Transition metal catalyzed selective cage B-H functionalization of carboranes has made significant progress in recent years, giving rise to the efficient synthesis of a large variety of cage B-functionalized carboranes including alkenylation, arylation, alkynylation, borylation, hydroxylation, acyloxylation, amination, and halogenation. However, the mechanisms of these catalytic B-X coupling reactions are not well understood. Herein, we describe the isolation and characterization of the catalytically relevant