We have synthesized and structurally characterized several dimetallaheteroborane clusters, namely, nido-[(Cp*Mo)2B 4SH6], 1; nido-[(Cp*Mo)2B 4SeH6], 2; nido-[(Cp*Mo)2B 4TeClH5], 3; [(Cp*Mo)2B 5SeH7], 4; [(Cp*Mo)2B6SeH 8], 5; and [(CpW)2B5Te2H 5], 6 (Cp* = η5-C5Me5, Cp = η5-C5H5). In parallel to the formation of 1-6, known [(CpM)2B5H9], [(Cp*M) 2B5H9], (M = Mo, W) and nido-[(Cp*M) 2B4E2H4] compounds (when M = Mo; E = S, Se, Te; M = W, E = S) were isolated as major products. Cluster 6 is the first example of tungstaborane containing a heavier chalcogen (Te) atom. A combined theoretical and experimental study shows that clusters 1-3 with their open face are excellent precursors for cluster growth reactions. As a result, the reaction of 1 and 2 with [Co2(CO)8] yielded clusters [(Cp*Mo)2B4H4E(μ3-CO) Co2(CO)4], 7-8 (7: E = S, 8: E = Se) and [(Cp*Mo)2B3H3E(μ-CO) 3Co2(CO)3], 9-10 (9: E = S, 10: E = Se). In contrast, compound 3 under the similar reaction conditions yielded a novel 24-valence electron triple-decker sandwich complex, [(Cp*Mo) 2{μ-η6:η6-B3H 3TeCo2(CO)5}], 11. Cluster 11 represents an unprecedented metal sandwich cluster in which the middle deck is composed of B, Co, and Te. All the new compounds have been characterized by elemental analysis, IR, 1H, 11B, 13C NMR spectroscopy, and the geometric structures were unequivocally established by X-ray diffraction analysis of 1, 2, 4-7, and 9-11. Furthermore, geometries obtained from the electronic structure calculations employing density functional theory (DFT) are in close agreement with the solid state structure determinations. We have analyzed the discrepancy in reactivity of the chalcogenato metallaborane clusters in comparison to their parent metallaboranes with the help of a density functional theory (DFT) study. © 2013 American Chemical Society.