J. Phys. Chem. 1985,89, 2171-2180 tionally omitted. Since this study was primarily to investigate the accuracy of using various types of Lo's in correlated methods, the test molecules employed were all quite small and highly symmetric. Since the LO methods do not exploit this symmetry and virtually no integrals are zero due to localization for such small systems, the LO calculations were actually more expensive than the corresponding S C F orbital calculations. We are currently working on an LO study of larger systems such as pentane and decane that will discuss in detail the relative timings, number of integrals, etc. Of course, one of the primary simplifications that may be introduced due to using localized orbitals is that the C C amplitudes t t b that actually need to be evaluated may be reduced
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to a much smaller number, since most are far smaller than in the delocalized S C F case.4 In these initial calculations, however, we have made no such simplification to permit complete comparisons with the usual delocalized S C F reference results. For realistic correlated applications to large molecules, the clusters such as @ should dominate, perhaps leading to a systematic classification of the various correlation contributions.
Acknowledgment. The authors thank Prof. M. C. Zerner for many helpful discussions. This research was supported, in part, by the Army Research Office under Contract No. DAAG2984-K-0025.
Interaction between Nlckel and the Ligand Groups Carbonyl, Water, and Phosphine Margareta R. A. Blomberg, Ulf B. Brandemark, Per E. M. Siegbahn,* Institute of Theoretical Physics, University of Stockholm, Vanadisvagen 9, S-113 46 Stockholm, Sweden
Kirsten Broch Mathisen, University of Tromso, N-9001 Tromso, Norway
and Gunnar Karlstrom Department of Physical Chemistry I, Chemical Center, University of Lund, S-220 07 Lund. Sweden (Received: August 8, 1984)
The interaction between nickel and three different types of ligands are studied at the CASSCF and contracted CI level. The carbonyl group is used as a model for a strong A acceptor and water as a ligand without A 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. The main reason for this is that the advantageous sd, hybridization on nickel, occurring already for one ligand, makes place for two ligands rather than one at a time. 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(H20). When A 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,) hybrid orbitals. Depending on the interaction the occupation of the hybridized orbitals ranges from 1.27, 0.73 for Ni(H20) to 1.94, 0.06 for NiCO. These near-degeneracy effects in general lead to very poor results for relative energies in the SCF approximation.
I. Introduction M~~~ important chemical reactions are catalyzed by transition metals, either homogeneously or heterogeneously, and the mechanisms of transition-metal catalysis is therefore a research area of great current interest. In homogeneous catalysis, which occurs in solution, the transition metal participates in the form of a metal complex and the ligands on the central metal atom can be used to modify the reaction conditions. In previous papers' simple models have been used to study the mechanisms of certain reaction steps in homogeneous transition-metal catalysis. It is our purpose to continue these studies, incorporating more complexity into our models. One step in this direction is to investigate the influence of different types of ligands on the reaction mechanisms. Such studies are in progress and in the present paper we present results from a study in which the metal-ligand interaction is investigated. The models used here are selected to be representativeof the different types of bonding which are present in the interaction between closed-shell molecules and transition metals. (1) M. Blomberg, U. Brandemark, L.Pettersson, and P. Siegbahn, In?.J . Quanrum Chem., 23, 855 (1983).
0022-3654/85/2089-2171$01.50/0
In previous studiesz4 the reductive elimination of R1-R2 from a transition metal has been studied as an important reaction step in processes LM ,
