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CPF, MCPF, and multireference CCI calculations have been done on nickel carbonyl and its positive and negative ions. For obtaining an accurate value of the dissociation energy De of Ni–CO it is important to correlate all the CO valence electrons. The difference in De with and without CO correlation is 17 kcal/mol which is about half of the total De. The best CCI calculation gave a dissociation energy of 33 kcal/mol in good agreement with experiments. The CPF and MCPF dissociation energies were 30 kcal/mol. The largest CCI calculation also gave a frequency shift between CO and NiCO in excellent agreement with experiments, 143 cm−1 compared to 142 cm−1. The effect of coupling the Ni–C and C–O motions is 20 cm−1 on the shift. The calculations on NiCO− show that the added electron enters into the σ symmetry and leads to an almost unchanged Ni–C bond length and a slight reduction of the dissociation energy. When NiCO is ionized on the other hand the removed electron in the σ symmetry causes a large increase of 0.8 a0 in the Ni–C bond length but does not change the dissociation energy.

Multireference CCI calculations have been performed for the diatomic molecules NiC, NiN, and NiO, and for the cluster systems NisC, Ni5N, and NisO. Of the diatomic molecules, NiN has by far the smallest dissociation energy, which we argue is primarily due to the small electron affinity of nitrogen. The bonding to nickel 3d, which is pronounced for the diatomic molecules, disappears almost entirely for the cluster systems. In the full geometry optimization of the Ni5X systems, including cluster relaxation, the final geometries are rather similar for the three adsorbates, all being adsorbed below the surface. The energy gain in penetrating the surface is much larger for carbon and nitrogen than it is for oxygen, however. This is in line with the experimentally determined surface geometries, where carbon and nitrogen adsorb close to the surface and reconstruct the surface, whereas oxygen stays further out and does not reconstruct the surface. For oxygen the restoring forces from neighboring atoms in the surface are larger than the driving force to reconstruct the surface. The origin of the difference between the adsorbates is that the 2pz orbital, which points toward the surface, is almost doubly occupied for oxygen, whereas this orbital is only partially occupied for carbon and nitrogen.

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Theoretical models for reductive elimination from transition metal containing molecules have been studied using large scale contracted CI calculations. Four different models were treated, namely, NiH2, PdH2, Ni(CH3)2, and Pd(H20)zHz. in order to study the effects of adding ligands, exchanging hydrogens with methyl groups, and comparing nickel and palladium. The most interesting result already appeared for the simplest system NiH2. A closed-shell-type ' A 1 state with a small bond angle of only 57O is bound compared to Ni and H2 with only a very small barrier for formation. The bond distance is short, shorter than in NiH, and the d orbitals are strongly involved in the binding. The hydrogen atoms bind both to nickel and to each other. With methyl groups rather than hydrogens, this double sided bonding situation is destroyed and Ni(CH3)Z has a negative binding energy with the carbon bonds pointing towards nickel. For PdH2 only a weakly bound complex between an essentially unchanged H Z and Pd was found. ThePond distance is very long. Adding H 2 0 ligands to Pd shortens the bond distance and significantly opens up the bond angle. The methods used in the investigation and the chemical implications of the results are discussed.