Linked and fused tungstaborane clusters:: Synthesis, characterization, and electronic structures of bis-{(η5-C5Me5W)2B5H8}2 and (η5-C5Me5W)2P{Fe(CO)3}nB6-nH10-n, n=0,1

in addition to (eta(5)-C5Me5W)(2)B5H9, (eta(5)-C5Me5W)(3)(mu-H)B8H8, and (eta(5)-C5Me5W)(2)B7H9 reported earlier, thermolysis of {eta(5)-C5Me5W}H3B4H8 (1) with BH3 center dot THF leads to the formation of bicapped closo-{eta(5)-C5Me5W}B5H9 (2) in parallel with a B-B linked dimer of {eta(5)-C5Me5W}(2)B5H9 (4) and bis-{eta(5)-C5Me5W}(2)B5H8}(2) (3). Reaction of 4 with a 3-fold excess of Fe-2(CO)(9) in hexane generates an iron analogue of 2, (eta(5)-C5Me5W)(2)B5H9Fe(CO)(3) (5) in 46% yield. All the metallaboranes, 2, 3, and 5, have been characterized by H-1, B-11 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 [{eta(5)-C5Me5Re}(2)B7H7], 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, W2B5, are joined by an exopolyhedral boron-boron bond.