 The room temperature 1H and 13C-{1H} NMR spectra of 6V also show the presence of the alkyne isomer [Mo(CO)2(h-Me3- SiC]] ] CSiMe3)(h-C6H3Me3)] 6A (Table 2); integration of the alkyne and vinylidene peaks in the 1H spectrum provides an estimated ratio of 5 : 1 for the vinylidene to alkyne isomers.The crystal structure of [Cr(CO)2{C]] C(SiMe3)2}(Á-C6H6)] 4V The 13C NMR and IR spectra provide strong evidence for the formulation of 1V–6V as vinylidene complexes. However, the crystal structure of 4V was determined in order to verify the structure and compare the structural parameters with those of other vinylidene complexes. Crystals of [Cr(CO)2{C]] C(SiMe3)2}(h-C6H6)] 4V were grown from a concentrated n-hexane solution cooled to 220 8C for 16 h.The molecular structure of 4V is shown in Fig. 1 and Fig. 1 The molecular structure of [Cr(CO)2{C]] C(SiMe3)2}(h-C6H6)] 4V showing the atom labelling scheme. Hydrogen atoms have been omitted for clarity. Table 3 Selected bond lengths (Å) and angles (8) for [Cr(CO)2- {C]] C(SiMe3)2}(h-C6H6)] 4V Cr–C(1) Cr–C(2) Cr–C(3) Cr–C(4) Cr–C(5) Cr–C(6) ave. Cr–Carene a Si(1)–C(11) Si(1)–C(12) Si(1)–C(13) Si(2)–C(14) Si(2)–C(15) Si(2)–C(16) Cr–C(9)–C(10) C(9)–C(10)–Si(1) C(9)–C(10)–Si(2) Cr–C(7)–O(1) Cr–C(8)–O(2) 2.229(6) 2.204(7) 2.206(8) 2.230(8) 2.215(7) 2.228(6) 2.216(8) 1.848(7) 1.854(7) 1.834(9) 1.832(8) 1.814(11) 1.810(10) 176.5(4) 119.3(4) 117.1(4) 179.5(7) 177.1(6) Cr–C(7) Cr–C(8) ave.Cr–CO Cr–C(9) C(9)–C(10) C(10)–Si(1) C(10)–Si(2) ave. C(10)–Si(Me) C(7)–O(1) C(8)–O(2) Si(1)–C(10)–Si(2) C(7)–Cr–C(8) C(7)–Cr–C(9) C(8)–Cr–C(9) 1.837(7) 1.838(9) 1.838(9) 1.854(5) 1.297(7) 1.872(5) 1.883(5) 1.878(5) 1.155(7) 1.145(8) 123.6(3) 85.9(3) 90.5(3) 86.2(3) a The error given for the mean values is the largest individual standard deviation in the set of values averaged.important bond lengths and angles are listed in Table 3. The benzene ligand eVectively occupies three co-ordination sites of a pseudo-octahedral geometry; the Cr–Carene distances are in the range 2.204–2.230 Å. The metal–carbonyl angles are essentially linear and the Cr–C(O) lengths [average 1.838(9) Å] are similar to those of other neutral arene chromium complexes such as [Cr(CO)3(h-C6H6)] (1.844 Å).12 The almost linear Cr–C(9)–C(10) angle [176.5(4)8] and the short distances Cr– C(9) [1.854(5) Å] and C(9)–C(10) [1.297(7) Å] are consistent with the presence of metal–carbon and carbon–carbon double bonds, as observed in other vinylidene complexes.7 The geometry around C(10) (Cb) is essentially trigonal (angles 117.1– 123.68), consistent with sp2 hybridisation. The large size of the SiMe3 group is reflected in the Si(1)–C(10)–Si(2) angle [123.6(3)8].The silyl atoms of the SiMe3 substituents of the vinylidene ligand lie in a plane perpendicular to an approximate mirror plane lying through AR–Cr–Ca-Cb (AR = centre of the arene), the orientation observed for the isoelectronic complex [Mn- (CO)2(C]] CMe2)(h-C5H5)].13 Molecular orbital studies on [Mn- (CO)2(C]] CR2)(h-C5H5)] have been carried out by HoVmann14 and Fenske.15 The valence orbitals in the metal fragment [Mn(CO)2(h-C5H5)] are derived from those of [Mn(CO)3- (h-C5H5)]; the two most important orbitals involved in bonding to the vinylidene ligand are shown in Fig. 2a. Two symmetrical orientations are possible for the vinylidene, coinciding with the symmetry plane (vertical, the xz plane in Fig. 2b) or bisecting the symmetry plane (horizontal, the yz plane in Fig. 2c). In the horizontal orientation the p and p* orbitals on the vinylidene interact with the metal-based orbitals of a9 symmetry, and the vacant vinylidene C(p) orbital interacts with the metal orbitals of a0 symmetry. In the vertical orientation the combinations are reversed.Thus, the C–C p and p* orbitals overlap with the metal orbitals a0 and the C(p) orbital overlaps with the metal a9 orbitals. For the complexes [M(CO)2(C]] CR2)(h-C5H5)]z1 (M = Fe, z = 1; M = Mn, z = 0) the interaction of the p orbital on Ca with the metal a0 orbital sets the ground state orientation as horizontal (Fig. 2a). However, this orientation is only marginally lower in energy than the vertical orientation and the energy barrier to rotation of the vinylidene ligand is ca. 17 kJ mol21 for [Mn(CO)2(C]] CR2)(h-C5H5)].15 The horizontal orientation appears to be similarly favoured in the arenechromium complex 4V. Electrochemistry The CVs of 2V–6V, at a platinum electrode in CH2Cl2 and at a scan rate of 200 mV s21, are similar to that of 1V,1 showing an irreversible oxidation wave, in the range 0.18 to 0.44 V, (Table 1) accompanied by a reversible product wave, between 20.24 and 0.13 V.Decreasing methylation of the arene increases the potential of both the irreversible oxidation wave and the Fig. 2 (a) Important orbitals of the M(CO)2(h-C5H5) fragment; (b) horizontal and (c) vertical orientations for complexes [M(CO)2- (C]] CR2)(h-C5H5)]z (M = Mn, z = 0; M = Fe, z = 1).694 J. Chem. Soc., Dalton Trans., 1999, 691–698 reversible product wave, by ca. 30 mV per methyl substituent, reflecting the reduced electron density at the metal.The molybdenum complexes are significantly more diYcult to oxidise than their chromium analogues (by ca. 170 mV). The irreversible oxidation wave corresponds to the formation of the cationic vinylidene complex [M(CO)2{C]] C(SiMe3)2}- (h-arene)]1 and its isomerisation to the cationic alkyne complex [M(CO)2(h-Me3SiC]] ] CSiMe3)(h-arene)]1. That the product wave is reversible shows that the isomerisation of [M(CO)2- (h-Me3SiC]] ] CSiMe3)(h-arene)] to [M(CO)2{C]] C(SiMe3)2}- (h-arene)] is slow on the CV timescale.The more positive potential for the oxidation of the vinylidene complex (cf. that for the alkyne complex) reflects a more electron-deficient metal centre. As noted above, NMR and IR spectroscopy suggested that the vinylidene complex [Mo(CO)2{C]] C(SiMe3)2}(h-C6H3Me3- 1,3,5)] 6V slowly equilibrates with the alkyne isomer [Mo(CO)2- (h-Me3SiC]] ] CSiMe3)(h-C6H3Me3-1,3,5)] 6A. The voltammetry of 6V provides conclusive evidence that thermally induced vinylidene-to-alkyne isomerisation does occur in solution (as well as the oxidatively-induced isomerisation process). Fig. 3a shows the CV of 6V ran immediately following the addition of a pure solid sample to the electrochemical cell; it is similar to those observed for the other vinylidene complexes 2V–5V. However, over ca. 20 min a second oxidation wave appears at a potential associated with the oxidation wave of the alkyne complex 6A (Fig. 3b). At this point the IR spectrum of the solution in the electrochemical cell showed the two new bands, at 1898 and 1818 cm21, assigned to the alkyne isomer.The formation with time of the alkyne complex 6A was also studied by using a rotating platinum electrode. Fig. 4a shows the rotating platinum electrode voltammogram (rpev) ca. 45 s after adding 6V to the electrochemical cell; there is only one wave, corresponding to the oxidation of the vinylidene complex. After 5 min a second wave, at the potential anticipated for Fig. 3 CV of [Mo(CO)2{C]] C(SiMe3)2}(h-C6H3Me3-1,3,5)] 6V, (a) after ca. 30 s, (b) after 20 min. Fig. 4 Voltammograms of [Mo(CO)2{C]] C(SiMe3)2}(h-C6H3Me3- 1,3,5)] 6V, from 20.2 to 0.6 V, at a rotating platinum electrode (a) after 45 s, (b) after 2 h. the alkyne complex 6A, begins to appear; Fig. 4b shows the rpev after 2 h by which time equilibrium had been reached. The relative wave heights provide an estimated vinylidene : alkyne ratio at equilibrium of 5 : 1, in good agreement with the results of 1H NMR spectroscopy.For the majority of alkyne and vinylidene complexes, the interconversion of the two bonding modes of the C2 fragment is not observed although in some syntheses of vinylidene complexes from terminal alkynes and a d6 metal centre an intermediate alkyne complex has been detected.16,17 Where isomerisation has been reported, alkyne-to-vinylidene rearrangements are far more common than the reverse vinylidene-to-alkyne isomerisation and the equilibrium between alkyne and vinylidene usually favours one isomer outright.However, the terminal alkyne complex [Re(PPh3)(h-ButC]] ] CH)(NO)(h-C5H5)]1, when heated to 80 8C for 2 h, undergoes alkyne-to-vinylidene isomerisation to give mostly [Re(PPh3)(C]] CHBut)(NO)(h- C5H5)]1; a small amount (ca. 14% by 1H NMR spectroscopy) of [Re(PPh3)(h-ButC]] ] CH)(NO)(h-C5H5)]1 remained even after 7 h. The vinylidene complex [Re(PPh3)(C]] CHBut)(NO)(h- C5H5)]1 has also been made by protonating the alkynyl complex [Re(PPh3)(C]] ] CBut)(NO)(h-C5H5)] with H[BF4] at 278 8C where no alkyne complex is formed.However, heating this cation to 80 8C for 2 h resulted in the same ratio of vinylidene and alkyne isomers, confirming that thermal equilibrium occurs at 80 8C.18 The vinylidene complexes 2V–6V are favoured over the alkyne complexes 2A–6A in that the repulsive interactions between the t2g orbitals (assuming octahedral symmetry) of the d6 metal and the filled alkyne p^ orbital are alleviated.Given the (unique) thermal isomerisation of 6V to 6A, the repulsive interaction appears to be alleviated by reducing the number of methyl groups on the arene and by replacing chromium by molybdenum. Chemical redox reactions The chemical oxidation of 2V–4V was readily eVected by adding one equivalent of [Fe(h-C5H5)2][PF6] to a CH2Cl2 solution of the chromium complexes at 0 8C. In each case, the IR spectrum of the resulting yellow solution shows two new carbonyl Table 4 IR spectroscopic and electrochemical data for alkyne complexes IRa/cm21 Complex 1Ab 1A1b 2A 2A1 3A 3A1 4A 4A1 5A 5A1 6A 6A1 7Ad 7A1 8Ad 8A1 9Ad 9A1 n(CO) 1874, 1799ms 2015ms, 1950 1891, 1817ms 2027ms, 1961 1898, 1826ms 2040ms, 1959 1911, 1841ms 2046ms, 1981 1879, 1802ms 2001, 1941 1898, 1818 2013, 1957 1937, 1861 2060, 2010 1889, 1811 2011, 1965 1894, 1821 2034, 1976 n(C]] ] C) — 1764w — 1785w — 1791w — 1810w — 1736w ———————— E89/V 20.24 c 20.13 c 20.13 c 20.03 c 20.01 c 0.13 c 0.12 20.24, 0.89 20.18 a Strong absorptions (s) in CH2Cl2 unless otherwise stated, m = medium, w = weak.b From ref. 1. c Potential data taken from the reversible product wave observed in the cyclic voltammogram of the isomeric vinylidene complex, in CH2Cl2. d [Cr(CO)2(h-RC]] ] CR9)- (h-C6Me6)] R =R9 =O2CEt 7A, R = R9 = C6H4OMe-4 8A, R = Ph, R9 = H 9A.J. Chem. Soc., Dalton Trans., 1999, 691–698 695 bands shifted to higher energy (by ca. 90 cm21) (Table 4), consistent with the formation of a cationic complex. Furthermore, as in the oxidation of 1V, the n(C]] C) band in 2V–4V is replaced by a much weaker band at higher energy (1785 to 1810 cm21) assigned to n(C]] ] C), indicating the formation of the cationic alkyne complexes [Cr(CO)2(h-Me3SiC]] ] CSiMe3)(h-arene)][PF6] (arene = 1,2,3,5-Me4H2C6 2A1, 1,2,3-Me3H3C6 3A1 or C6H6 4A1). The chemical oxidation of the molybdenum complexes 5V and 6V was carried out in a similar manner to that of the chromium complexes 2V–4V, reaction with one equivalent of [Fe(h-C5H5)2][PF6] resulting in an immediate colour change from orange to pale yellow.With 5V, the IR spectrum showed two new carbonyl bands at 2001 and 1941 cm21 and a weak n(C]] ] C) band at 1736 cm21 consistent with the formation of the cationic alkyne complex [Mo(CO)2(h-Me3SiC]] ] CSiMe3)(h-C6- Me6)]1 5A1. However, the oxidation of 6V did not proceed as cleanly and the IR spectrum of the product solution contained five peaks, at 2050, 2013, 1988, 1957 and 1895 cm21.The strongest two peaks, at 2013 and 1957 cm21, are assigned to n(CO) for the alkyne complex [Mo(CO)2(h-Me3SiC]] ] CSiMe3)- (h-C6H3Me3-1,3,5)][PF6] 6A1 by comparison with those of 5A1 but the other peaks remain unidentified. Attempts to isolate 2A1–6A1 were unsuccessful; their stability decreases with decreasing methylation of the arene and, more strikingly, on changing the metal from chromium to molybdenum.In order to study the stability of the neutral alkyne complexes 2A–6A, solutions of the cations 2A1–6A1 were generated at 0 8C from the neutral vinylidene complexes 2V–6V as described above. Addition of one equivalent of the mild reducing agent [NBun 4][BH4] then resulted in an immediate colour change from yellow to orange. The IR spectrum of each solution showed four new carbonyl bands at lower energy than those of 2A1– 6A1. Two of the bands correspond to those of the neutral vinylidene complexes 2V–4V.The other peaks, at slightly lower energy (ca. 25 cm21) than those of the vinylidene complex, are assigned to the neutral alkyne complexes [Cr(CO)2(h-Me3SiC]] ] CSiMe3)(h-arene)] (arene = C6H2Me4-1,2,3,5 2A, C6H3Me3- 1,2,3 3A or C6H6 4A). Over ca. 20 min the peaks due to the alkyne complexes 2A–4A are replaced by those of the corresponding vinylidene complexes as the alkyne-to-vinylidene isomerisation slowly occurs.The reduction of the molybdenum complex 5A1 with one equivalent of [Co(h-C5H5)2] gave an immediate colour change from yellow to orange, the resulting solution showing only two IR carbonyl bands, at 1879 and 1802 cm21, assigned to the alkyne complex [Mo(CO)2(h-Me3SiC]] ] CSiMe3)(h-C6Me6)] 5A. These bands were then replaced by those of the neutral vinylidene complex 5V over the next hour. Although the oxidation of [Mo(CO)2{C]] C(SiMe3)2}(h-C6H3Me3-1,3,5)] 6V with [Fe(h-C5H5)2][PF6] does not proceed cleanly, as noted above, addition of approximately one equivalent of [Co(h-C5H5)2] to the reaction mixture resulted in the replacement of the carbonyl bands assigned to 6A1 by four bands which can be assigned to the neutral alkyne complex 6A and the vinylidene complex 6V.The oxidation and reduction sequences described above are similar to those observed for [Cr(CO)2{C]] C(SiMe3)2}(h-C6- Me6)] 1V and likewise can be represented by a square scheme (Scheme 1).The qualitative chemical oxidation and reduction studies noted above allow the identification of three members of the square scheme (the vinylidene cations have not been detected by IR spectroscopy, but their ESR spectra are described below). Moreover, it is clear that isomerisation of the neutral alkyne complexes to the vinylidene isomers is much slower than that of the cationic vinylidene to the alkyne containing cation. This has been quantified for 1V1 but the present qualitative study suggests that alkyne-to-vinylidene isomerisation is slower for the d6 molybdenum complexes 5A and 6A than for the chromium complexes 2A–4A.ESR spectroscopic studies of the paramagnetic vinylidene and alkyne complex cations The electronic structures of the paramagnetic vinylidene and alkyne complexes described above have been probed by ESR spectroscopy. At low temperatures the vinylidene-to-alkyne isomerisation is retarded and the cationic vinylidene complexes [M(CO)2{C]] C(SiMe3)2}(h-arene)]1 can be detected by ESR spectroscopy.The complexes 2V1–6V1 were generated in situ by adding solid [Fe(h-C5H5)2][PF6] to frozen CH2Cl2 solutions of 2V–6V at 77 K. The frozen solution was transferred to the cavity of the ESR spectrometer and allowed to warm to 190 K; after ca. 10 min, a spectrum assignable to the vinylidene cation was observed. These spectra consist of a relatively broad line, ·gÒ ª 2.014–2.015, usually accompanied by one or more satellites assigned to coupling to 53Cr (I = 3/2, 9.5% abundance) or 95,97Mo (I = 5/2, 25.5% abundance). A sharper line, also accompanied by satellites, is observed at higher field and assigned to the alkyne complex, [M(CO)2(h-Me3SiC]] ] CSiMe3)(h-arene)]1 2A1–6A1.(Fig. 5 shows the ESR spectrum of 4V1 accompanied by a small amount of 4A1.) At 190 K, the alkyne lines are much weaker than those assigned to the vinylidene complex, but at higher temperatures, they increase in intensity at the expense of the vinylidene lines, and at 250 K or above, only the spectrum of the alkyne complex is observed.ESR parameters for 1V1– 6V1 and 1A1–6A1 are given in Table 5. For comparison, parameters for [Cr(CO)2(h-RC]] ] CR9)(h-C6Me6)]1 (R = R9 = CO2Et 7A1, C6H4OMe-4 8A1; R = Ph, R9 = H 9A1),2 generated directly from the neutral alkyne complexes 7A–9A by oxidation with [Fe(h-C5H5)2][PF6], are given in Table 6, and g components, measured from frozen CH2Cl2–thf solution spectra of 1V1,1 1A1, 7A12 and 8A1, are given in Table 7.The 53Cr or 95,97Mo satellites observed in the spectra of 1A1– 9A1 exhibit a dependence of line width on mI, the nuclear spin quantum number, with the high-field lines substantially broader than those at low field. The lowest-field feature is usually Scheme 1 Square scheme for redox-induced vinylidene–alkyne isomerisations. M OC C C R R C O Men M OC C C R R C O Men + –e– +e– M OC C O Men M OC C O Men + –e– +e– CR CR CR CR Fig. 5 The ESR spectrum of [Cr(CO)2{C]] C(SiMe3)2}(h-C6H6)]1 4V1 in CH2Cl2 at 190 K. The asterisk (*) marks the spectrum of [Cr(CO)2- (h-Me3SiC]] ] CSiMe3)(h-C6H6)]1 4A1.696 J. Chem. Soc., Dalton Trans., 1999, 691–698 Table 5 Isotropic ESR parameters for [M(CO)2{C]] C(SiMe3)2}(h-arene)]1 and [M(CO)2(h-Me3SiC]] CSiMe3)(h-arene)]1 in CH2Cl2 M Cr Cr Cr Cr Mo Mo arene C6Me6 C6H2Me4-1,2,3,5 C6H3Me3-1,3,5 C6H6 C6Me6 C6H3Me3-1,3,5 Complex 1V1a 2V1 3V1 4V1 5V1 6V1 ·gÒ 2.0139(2) 2.0155(2) 2.0150(1) 2.0140(2) 2.0412(1) 2.0436(6) ·aMÒ/G 13.1(1) 12.8(1) 13.15(3) 13.09(6) 18.1(3) 18.5(4) Complex 1A1 2A1 3A1 4A1 5A1 6A1 ·gÒ 1.9983(1) 1.9991(1) 1.9989(1) 1.9980(1) 2.0140(1) 2.0160(1) ·aMÒ/G 16.77(1) 16.8(3) 16.79(2) 17.02(1) 29.65(3) 29.74(4) ·aHÒ/G — 1.50(5) (2 H) — 1.418(5) (6 H) — 2.2(1) (3 H) a From ref. 1. sharper than the central line and, in some spectra of alkyne complexes, e.g. that of [Cr(CO)2(h-Me3SiC]] ] CSiMe3)(h-C6H6)]1 4A1 (Fig. 6), arene proton hyperfine coupling is resolved comparable to that observed in the spectra of the sandwich complexes [M(h-arene)2]1 (M = Cr, Mo),19,20 anisotropic ESR parameters for which are given in Table 7.Extended Hückel MO calculations were performed on [Cr- (h-C6H6)2]1, [Cr(CO)2(h-HC]] ] CH)(h-C6H6)]1, and [Cr(CO)2- (C]] CH2)(h-C6H6)]1 using idealised coordinates derived from the crystal structures of the Cr(0) complexes [Cr(h-C6H6)2],21 [Cr(CO)2(h-PhC]] ] CPh)(h-C6Me5H)] 4 and 4V respectively, and parameters collated by Alvarez.22 The results for [Cr(h-C6H6)2]1 are essentially identical to those reported by Muetterties et al.23 The a1g SOMO is nearly purely metal dz2, and the e2g pair, largely metal dx2 2 y2 and dxy, lies at slightly lower energy.Metal dxz and dyz mainly contribute to weakly anti-bonding, empty MOs. The frontier orbitals for the alkyne and vinylidene complexes show family resemblances to those of [Cr(h-C6H6)2]1 although the lower symmetry allows considerable d-orbital hybridisation; the principal interactions for the Cr(CO)2(h-C6- Fig. 6 The ESR spectrum of [Cr(CO)2(h-Me3SiC]] ] CSiMe3)(h-C6H6)]1 4A1 in CH2Cl2 at 250 K, showing hyperfine coupling to the arene protons and the variation in line width of the 53Cr satellites. Table 6 Isotropic ESR parameters for [Cr(CO)2(h-RC]] ] CR9)(h-C6- Me6)]1 in CH2Cl2 Complex 7A1a 8A1 9A1 R CO2Et C6H4- OMe-4 Ph R9 CO2Et C6H4- OMe-4 H ·gÒ 1.9936(1) 1.9956(1) 1.9964(1) ·aCrÒ/G 16.78(3) 14.38(2) 16.12(2) ·aXÒ/G 9.1(1) (2 C) — 4.17(5) (1 H) a From ref. 2. Table 7 Anisotropic ESR parameters for arene metal complexes Complex [Cr(h-C6H6)2]1 [Mo(h-C6H6)2]1 1V1 1A1 7A1 8A1 gx 1.978 1.977 1.993 1.994 1.995 1.996 gy 1.978 1.977 2.020 1.986 1.979 1.983 gz 2.002 1.998 2.030 2.017 2.007 2.009 Ref. 19 20 1 This work 2 This work H6) vinylidene complex are similar to those shown in Fig. 2 for the isoelectronic [Mn(CO)2(h-C5H5)]1 fragment. For both the alkyne and vinylidene complexes, the a9 SOMO is significantly delocalised into ligand p orbitals and the metal contribution is a dz2/dx2 2 y2/dxz hybrid: 28, 12 and 2% for the vinylidene complex, 43, 11 and 0.3% for the alkyne complex; in either case, the metal contribution is best described as dz2 2 x2.The next two filled MOs, related to the e2g pair for [Cr(h-C6H6)2]1, are an a9 MO with metal dx2 2 y2/dz2/dxz character (34, 16 and 3% for the vinylidene complex, 44, 11 and 0.4% for the alkyne complex, in either case best described as dy2) and an a0 MO with metal dxy/dyz character (44 and 2% for the vinylidene complex, 45 and 6% for the alkyne complex), similar to the a0 orbital of Fig. 2. These orbitals are very similar in shape to those found by HoVmann14 for [Mn(CO)2(HC]] ] CH)(h-C5H5)] and [Mn(CO)2- (C]] CH2)(h-C5H5)], but the energy order of the two a9 molecular orbitals is inverted. The dz2 character of the SOMO is confirmed for the alkyne complexes by the appearance of hyperfine coupling to the arene ring protons.The proton couplings observed in the ESR spectra of [Cr(h-C6H6)2]1 (3.46 G),19 [Mo(h-C6H6)2]1 (4.45 G),20 and related species are positive in sign 24 and have been interpreted as resulting from direct delocalisation of the predominantly metal dz2 SOMO into the s-orbitals of the arene; 25 the usual mechanism whereby proton couplings in arene radical ions arise from spin polarisation by p spin density is of negligible importance in these systems.If we assume that the arene proton coupling is proportional to the metal dz2 character, the observed 3.46 G coupling for [Cr(h-C6H6)2]1, together with the EHMO calculations, suggests a coupling of 1.6 G for [Cr(CO)2(h-HC]] ] CH)(h-C6H6)]1 and 1.4 G for [Cr(CO)2(C]] CH2)(h-C6H6)]1. This prediction is in reasonable agreement with the observed couplings for 2A1 and 4A1. No proton coupling could be resolved for the vinylidene complexes, in part because the isotropic spectra could be observed only at relatively low temperatures where lines are too broad to resolve the proton coupling.The larger proton coupling found for 6A1 is consistent with the larger coupling observed for [Mo(h-C6H6)2]1. Spin–orbit coupling of the dz2 SOMO with dxz and dyz for [Cr(h-C6H6)2]1 leads to g^ < ge, g|| = ge. In addition to similar contributions for the alkyne and vinylidene model complexes, coupling of the dx2 2 y2 SOMO contribution with dxy leads to gz slightly larger than the free-electron value. For the alkyne complex, the dxz contributions to empty MOs still dominate so that gy remains less than ge, but for the vinylidene complex, the dxz contributions to the SOMO and to the filled MOs lead to gy slightly larger than ge.In general, these results are in good agreement with the experimental results and allow us to make tentative assignments of the g-matrix components: for the alkyne complexes, gz > gx > gy; for the vinylidene complexes, gz > gy > gx.The mI-dependence of the satellite line widths is similar to that observed in the spectra of [Cr(h-C6H6)2]1 and related species.26 Line widths can be expressed by eqn. (1) where Width = a 1 bmI 1 gmI2 (1) the parameters a, b and g are related to the anisotropies of theJ. Chem. Soc., Dalton Trans., 1999, 691–698 697 g- and metal-hyperfine matrices.27 In the case of [Cr(h-C6H6)2]1, it is known that ·aCrÒ is positive so that the observed increase in width with increasing magnetic field implies that b is negative.(The nuclear magnetic moments of 53Cr and 95,97Mo are negative so that a spin polarisation mechanism is expected to lead to positive isotropic coupling constants.) For w0tr @ 1, |·AÒ|/w0 ! 1, the parameter b is given by eqn. (2), where tr is the b = 4– 15 B0(b Dg 1 4c dg) tr (2) rotational correlation time, w0 is the angular microwave frequency, and the other parameters are given by eqns.(3a–d). For b = 2 3 – [Az 2 ��� (Ax 1 Ay)] (3a) c = 1– 4 (Ax 2 Ay) (3b) Dg = 2pmB h [gz 2 ��� (gx 1 gy)] (3c) dg = pmB h (gx 2 gy) (3d) [Cr(h-C6H6)2]1, c = dg = 0, Dg > 0 and, for the dz2 SOMO, we expect Az, and thus b, to be negative, consistent with the observed line widths. Assuming ·ACrÒ is also positive for the alkyne and vinylidene cations, we again have b < 0. With the g-component assignments given in Table 7, Dg > 0 for both alkyne and vinylidene complexes, dg > 0 for alkyne complexes, and dg < 0 for vinylidene complexes.If we describe the metal SOMO contribution as dz2 2 x2, we expect a dipolar contribution 1Q to Ay, 2Q/2 to Ax and Az; thus b = 2Q/2, c = 23Q/8. Substituting numbers in eqn. (3c), (3d) and (2), we find balkyne ! bvinylidene < 0, consistent with the experimental results. Coupling to a single proton was observed in the ESR spectrum of [Cr(CO)2(h-PhC]] ] CH)(h-C6Me6)]1 9A12 and to two 13C nuclei in the spectrum of [Cr(CO)2(EtCO2C]] ] CCO2Et)(h- C6Me6)]1 7A1.Lines in the spectra of [Cr(CO)2(h-4-MeOC6- H4C]] ] CC6H4OMe-4)(h-C6Me6)]1 8A1 are significantly broader than those of 7A1 and are noticeably non-Lorentzian, suggesting unresolved hyperfine coupling to the phenyl ring protons. These results are consistent with the extensive delocalisation of the SOMO into the alkyne p-orbitals predicted by EHMO calculations. Indeed, the EHMO results suggest a carbon 2pp spin density of 0.074, nearly large enough to explain the proton coupling for 9A1 in terms of the usual indirect polarisation mechanism.Since the 53Cr or 95,97Mo couplings arise primarily through polarisation of inner-shell s-orbitals by 3d or 4d metal spin density, it is tempting to use these couplings as measures of the metal d-orbital character in the SOMOs. Since ·aCrÒ = 18.1 G for [Cr(h-C6H6)2]1,19 this measure would suggest a SOMO with nearly as much Cr 3d character for the alkyne complexes and only slightly smaller values for the vinylidene complexes.Unfortunately, in all cases the SOMOs belong to totally symmetric representations and metal 4s character is permitted. This leads to a negative contribution to ·aCrÒ, 2267 r4s G,28 cancelling part of the dominant polarisation contribution. EHMO calculations suggest that metal 4s SOMO character is suf- ficiently variable that the metal coupling cannot be used as a reliable measure of metal 3d SOMO character. Conclusions UV irradiation of [M(CO)3(h-C6Me6 2 nHn)] (M = Cr, Mo) with the alkyne Me3SiC]] ] CSiMe3 generally gives the vinylidene complexes [M(CO)2{C]] C(SiMe3)2}(h-C6HnMe6 2 n)] rather than the alkyne complexes [M(CO)2(h-Me3SiC]] ] CSiMe3)(h- C6HnMe6 2 n)].However, the molybdenum complex [Mo(CO)2- {C]] C(SiMe3)2}(h-C6H3Me3-1,3,5)] reaches thermal equilibrium with the alkyne complex [Mo(CO)2(h-Me3SiC]] ] CSiMe3)- (h-C6H3Me3-1,3,5)] in solution. By contrast, for the more substituted arene complexes such as [Mo(CO)2{C]] C(SiMe3)2}- (h-C6Me6)], no vinylidene-to-alkyne isomerisation is observed.One-electron oxidation of the vinylidene complexes [M(CO)2{C]] C(SiMe3)2}(h-C6HnMe6 2 n)] (M = Mo, Cr) gives the cationic alkyne complexes [M(CO)2(h-Me3SiC]] ] CSiMe3)- (h-C6HnMe6 2 n)]1 via a fast redox-induced vinylidene-to-alkyne isomerisation. However, on reduction the neutral alkyne complex [M(CO)2{C]] C(SiMe3)2}(h-C6HnMe6 2 n)] is formed which slowly isomerises to the neutral vinylidene complex.Analysis of the ESR spectra of [M(CO)2{C]] C(SiMe3)2}- (h-C6HnMe6 2 n)]1 and [M(CO)2(h-Me3SiC]] ] CSiMe3)(h-C6Hn- Me6 2 n)]1 shows the unpaired electron to be extensively delocalised on to the alkyne, in agreement with the results of EHMO calculations. Experimental The preparation, purification and reactions of the complexes described were carried out under an atmosphere of dry dinitrogen using dried, distilled and deoxygenated solvents; reactions were monitored by IR spectroscopy where necessary.The compounds [M(CO)3(h-arene)] (M = Cr29 or Mo30), [Cr(CO)2- {C]] C(SiMe3)2}(h-C6Me6)],1 [Cr(CO)2(h-RC]] ] CR9)(h-C6Me6)] (R = R9 = Ph, 4-MeOC6H4C]] ] CC6H4OMe-4, CO2Et; R = Ph, R9 = H)3 and [Fe(h-C5H5)2][PF6] 31 were prepared by published methods. IR spectra were recorded on a Nicolet 5ZDX FT spectrometer. 1H and 13C NMR spectra were recorded on JEOL GX270, l300 or GX400 spectrometers with SiMe4 as internal standard.X-Band ESR spectra were recorded using a Bruker ESP-300E spectrometer equipped with a Bruker variabletemperature accessory and a Hewlett Packard 5350B microwave frequency counter. The field calibration was checked by measuring the resonance of the diphenylpicrylhydrazyl (dpph) radical. Cyclic voltammetry was carried out as previously described; 32 for rpev the platinum electrode was rotated at 600 r.p.m. Under the conditions used for voltammetry, E89 for the one-electron reduction of [Co(h-C5H5)2][BF4], added to the test solutions as an internal calibrant, is 20.87 V.(On this scale, the one-electron oxidation of ferrocene occurs at 0.47 V.) Microanalyses were carried out by the staV of the Microanalytical Service of the School of Chemistry, University of Bristol. Syntheses [Cr(CO)2{C]] C(SiMe3)2}(Á-C6H2Me4-1,2,3,5)] 2V. A stirred solution of [Cr(CO)3(h-C6H2Me4-1,2,3, 1.85 mmol) and Me3SiC]] ] CSiMe3 (2 cm3, 8.90 mmol) in thf (70 cm3) was irradiated under UV light for 5 h while a slow purge of nitrogen (ca. 1 bubble per second) was passed through the mixture. The resulting brown-orange solution was evaporated to dryness in vacuo and the residue extracted into n-hexane (70 cm3). After filtration through Celite, the orange solution was reduced in volume to ca. 20 cm3 and then stored at 220 8C for 18 h giving the product as orange needles, yield 0.30 g (39%). The solid complex is stable in air for several weeks.It dissolves in most organic solvents to give moderately air-sensitive solutions. The complexes [Cr(CO)2{C]] C(SiMe3)2}(h-C6H3Me3-1,2,3)] 3V and [Cr(CO)2{C]] C(SiMe3)2}(h-C6H6)] 4V were prepared similarly; UV irradiation was carried out in n-hexane (for 6 h and 27 h respectively) and the reaction mixture was filtered, evaporated to low volume in vacuo and cooled to induce crystallisation. Air stability, both in the solid state and in solution, decreases with decreasing methylation of the arene ligand.[Mo(CO)2{C]] C(SiMe3)2}(Á-C6Me6)] 5V. A stirred solution of [Mo(CO)3(h-C6Me6)] (0.50 g, 1.46 mmol) and Me3SiC]] ] CSiMe3 (2 cm3, 8.90 mmol) in benzene (120 cm3), purged with a stream of N2, was irradiated under UV light for 16 h to give an698 J. Chem. Soc., Dalton Trans., 1999, 691–698 orange solution which was evaporated to dryness in vacuo. The residue was extracted into diethyl ether (180 cm3) and filtered through Celite to give an orange solution which was treated with n-hexane (80 cm3).Removal of the diethyl ether in vacuo and cooling to 220 8C for 5 h gave the product as orange needles, yield 0.19 g (27%). The solid complex is stable in air for ca. 1 d and soluble in most organic solvents (sparingly in nhexane, insoluble in MeCN) to give solutions which decompose in air in minutes. The complex [Mo(CO)2{C]] C(SiMe3)2}(h-C6H3Me3-1,3,5)] 6V was prepared similarly, in n-hexane. The reaction mixture also contains small amounts of [Mo(CO)2(h-Me3SiC]] ] CSiMe3)- (h-C6H3Me3-1,3,5)] but crystallisation of the filtered solution at 220 8C gave pure orange-red needles of [Mo(CO)2- {C]] C(SiMe3)2}(h-C6H3Me3-1,3,5)].The solid is stable in air for a few hours; solutions in organic solvents are air-sensitive. Structure determination of [Cr(CO)2{C]] C(SiMe3)2}(Á-C6H6)] 4V Many of the details of the structure analysis of [Cr(CO)2- {C]] C(SiMe3)2}(h-C6H6)] 4V are presented in Table 8. Crystal decay of ca. 35% was observed over the period of data collection; an appropriate correction was made. CCDC reference number 186/1336. Acknowledgements We thank the EPSRC for a Postdoctoral Research Associateship (to T. J. P.), the Spanish Ministerio de Educacion y Ciencia for an FPU (Becas en el extranjero) grant (to A. J. M.) and the University of Bristol for a Postgraduate Scholarship (to I. M. B.). References 1 N. G. Connelly, W. E. Geiger, M. C. Lagunas, B. Metz, A. L. Rieger, P.H. Rieger and M. J. Shaw, J. Am. Chem. Soc., 1995, 117, 12202. Table 8 Crystal and refinement data for complex 4V Formula M Crystal system Space group (no.) a/Å b/Å c/Å b/8 T/K U/Å3 Z m/mm21 Reflections collected Independent reflections (Rint) Goodness-of-fit on F2 Final R indices [I > 2s(I)]: R1, wR2 C16H24CrO2Si2 356.53 Monoclinic C2/c (15) 34.627(7) 6.8137(14) 16.501(3) 94.67(3) 293(2) 3880.3(14) 8 0.714 3441 3373 (0.0468) 1.138 0.0662, 0.1419 2 N. G. Connelly, A. G. Orpen, A.L. Rieger, P. H. Rieger, C. J. Scott and G. M. Rosair, J. Chem. Soc., Chem. Commun., 1992, 1293. 3 N. G. Connelly and G. A. Johnson, J. Organomet. Chem., 1974, 77, 341. 4 I. M. Bartlett, N. G. Connelly, A. G. Orpen, M. J. Quayle and J. C. Rankin, Chem. Commun., 1996, 2583. 5 R. Davis and L. A. P. Kane-Maguire, in Comprehensive Organometallic Chemistry, eds. G. W. Wilkinson, F. G. A. Stone and E. W. Abel, Pergamon Press, Oxford, 1982, vol. 3, 1035. 6 R. Davis and L. A.P. Kane-Maguire, in Comprehensive Organometallic Chemistry, eds. G. W. Wilkinson, F. G. A. Stone and E. W. 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