The electronic structure of Ni– and Ni2–ethylene cluster complexes

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Abstract
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 NiC2H4 and Ni2C2H4 species. The π‐NiC2H4 and π‐Ni2C2H4 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 3d94s1 with the 4s hybridized (by the metal 4p) away from the ethylene molecule, thereby allowing the π 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‐Ni2C2H4 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.

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