Theoretical studies of rotational barriers of vinylidene ligands in the five-coordinate complexes M(X)Cl(=C=CHR)L-2 (M = Os, Ru; L = phosphine)

Rotational barriers of vinylidene ligands in the five-coordinate complexes M(X)Cl(=C=CHR)L-2 (M = Os, Ru; L = phosphine) have been investigated by den sity functional theory calculations at the level of B3LYP. The effects of l igand X, transition metal M, and substituent R on the barriers have been ex amined. The results show that the rotational barriers increase with X from having pi -acceptor, sigma -donor to having pi -donor properties. Ligands ( X) with pi -acceptor properties stabilize the transition state structures t hrough interactions with the d orbital used for metal-vinylidene pi bonding in the most stable conformations and, therefore, give smaller rotational b arriers. Studies of the influence of different substituents R show that the rotational barriers also increase with the electron donation abilities of R. The rotational barriers for Os complexes are generally higher in compari son to those of the Ru analogues. This result is related to the stronger os mium-ligand interactions because of the more diffuse d orbitals of the heav ier metal.