Metallaboranes of the Early Transition Metals: Direct Synthesis and Characterization of [{eta(5)-C5Me5)Ta}(2)BnHm] (n=4, m=10; n=5, m=11), [{(eta(5)-C5Me5)Ta}(2)B5H10(C6H4CH3)], and [{(eta(5)-C5Me5)TaCl}(2)B5H11]

Reaction of [Cp*TaCl4] (Cp* = eta(5)-C5Me5) with a sixfold excess of LiBH4 center dot thf followed by BH3 center dot thf in toluene at 100 degrees C led to the isolation of hydrogen-rich metallaboranes [(CP*Ta)(2)B4H10] (1), [(CP*Ta)(2)B5H11] (2), [(CP*Ta)(2)B5H10(C6H4CH3)] (3), and [(Cp*TaCl)(2)B5H11] (4) in modest yield. Compounds 1-3 are air- and moisture-sensitive but 4 is reasonably stable in air. Their structures are predicted by the electron-counting rules to be a bicapped tetrahedron (1), bicapped trigonal bipyramids (2, 3), and a nido structure based on a closo dodecahedron (4). Yellow tantalaborane 1 has a nido geometry with C-2v symmetry and is isostructural with [(Cp*M)(2)B4H8] (M = Cr and Re); whereas 2 and 3 are C-3v-Symmetric and isostructural with [(Cp*M)(2)B5H9] (M = Cr, Mo, W) and [(Cp*ReH)(2)B5Cl5]. The most remarkable feature of 4 is the presence of a hydride ligand bridging the ditantalum center to form a symmetrical tantalaborane cluster with a long Ta-Ta bond (3.22 angstrom). Cluster 4 is a rare example of electronically unsaturated metallaborane containing four TaHB bonds. All these new metal-laboranes have been characterized by mass spectrometry, H-1, B-11, and C-13 NMR spectroscopy, and elemental analysis, and the structural types were unequivocally established by crystallographic analysis of 1-4.