Isomerism and fluxional behaviour in η 2 -vinyl complexes resulting in inversion of configuration at an asymmetric carbon atom. The crystal and molecular structures of two isomeric forms of [MoCl{η 2 -C(CF 3)C(CF 3)(PEt 3)} (CF 3CCCF 3)(η 5 -C 5H 5)

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η 2 -Vinyl complexes [MCl{η 2 -C(CF 3)C(CF 3)L} (CF 3CCCF 3)(η 5 -C 5H 5)] (M = Mo, W: L = tertiary phosphine or phosphite or pyridine) have been shown by NMR and X-ray diffraction studies to exist in two distinct isomeric forms which exhibit (a) different orientations of the η 2 -vinyl ligand (b) different configurations at the asymmetric carbon atom of the η 2 -vinyl ligand: variable temperature NMR studies reveal fluxional behaviour thought to involve reversible inversion of configuration at this carbon atom.