Aspects of anionic framework formation. Clustering of p-block elements

High electron delocalization and coordination numbers are typical of metals. For alloys in which the electronegativities of the components are sufficiently different, electron transfer brings the electronegative elements into new electronic configurations, This transforms metallic architectures either into classical anionic frameworks (Zintl phases) obeying the octet and 8-N valence rules, or into various complex networks in which classical valence rules no longer bold. Intermediate phases with partially delocalized bonding may form with clusters which mostly satisfy Wade's electron-counting rules. Generally, these phases have closed-shell electronic structures and the expected semiconducting properties. However, some of them display poor metallic behaviour,wing to a few additional and somewhat delocalized electrons. The intermetallic chemistry of group 13 elements with alkali metals provides an interesting overview of various structural and electronic situations. Bonding within naked clusters and anionic cages of p-block elements clearly illustrates the transition between the metallic and molecular states. (C) 1998 Elsevier Science S.A. All rights reserved.