Non-spectator behavior of carborane ligands in icosahedral metallacarboranes

Icosahedral metallacarboranes with closo-3,1,2-MC₂B₉ frameworks are formally derived by η⁵ coordination of the open face of a nido-7,8-C₂B₉ cage to a metal atom. Recent work has established that the boron vertices of these faces readily form exo-polyhedral bonds of various kinds. These include linkages to other metal-ligand systems via either three-center two-electron B---H metal of two-center B-metal σ interactions, as well as attachments to organic groups with B---C or B---O bonds. Many new types of molecular structure have been identified, thereby opening a new domain of metallacarborane chemistry which merits further study.