closoboranes

Closo-boranes, or closo borane clusters, are a class of boron-hydride compounds in which boron atoms form a closed, three-dimensional polyhedral framework. The name closo denotes a closed polyhedron. According to Wade's rules, these clusters have the maximum possible skeletal bonding for a given number of vertices, which imparts high symmetry and electron delocalization and generally contributes to their stability.

Prototypical closo boranes include the dodecaborane dianion B12H12^2-, which adopts an icosahedral B12H12^2- cage, and B6H6^2-, with octahedral symmetry. A family of closo boranes exists for various n, described by the general BnHn^2- formula, with structures ranging from octahedral to icosahedral as n increases.

Carboranes are heteroboranes in which one or more boron atoms are substituted by carbon within the closo framework. The neutral closo carborane C2B10H12 occurs in three isomeric forms—ortho-, meta-, and para-carborane—referring to the relative positions of the two carbons in the icosahedral cage. Carboranes are noted for thermal stability and chemical inertness, while providing a boron-rich platform for functionalization.

Chemistry of closo boranes centers on reactions at boron vertices, including halogenation, hydroxy-, alkyl-, and transition-metal substitutions. Their stability arises from delocalized bonding within the closed cage, consistent with Wade's rules. Neutral and anionic closo clusters can be interconverted under appropriate redox and substituent conditions.

Applications span materials science, catalysis, and medicinal chemistry. Closed borane frameworks serve as ligands for transition metals and as boron sources in boron neutron capture therapy (BNCT). Carboranes are used as building blocks in pharmaceuticals and as protective groups due to their robustness.