Second moment scaling, metallic and covalent structure rationalization and electron counting rules

Abstract

In this paper we review how a modified tight-binding method, which is based on second moment scaling of the electronic density of states, can be used to rationalize both crystal structure and electron counting rules. The electron counting rules considered are the Hückel 4 N + 2 rule, VSEPR model, the octet rule, Wade's rule for electron deficient clusters and the Hume-Rothery electron concentration rules for pure metals and alloys. We consider covalent and metallic structures based on elements from group 6 to group 17 of the periodic table. Finally we show how both the metallic and covalent bonding found in such systems can be placed into a single cohesive bonding picture. The method of moments proves useful in this analysis.