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 d(M-C) and/or d(C=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 sigma-bonding as well as the pi-back-bonding. Instead, the explanation and prediction using the NBO analyses of electron populations in pi*-orbitals of olefin ligands and of the second-order charge transfer interactions between metal d-orbitals and olefins pi*-orbitals could be more efficient and correct. (C) 2004 Elsevier B.V. All rights reserved.