The metal–carbonyl bond in Ni(CO)4 and Fe(CO)5: A clear-cut analysis

Abstract

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 3d¹⁰ 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 s and 4p populations arise because of artifacts of the population analysis and give misleading information about the nature of the bonding.