AN ENERGY CRITERION FOR DETERMINING RELATIVE SIGMA-CONTRIBUTIONS AND PI-CONTRIBUTIONS IN TRANSITION-METAL CARBONYL BONDING

In this paper a new criterion is presented for determining the contribution of a particular class or group of orbitals to a chemical bond. The new criterion is the diatomic energy contribution of particular orbitals in a particular bond (any diatomic atom pair) to the total molecular energy. It is a unique feature of NDO (neglect of diatomic overlap) type procedures that the total energy can be divided entirely into monatomic and diatomic energy terms. Here this method is applied to determining the contribution of sigma and pi-orbitals to the cohesive energy of CO adsorbed on a cluster of 12 iron atoms. While recent theoretical calculations have tended to downplay the role of the sigma-orbitals in metal-CO bonding, the results here show that the largest contribution to the stability of the metal-carbon bond is provided by the sigma-electrons.