The reactions of dimetallaoctaboranes(12) [(Cp*M)(2)B6H10] [M = Co (1) or Rh (2); Cp* = (eta)-C5Me5] with different chalcogen sources, such as Li[BH2E3] and Li[BH3EPh] (E = S, Se, or Te), led to two unique reaction outcomes. For example, the formation of 10-vertex nido-[(Cp*M)(2)B6E2H6] (3, M = Co, E = S; 4, M = Co, E = Se; 5, M = Co, E = Te; 6, M = Rh, E = Se) from compounds 1 and 2 is a typical representation of a cluster growth reaction, while the formation of arachno[(Cp*Co)(2)B6H9(EPh)] [E = S (9), Se (10), or Te (11)] is a rare method that yielded arachno clusters, keeping the core geometry identical. The formation of arachno-9-11 is a unique method that converts disobedient cluster 1 to obedient clusters 9-11. Further, the reactivity of nido-4 with various metal carbonyls presented sequential cluster growth reactions, which afforded 11-vertex nido[(Cp*Co)(2)B6Se2H6{Fe(CO)(3)}] (7) and 13-vertex fused closo-[(Cp*Co)(2)B6Se2H6{Ru-3(CO)(8)}] (8). The core geometry of nido-7 is uncommon and very similar to that of [C2B9H11](2-) with a unique open pentahapto-coordinating five-membered face.