Mono-, di- and tri-metallic complexes of trimethylenemethane: an EHMO study of the TMM rotational barrier

The rotational barrier of a trimethylenemethane ligand relative to an Fe(CO)2L unit, where L  CO or a phosphine, is analyzed by molecular orbital calculations at the extended Hückel level. It is shown that the original Hoffmann-Albright model fits well for carbonyl complexes, but yields barriers for phosphine complexes that are too high. A rationale is presented to account for the different behavior of the bimetallic complexes Cp2M′M″(CO)2(TMM), where M′ M″ Ru, 2, (trimethylenemethane (TMM) non-fluxional), and for M′ W and M″  Ni, 3, where the TMM rotates. Finally, calculations reveal that the trimetallic complex [(TMM)CO3(CO)9]+ should be a stable and non-fluxional system with an eclipsed conformation.