In addition to (η 5-C 5Me 5W) 25H 9, (η 5-C 5Me 5W) 3(μ-H)B 8H 8, and (η 5-C 5Me 5W) 2B 7H 9 reported earlier, thermolysis of {η 5-C 5Me 5 W}H 3B 4H 8 (1) with BH 3·THF leads to the formation of bicapped closo-{η 5C 5Me 5W} 2B 6H 10 (2) in parallel with a B-B linked djmer of {η 5-C 5Me 5W} 2B 5H 9 (4) and bis-{η 5-C 5Me 5W) 2B 5H 8} 2 (3). Reaction of 4 with a 3-fold excess of Fe 2(CO) 9 in hexane generates an iron analogue of 2, (η 5-C 5Me 5W) 2B 5H 9Fe(CO) 3 (5) in-46% yield. AU the metallaboranes, 2, 3, and 5, have been characterized by 1H, 11B NMR spectra and crystal structure determinations. The structures of 2 and 5 appear to follow the regular electron counting rules as they possess seven skeletal electron pairs (sep) appropriate for bicapped octahedral structures. However, their electronic structures revealed by a molecular orbital analysis is more closely related to that of [{η 5-C 5Me 5Re} 2B 7H 7], a cluster that does not obey the classical electron counting rules. Clusters 2 and 5 are notable examples of closo clusters containing four bridging hydrogens. 3 is a coupled-cage structure in which two bicapped trigonal bipyramidal frameworks, W 2B 5, are joined by an exopolyhedral boron-boron bond. © 2007 American Chemical Society.