Metallaborane reaction chemistry. Macropolyhedral metallaheteroborane synthesis by direct heteroatom insertion. Formation of the twenty-vertex [(η5-C5Me5)IrB18H19S]− anion from nineteen-vertex syn-[(η5-C5Me5)IrB18H20]

Syn-[(η5-C5Me5)IrB18H20] (1), from [{IrCl2(η5-C5Me5)}2] and syn-B18H22 with base, consists of nido-type twelve-vertex {IrB11} and ten-vertex {B10} subclusters fused with a common {B3} triangular face. Reaction of 1 with excess elemental sulfur and base results in the generation of the iridathiaborane anion [(η5-C5Me5)IrB18H19S]− (2), which consists of a nido-type twelve-vertex {IrB11} subcluster and an arachno-type ten-vertex {SB9} subcluster fused with a {B2} edge in common. The reductive addition of electron-rich sulfur results in a loss in the degree of intimacy of the intercluster fusion together with a nido→arachno conversion of the non-metalla subcluster character.