Oxidatively induced isomerisation of vinylidene ligands to alkynes: ESR spectra of paramagnetic vinylidene and alkyne arene metal complexes

UV irradiation of [M(CO)(3)(eta-arene)] and Me3SiC=CSiMe3 gives [M(CO)(2){C =C(SiMe3)(2)}(eta-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)}(eta-C6H6)] 4V confirms the presence of the vinylidene ligand; the complex has approximate C-s symmetry with th e C(SiMe3)(2) plane orthogonal to the arene(centroid)-Cr-CalphaCbeta plane. Voltammetry and IR and NMR spectroscopy show that in solution [Mo(CO)(2){C =C(SiMe3)(2)}(eta-C6H3Me3-1,3,5)] 6V thermally equilibrates with the alkyne isomer [Mo(CO)(2)(eta-Me3SiC=CSiMe3)(eta-C6H3Me3-1,3,5)] 6A. The vinyliden e complexes [M(CO)(2){C=C(SiMe3)(2)}(eta-arene)] 2V-6V undergo one-electron oxidation to the alkyne cations [M(CO)(2)(eta-Me3Si=CSiMe3)(eta-arene)](+) 2A(+)-6A(+) via fast, redox-induced vinylidene-to-alkyne isomerisation. Th ese cations are reduced to the neutral alkyne complexes [M(CO)(2)(eta-Me3Si C=CSiMe3)(eta-arene)] 2A-6A which slowly isomerise thermally to the neutral vinylidene complexes 2V-6V. Paramagnetic vinylidene and alkyne complex cat ions have been characterised by ESR spectroscopy; unpaired electron density is extensively delocalised from the metal centre to the C-2 ligand, in agr eement with the results of EHMO calculations.