Chemistry of Unsaturated Group 6 Metal Complexes with Bridging Hydroxy and Methoxycarbyne Ligands. 1. Synthesis, Structure, and Bonding of 30-Electron Complexes

Abstract

The new cationic alkoxy and hydroxycarbyne complexes [M2Cp2(μ-COR)(μ-PR‘2)2]BF4 (Cp = η5-C5H5; M = W, R = Me, R‘ = Ph; M = Mo, R = Me and H, R‘ = Et) and [Mo2Cp2(μ-COR)(μ-COR‘)(μ-PCy2)]BF4 (R = Me; R‘ = H, Me, Et) are obtained in high yield by the reaction of the corresponding neutral monocarbonyl precursors with either [Me3O]BF4 or HBF4·OEt2 in dichloromethane. While the methoxycarbyne complexes are stable at room temperature, the analogous hydroxycarbyne species are thermally unstable, and above ca. 253 K they experience a hydrogen migration from oxygen to metal, to give the new hydride carbonyl complexes [Mo2Cp2(H)(μ-PEt2)2(CO)]BF4 and [Mo2Cp2(H)(μ-COMe)(μ-PCy2)(CO)]BF4 as major products. The electronic structure and bonding in some of these face-sharing bioctahedral complexes were studied by means of density functional theory. These calculations allow us to describe the intermetallic interaction present in the carbonyl-bridged 30-electron complexes by a configuration of type σ2δ4, with the δ orbitals being involved in π back-bonding to the carbonyl bridges. Upon methylation of the latter ligands, these δ-bonding orbitals become more delocalized over the Mo2C(carbyne) triangle and constitute the π-bonding component of the metal−carbyne bond. Therefore, a partial reduction of the direct metal−metal overlap occurs upon formation of these methoxycarbyne complexes, which still retain some multiplicity in the corresponding C−O bonds. The topological analysis of the electron density under the AIM scheme supports the above description of the Mo−Mo, Mo−C, and C−O bonds in these complexes.