Structure and Bonding in the Unsaturated Hydride- and Hydrocarbyl-Bridged Complexes [Mo₂(η⁵-C₅H₅)₂(μ-X)(μ-PCy₂)(CO)₂] (X = H, CH₃, CH₂Ph, Ph). Evidence for the Presence of α-Agostic and π-Bonding Interactions^†

The reactions of the triply bonded anion [Mo₂Cp₂(μ-PCy₂)(μ-CO)₂]^- (Li⁺ salt) with [NH₄]PF₆, MeI, and PhCH₂Cl give, with good yields, the corresponding hydride- or alkyl-bridged derivatives [Mo₂Cp₂(μ-X)(μ-PCy₂)(CO)₂] (X = H, Me, CH₂Ph). The related phenyl complex [Mo₂Cp₂(μ-Ph)(μ-PCy₂)(CO)₂] can be obtained upon reaction of the above anion with Ph₃PbCl. According to the corresponding X-ray diffraction studies, the latter complex displays its phenyl group bonded to the dimetal center exclusively through the ipso carbon atom, while the methyl and benzyl complexes adopt an asymmetric α-agostic structure whereby one of the C−H bonds of the bridgehead carbon is bound to one of the molybdenum atoms. The intermetallic distances remain quite short in all cases, 2.56−2.58 Å. In solution, the hydride complex exhibits dynamic behavior involving mutual exchange of the carbonyl ligands. The alkyl derivatives behave similarly to each other in solution and also exhibit dynamic behavior, possibly implying the presence of small amounts of a nonagostic structure in equilibrium with the dominant α-agostic structure. Density functional theory calculations (B3LYP, B3PW91) correctly reproduce the experimental structures, and predict an α-agostic structure for both the methyl and benzyl complexes. The bonding in the above hydride and hydrocarbyl complexes was analyzed using molecular orbital, atoms in molecules, and natural bond orbital methodologies. The intermetallic binding in the hydride complex can be thus described as composed of a tricentric (Mo₂H) plus two bicentric (Mo₂) interactions, the latter being of σ and π types. In the hydrocarbyl-bridged complexes, analogous tricentric (Mo₂C), and bicentric (Mo₂) interactions can be identified, but there are additional interactions reducing the strength of the intermetallic binding, these being the α-agostic bonding in the case of the alkyl complexes and a π-donor interaction from the π-bonding orbitals of the hydrocarbon ring into suitable metal acceptor orbitals, in the case of the phenyl complex. The strength of these additional interactions have been estimated by second-order perturbation analysis to be of 70.3 (Me), 89.2 (CH₂Ph), and 52.2 (Ph) kJ mol^-1, respectively.