Borane Polyhedra Beyond Icosahedron

Polyhedral boron cluster chemistry has been dominated over the last six decades by icosahedral borate dianions, carboranes, metallacarboranes and metallaboranes owing to their high stabilities and rich chemistry. After the first isolation of the closo-borate dianion [B10H10]2− in 1959, many other closo-[BnHn]2− anions (n = 5–9, 11 and 12) were synthesized by Hawthorne, Lipscomb, Muetterties and others. However, the first crystallographic characterization of icosahedral K2[B12H12] by Lipscomb in 1960 stimulated the chemistry and bonding investigations of these closo-[BnHn]2− (n = 6–12) anions. Interestingly, although the closo-[B7H7]2− was synthesized and partially characterized, the X-ray structure of the heptaborane dianion was not established until very recently. Bernhardt and co-worker have fully characterized closo-[B7H7]2− very recently and subsequently, we have isolated a transition metal guarded closo-heptaborane. After exploring the lower borane series, chemists have started envisaging supraicosahedral boranes. Although theoretical calculations by Lipscomb, Jemmis and Schleyer suggested that closo-borate dianions beyond icosahedron [BnHn]2− (n ˃ 12) are stable, the parent closo-borate dianions [BnHn]2− (n ˃12) have not been isolated yet. Following the first isolation of a supraicosahedral metallacarborane in 1971, carboranes, metallacarboranes and metallaboranes have been synthesized with supraicosahedral geometries. The bonding and characteristic electron counts of these supraicosahedral clusters are quite different from those of the lower nuclearity clusters. This chapter describes the synthesis, structural features and electron counts of 13–16-vertex borane, carborane, metallacarborane and metallaborane polyhedral molecules.