A series of CASSCF calculations were performed on the ground states of NiCO and FeCO. The contributions of the σπ 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 π donation from the metal 3dπ to the CO2π and p repulsion between the metal σ electrons and the CO5σ lone pair and, at the same time, emphasizes the importance of the synergistic σπ interactions between the metal and the CO group. The relative importance of σπ interactions depends on the nature of the metal. In the case of NiCO, it is the π donation from Ni 3dπ to CO2π that makes the σ largest contribution to the formation of the Ni-CO bond, while in the case of FeCO, it is the correlation of σ electrons that holds the metal and CO together.