Theoretical studies of the relative stabilities of transition metal alkylidyne (CH3)2M(CH)(X) and bis(alkylidene) (CH3)M(=CH2)2(X) complexes

Density functional molecular orbital calculations at the B3LYP level were performed to investigate the relative stabilities of the tautomeric pairs of transition metal alkylidyne (CH3)(2)M(=CH)(X) and bis(alkylidene) (CH3)M(=CH2)(2)(X) complexes (M = W, Mo, Os, and Ru; X = Cl, CH3, CF3, SiH3, and SiF3). Calculation results indicate that the relative stabilities of the bis(alkylidene) tautomers increase with the increasing pi-accepting ability of X for the W and Mo complexes. When X is a silyl ligand, it is found that the tautomeric pair for W or Mo have similar stabilities. These results have been explained in terms of pi interaction between ligand X and the electron density in the metal-alkylidyne/alkylidene bonds. Far the Os and Ru complexes, the bis(alkylidene) tautomers are found more stable no matter what X is. The stabilities of the bis(alkylidene) tautomers for these d(2) metal complexes have been related to the bonding characteristic of the orbital that accommodates the two metal d electrons.