We report an unambiguous analysis of the metal-carbonyl bonding in Ni(CO)4 and Fe(CO)5 which gives new and clear insight into the importance of various contributions to the interaction. Three aspects of the interaction are of particular importance: (i) The frozen orbital repulsion between the metal 4s and the CO is large. This strongly favors the Ni 3d 10 and Fe 3d8 occupations as the starting point for the interaction; (ii) The metal to CO π donation is energetically much more important than the CO to metal σ donation. For Fe(CO)5, the CO σ donation is significant because the 3d shell is not filled and it is able to accept charge; for Ni(CO)4, the Ni d shell is filled and the CO σ donation is very small; (iii) The metal 4s and 4p orbitals make a very small contribution < 0.4 eV to the interaction energy. The large 4s and 4p populations arise because of artifacts of the population analysis and give misleading information about the nature of the bonding. © 1984 American Institute of Physics.