The novel versatile cobalt(I) tris-carbene complex [(TIMENxyl)Co]Cl (1) (where TIMEN = (tris[2(3-arylimidazol-2-ylidene)ethyl]amine) reacts with CO, one-electron oxidizers such as CH2Cl2, and O-2 to yield the cobalt complexes [(TIMENxyl)Co(CO)]Cl (2), [(TIMENxyl)Co(Cl)]Cl (3), and peroxo species [(TIMENxyl)Co(O-2)](BPh4) (5). All new complexes were fully characterized by H-1 NMR, UV/vis, and IR spectroscopy as well as superconducting quantum interference device (SQUID) magnetization measurements and single-crystal X-ray crystallography. The nucleophilic character of the eta(2) -bound dioxygen ligand in 5 was confirmed by density functional theory (DFT) studies and allows for oxygen-transfer reactions with electron-deficient organic substrates, such as benzoyl chloride.