Coulomb correlation energy versus covalence in transition-metal compounds

We have performed optical and photoemission studies on Mott-Hubbard and charge transfer insulators with formal ionic configurations 3d3 (CrCl3, CrBr3) and 3d8 (NiCl2, NiBr2, NiI2). The photoemission spectra have been analyzed in terms of a cluster model leading to estimates of the on-site Coulomb repulsion energy, charge transfer energy and hybridization energy parameters. The ionicity parameter fi in Cr and Ni compounds has been calculated by the means of the Phillips-Van Vechten theory: its value is about 0.80 in Cr halides, and it varies from 0.70 in NiI2 to 0.80 in NiCl2 in Ni halides. We have also considered the ionicity scale fiDT of the dielectric theory. The two scales allow a good understanding of the chemical bond character and electron correlation in these materials.