摘要：

Alkylidene complexes of electron-deficient transition metals display an interesting structural deformation in which the carbene appears to pivot in place while the Cα-H bond weakens. A molecular orbital analysis of these carbene complexes traces the deformation to an intramolecular electrophilic interaction of acceptor orbitals of the metal with the carbene lone pair. Bulky substituents on the metal and carbene protect the metal center from intermolecular reactions and control the extent of carbene pivoting. While a secondary interaction weakens the C-Hα, bond and attracts the α-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 neighboring 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.

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