Structure and bonding of d8 allyl complexes M(η3-allyl)L3 (M = Co, Rh, Ir;: L = phosphine or carbonyl)

Density functional theory calculations were used to study structure and bonding of d(8) five-coordinate allyl complexes M(eta(3)-allyl)L-3 (M = Co, Rh, Ir; L = phosphine or carbonyl). In these pseudo-square-pyramidal d(8) complexes, we found that only the exo structures correspond to energy minima on the potential energy surface. The exo structures are able to maximize the metal(d)-to-allyl(pi*) back-bonding interaction. The calculations predicted that the endo structures for the Ir and Co complexes are transition states for interconversion of two different exo structures. Complexes such as Ir(eta(3)-allyl)(PMe3)(3) having only phosphines as the ancillary ligands possess the strongest metal-allyl bonding interaction, while complexes such as Co(eta(3)-allyl)(CO)(3) having only carbonyls have the weakest interactions. We also studied the eta(3) -> eta(3) ->eta(3) and eta(3) -> eta(1) -> eta(3) rearrangements of the allyl ligand and found that both the rearrangement mechanisms are energetically feasible for the d(8) complexes studied in this paper.