Inner and outer phases of antibonding orbitals of transition metal bonds: olefin metathesis

The frontier orbital theory is applied to ruthenium olefin metathesis. The formal [2+2] cycloaddition step, that is, the key step involved in the catalytic cycle of the reaction, is found to be favored by the phases of the HOMO and LUMO, in sharp contrast to [2+2] cycloaddition reactions between olefins. In the LUMO of transition metal part, a d-orbital overlaps out of phase with the vacant p-orbital of the carbene in the inner space of the metal–carbon π bond as is expected, but the remote lobe of the d orbital overlaps in phase in the outer space of the bond. This is a characteristic feature of the antibonding orbitals of transition metal bonds. The outer orbital phase plays more important role in the interaction. gnificantly enhanced activity of the ‘second-generation’ catalysts or N-heterocyclic carbenes (NHC’s) comes from the high HOMO energy.