Interaction between nickel and the ligand groups carbonyl, water, and phosphine

The interaction between nickel and three different types of ligands are studied at the CASSCF (Complete Active Space Self-Consistent Field) and contracted CI (Configuration Interaction) level. The carbonyl group is used as a model for a strong ..pi.. acceptor and water as a ligand without ..pi.. bonding. The phosphine group has an intermediate character. The effect on the binding of adding one, two, and in the case of carbonyl four ligands is studied in detail. For water and phosphine the binding energy per ligand is larger with two ligands than with only one ligand. There are also inductive effects on nickel which are quadratic in the field strength of the ligand field. Large near-degeneracy effects are found for all studied systems except Ni(H/sub 2/O). When ..pi.. bonds are formed involving the metal 3d orbitals, the corresponding antibonding orbital will always have a high occupation. Even more critical for the binding is the fractional distribution of the two electrons among the pair of (4s,3d/sub sigma/) hybrid orbitals. Depending on the interaction, the occupation of the hybridized orbitals ranges from 1.27, 0.73 for Ni(H/sub 2/O) to 1.94, 0.06 for NiCO. These near-degeneracy effects in general lead to very poor results for relativemore » energies in the SCF approximation. 33 references, 4 figures, 3 tables.« less