The electrical conductivity and optical absorption data for many compounds of the transition metals suggest that there is a trend from ionic insulators, to metals, to covalent semiconductors as the overlap of atomic orbitals is increased. In this paper it is shown that a correlation exists between the electrical behavior of insulators and metals and the magnitude of the overlap integralsS(d&egr;d&egr;) andS(d&egr;p&pgr;).