Unknowns in the Chemistry of boron

Abstract

Structural links between benzenoid aromatics and graphite as well as saturated hydrocarbons and diamond are seen in high school text books. A similar understanding is only beginning to emerge in the chemistry of boron. Unlike benzenoid aromatics where the condensation is usually by edge-sharing, there are several ways of condensing polyhedral boranes. These include edge-sharing, triangular face sharing, four atom sharing and a single atom sharing. An electron counting mno rule similar to the Huckel 4n + 2 π electron counting rule will be presented for mono and condensed polyhedral boranes. Application of this rule shows that the structural details of β-rhombohedral boron, such as the vacancies and extra occupancies in the unit cell are a consequence of the electronic requirements rather than defects in the structure. While this is the beginning of a general understanding, there is a long way to go before the quantitative details emerge in this area, along with the synthesis and chemistry of a variety of condensed boranes. These ideas also provide a relationship between boron and fullerenes. Further, designing stable boron-rich candidates for physiological applications and materials for use in extreme conditions are areas that are waiting to be explored.