UNUSUAL VALENCES IN RARE-EARTH-HALIDES

The valence of an atom in a particular compound is considered unusual when there is a difference between the "localized" valence and the oxidation number. The degrees of delocalization of electrons may be different Total delocalization gives rise to a conduction band. There may also be partial delocalization with multicenter or cluster bonding. With such a definition, reduced rare-earth halides such as metallic LaI2 or semiconducting Pr2Br5 and Gd2Cl3 with [Gd6] clusters contain unusual valences but TmI2 does not. In addition to the generalities of the synthesis and properties of reduced rare-earth halides, the electronic structures have been calculated making use of the extended Huckel algorithm. These calculations aim to yield a better understanding of bonding and the conductivity properties of these compounds.