From the Metal to the Molecule-Ternary Bismuth Subhalides.

Subvalent compounds, that is, metal-rich substances in which the average oxidation state of the cation is smaller than would be expected from the (8-N) rule, have proved to be a rich source of unexpected structural and physical features. The extraordinary structural chemistry generally observed in subvalent compounds is a consequence of the low and often non-integer oxidation states of the metal atoms coupled with the low concentration of valence electrons. Both factors can lead to a wide-range of bonding types within the same compound. A characteristic of these compounds is the interplay between "metallic" regions, with delocalized electrons and mainly nonpolar bonds between the metal atoms, and "saltlike" regions, which are characterized by strong localization of the electrons and heteropolar exchange between the metal and nonmetal atoms. The volumes of the different structural regions as well as the extent to which they interpenetrate can vary from compound to compound. The ternary subhalides of bismuth belong to a new class of substances which cover the whole spectrum from partially oxidized "porous" metals, through one- and two-dimensional metals, up to semiconducting ionic or molecular cluster compounds. These subvalent compounds with their unusually high chemical stabilities provide excellent vehicles for further research and their potential is described in the following article.