CASSCF study of bonding in NiCO and FeCO

A series of CASSCF calculations were performed on the ground states of NiCO and FeCO. The contributions of the sigma/pi interactions are checked by ex amining the validity of the CASSC calculation to describe the molecule with a particular choice of the active space. The calculation results substanti ate that the stability of MCO is determined by a balance between pi donatio n from the metal 3d(pi) to the CO 2 pi and repulsion between the metal sigm a electrons and the CO 5 sigma lone pair and, at the same time, emphasizes the importance of the synergistic sigma/pi interactions between the metal a nd the CO group. The relative importance of sigma/pi interactions depends o n the nature of the metal. In the case of NiCO, it is the pi donation from Ni 3d(pi) to CO 2 pi that makes the largest contribution to the formation o f the Ni-CO bond, while in the case of FeCO, it is the correlation of sigma electrons that holds the metal and CO together. (C) 1999 John Wiley & Sons , Inc.