A COMPARISON OF THE BONDING IN THE 2ND-ROW TRANSITION-METAL OXIDES AND CARBENES

Calculations including electron correlation of all valence electrons have been performed for the sequence of second-row transition-metal oxides and carbenes. For the atoms to the right the bond strengths of the carbenes and the oxides are similar while for the atoms to the left the oxide bond strengths are much larger. The origin of this difference is the interaction between the oxygen lone pairs and empty 4d orbitals on the metal. With the large donation of electrons from the oxygen lone pair to the metal for the atoms to the left the bonding can almost be described as triple bond formation for YO, ZrO and NbO. For MoO, where a 4d(pi) orbital is singly occupied on the metal, the bond strength is much smaller than for the oxides to the left. For the metal carbenes two covalent bonds are formed. The ground state spin for the oxides to the right is higher than for the corresponding carbenes, which can be explained by the presence of the pi degeneracy for the oxides. The relevance of the present results for the epoxidation and the olefin metathesis reactions are discussed.