Electron correlation and hybridization in late-transition-metal compounds

In this article we discuss the transition-metal d-d Coulomb interaction U and the anion-to-cation charge transfer energy Delta for nickel halides and relate these to the observed bandgaps. In addition, we describe the degree of hybridization for NiCl2 and NiBr2 and transition-metal chlorides by considering the ionicity of the chemical bond of these compounds and the results obtained with the Phillips-Van Vechten dielectric theory. The degree of hybridization in transition-metal chlorides has been obtained by considering a simple model which utilizes the photon energy dependence of the photoionization cross-sections for the 3p band of the chlorine ions and the 3d band of the transition-metal ions. Good agreement between the two approaches has been found and a degree of hybridization between 0.15 and 0.25 for transition-metal chlorides is confirmed.