The (1-tert-butyl-2-methyl-1 H-1,2-azaborolyl)cobalt fragment (AbCo), available from AbCo(C2H4)2 (1), serves as a building block in a series of complexes and clusters. The mononuclear complexes AbCo(C2H4)PMe3 (2), AbCo(PMe3)2 (3), AbCo(C2H4)P(OMe)3 (4), AbCo(P(OMe)3)2 (5), AbCo(C2H4)CO (6), and AbCo(CO)2 (7) are easily prepared from 1. AbCo(CpH) (8) is formed from 1 and CpCo(C2H4)2 together with AbCp3Co4H4 (9). The tetranuclear cluster Ab4Co4H4 (10) results, if 1 is warmed to 80 °C without a solvent. Ab3(CO)3Co3 (11) is available by a co-thermolysis of 1 and 7. Thermolysis of 6 leads to the trinuclear cluster Ab3(CO)2Co3 (12). Photolytical degradation of 7 forms the dinuclear complex Ab2(CO)3Co2 (13). Mass spectrometry, IR spectroscopy, and NMR investigations serve to elucidate the molecular structures.