CASSCF Study of Bonding in NiCO

ABSTRACT: A series of CASSCF calculations were performed on the ground states of NiCO and FeCO. The contributions of the srp interactions are checked by examining the validity of the CASSC calculation to describe the molecule with a particular choice of the active space. The calculation results substantiate that the stability of MCO is determined by a balance between p donation from the metal 3d to the CO 2p andp repulsion between the metal s electrons and the CO 5s lone pair and, at the same time, emphasizes the importance of the synergistic srp interactions between the metal and the CO group. The relative importance of srp interactions depends on the nature of the metal. In the case of NiCO, it is the p donation from Ni 3d to CO 2p that makes thep largest contribution to the formation of the Ni—CO bond, while in the case of FeCO, it is the correlation of s electrons that holds the metal and CO together. Q 1999 John Wiley &amp