Density functional study on zerovalent lanthanide bis(arene)-sandwich complexes

Several zerovalent lanthanide bis(arene)sandwich complexes, Ln(eta(6)-C6H6) (2), Ln = La, Ce, Eu, Gd and Lu, have been studied by means of density func tional theory. The calculated geometries are in good agreement with experim ent The calculated dissociation energies of the bond Ln-(eta(6)-C6H6) may b e considerably underestimated, but they correctly reveal the variation regu larity. The bonding in these molecules call be described in terms of a rela tively weak pi-electron donation from benzene to Ln and a stronger electron back-donation from Ln 5d to the benzene pi* orbitals. During bond formatio n, there is electron promotion from Ln 6s to 5d instead of from 4f to 5d, i n opposition to the proposal of Anderson et al. The relativistic effect onl y slightly influences the molecular geometry, but decreases the bonding ene rgy considerably through lowering the Ln 6s level and raising the 5d level. It enhances the trend of the bonding energy to decrease along the lanthani de series.