The application of the metal atom technique to the synthesis of metallaboron clusters was investigated. In particular, the reactions of cobalt atoms were examined with three different boron hydrides, pentaborane(9), hexaborane( lo), and decaborane(l4), in the presence of a variety of reactants. Reactions of cobalt vapor and cyclopentadiene with pentaborane(9) gave a number of new and structurally novel metallaborane clusters including 1,2,3-(q-C5H5),Co3B5H, 8-o-(C5H9)l,2,3-(q-C5H5),Co3B5H4 and (y3-CO)-1,2,3-(q-C5H5)3C03B3H3 as well as a series of previously reported polyhedral borane metal clusters. Analogous reactions with hexaborane(l0) resulted in the isolation of the three previously reported cobaltaboranes 1 ,2-(q-C5H5)2C02B4H6, 2-(q-C5H5)CoB9Hlj, and 5-(q-C5H5)CoB9H13. Similarly, reactions with decaborane ( 14) gave 5-(q-C5H5)CoB9H13 along with the new isomer 6-(q-C5H5)CoB9HI3. The metal atom technique has also been shown to allow the direct conversion of boron hydrides and alkynes into metallacarborane clusters. For example, the new metallacarborane 2,5-(CH3)2-1,7,2,5-(q-C5H5)2C02C2B5H5, along with the previously reported metallacarboranes 2,3-(CH3),-1,2,3-(q-C5H5)CoC2B4H4 and 2,3-(CH3)2-1,7,2,3-(q-C5H5)2C02C2B3H3, was isolated in the reaction of cobalt atoms with cyclopentadiene, 2-butyne, and pentaborane(9). Similarly, the metallacarboranes 1,2-(CH3)2-3,1,2-(q-C5H5)CoC2B5H5, 6,7-(CH3)2-2,5,6,7-(q-C5H5)2C02C2B6H8, and 6,7-(CH3)2-2,4,6,7-(q-csH5)2co~c2B6H6 were all isolated in an analogous reaction employing hexaborane(l0). In a separate experiment it was found that the latter compound could be thermally converted to the new isomer 4,9-(CH3)2-2,7,4,9-(g-C5H5)2C02C2B6H6. The reaction of cobalt atoms with pentaborane (9) and diphenylacetylene gave a variety of metallacarboranes including 2,3-(C6H5),-1 ,2,3-(q-C5H5)CoC2B,H5 and 2,3-(C6H5)z-l ,7,2,3-(q-CSH5)CoC2B3H3, along with a unique four-carbon metallacarborane cluster, 4,5,7,8-(C6H,),-1 ,4,5,7,8-(q-C5H5)CoC4B3H3. Finally, it was found that in contrast to the results obtained with the smaller boron hyrides the reaction of cobalt atoms and cyclopentadiene with decarborane( 14) and 2-butyne or diphenylacetylene did not result in the formation of metallacarborane products.