Unusual metal-carbon-hydrogen angles, carbon-hydrogen bond activation, and α-hydrogen abstraction in transition-metal carbene complexes

Abstract

Alkylidene complexes of electron-deficient transition metals display an interesting structural deformation in which the carbine appears to pivot in place while the C_α-H bond weakens. A molecular orbital analysis of these carbine complexes traces the deformation to an intramolecular electrophilic interaction of acceptor orbitals of the metal with the carbine lone pair. Bulky substituents on the metal and carbine protect the metal center from intermolecular reactions and control the extent of carbine pivoting. While a secondary interaction weakens the C-H_α bond and attracts the a-hydrogen to the metal, full transfer of hydride to the metal is a forbidden reaction, at least for a five-coordinate, 14-electron complex. The metal-hydrogen bonding interaction guides the hydride to a neighbouring alkyl group, facilitating an α-elimination mode characteristic of the reactions of these compounds. The complexed carbene centers are unusually electron-rich, nucleophilic, by comparison with 18-electron d⁶ stabilized carbene complexes. This is a consequence of an extremely effective Ta-C overlap.