The electronic structure of Ni- And Ni₂-ethylene cluster complexes

The electronic structure of metal cluster-ethylene complexes has been investigated by carrying out ab initio bonding pair-correlated, self-consistent field, and configuration interaction (CI) calculations on the NiC ₂H₄ and Ni₂C₂H₄ species. The π-NiC₂H₄ and π-Ni₂C₂H ₄ cluster complexes are found to be bound, the former only with CI, while diσ-Ni₂C₂H₄ has only a repulsive Ni₂-C₂H₄ ground state potential curve. The bonding in the π-type cluster complexes can be described as follows: the metal atom configuration is 3d ⁹4s¹ with the 4s hybridized (by the metal 4p) away from the ethylene molecule, thereby allowing the σ orbital to form a dative σ bond with the metal atom. The bonding interaction is promoted by the presence of a second nickel atom behind the first one, leading to a 4s orbital electron deficiency of the bonded nickel atom and thus making this nickel atom a better electron acceptor. Back donation from the occupied metal 3d into the ethylene π* molecular orbital also takes place to some extent, and thus both features of the classical Dewar-Chatt-Duncanson model are observed. The π-Ni₂C ₂H₄ species is analyzed in terms of the addition of a bare nickel atom to a π-NiC₂H₄ cluster complex, with concomitant stabilization of the orbitals of the bonded nickel atom. A study of the excited electronic states of π-NiC₂H₄ shows that low-lying 4s→π* and 3d→π*(M→L) charge transfer transitions are predicted. The former is not observed experimentally, probably due to the diffuse nature of the 4s orbital. The relationship between small cluster-ethylene complex systems and ethylene chemisorption on a nickel metal surface is discussed.