The Electronic Structure of Metal Dimers and Metal Clusters: The Eighteen-Electron Rule vs. Skeletal Electron-Pair Counting

Abstract

The 18-electron or effective atomic number rule (EAN)¹ has been extremely valuable to organometallic chemists in directing their thinking toward new compounds and “explanations” of the structure and bonding in known compounds. In metal dimers one often postulates the degree of metal-to-metal bonding by enforcing the 18-electron rule. As chemists made larger and larger clusters, there came the realization that the 18-electron rule was difficult, if not impossible to enforce. This problem lead to the formulation of skeletal electron pair counting (SEPC)² and cluster valence electron counting (CVEC).³ A strong connection between the bonding in main group clusters such as the boranes and that in organometallic or low oxidation state transition metal clusters can be made through the isolobal principle.^4,5 Again SEPC and the isolobal principle have lead to suggestions for new compounds and new synthetics routes, as well as “explanations” for the structure of existing compounds.