NBO analyses of the back-bonding in metal–olefin complexes

The characteristics of back-bonding in metal–olefin complexes have been studied by using the NBO analyses. In metal–olefin complexes, the evidence for back-bonding is only possible from the dM–C and/or dCC bond length variations. However, these bond lengths might be ambiguous in explaining the extents of back-bonding, since both bond lengths are dependent upon the extents of the σ-bonding as well as the π-back-bonding. Instead, the explanation and prediction using the NBO analyses of electron populations in π*-orbitals of olefin ligands and of the second-order charge transfer interactions between metal d-orbitals and olefins π*-orbitals could be more efficient and correct.