Chemical bond and hybridization in nickel compounds

By means of ultraviolet reflectance and valence band photoemission one can get a scale of ionicity for nickel halides. The fraction of ionic-character of the chemical bond, from 0.72 in NiI2 to 0.80 in NiCl2, evaluated with the Phillips theory has been related to the ground state d electron counts (n(dn)) calculated from Ni2p core photoemission. While the chemical trends of cluster model parameters are in general well reproduced, a systematic discrepancy in band gap values (2-3 eV) has been found by comparing core-level results with optical and photoconductivity data.