We report the synthesis, isolation and structural characterization of several moderately air stable nido-metallaboranes that represent boron rich open cage systems. The reaction of [Cp*CoCl]₂, (Cp* = η⁵-C₅Me₅), with [BH₃·thf] in toluene at ice cold temperature, followed by thermolysis in boiling toluene produced [(Cp*Co)B₉H₁₃], 1 [(Cp*Co)₂B₈H₁₂], 2 and [(Cp*Co)₂B₆H₁₀] 3. Building upon our earlier reactivity studies on rhodaboranes, we continue to explore the reactivity of dicobalt analogues of octaborane(12) cluster 3 with [Fe₂(CO)₉] and [Ru₃(CO)₁₂] at ambient conditions that yielded novel fused clusters [Fe₂(CO)₆(Cp*Co)₂B₆H₁₀], 4 and [Ru₄(CO)₁₁(Cp*Co)₂B₃H₃], 5 respectively. In an attempt to synthesize a heterometallic metallaborane compound we performed the reaction of [(Cp*Rh)₂B₆H₁₀], 6 with [Cp*IrH₄] that yielded a Ir–Ir double bonded compound [(Cp*Ir)₂H₃][B(OH)₄], 7. All the new compounds have been characterized by IR, ¹H, ¹¹B, ¹³C NMR spectroscopy, and the molecular structures were unambiguously established by X-ray diffraction analysis.