Chemistry of unsaturated group 6 metal complexes with bridging hydroxy and methoxycarbyne ligands.: 1.: Synthesis, structure, and bonding of 30-electron complexes

The new cationic alkoxy and hydroxycarbyne complexes [M2Cp2(mu-COR)(mu-PR'(2))(2)]BF4 (Cp = eta(5)- C5H5; M = W, R Me, R' Ph; M = Mo, R = Me and H, R' = Et) and [Mo2Cp2(mu-COR)(mu-COR')-(mu-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 center dot 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)(mu-PEt2)(2)(CO)]BF4 and [Mo2Cp2(H)(mu-COMe)-(mu-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 sigma(2)delta(4), with the delta orbitals being involved in T back-bonding to the carbonyl bridges. Upon methylation of the latter ligands, these delta-bonding orbitals become more delocalized over the Mo2C-(carbyne) triangle and constitute the pi-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.