The novel versatile cobalt(I) tris-carbene complex [(TIMEN^xyl)Co]Cl (1) (where TIMEN = (tris[2-(3-arylimidazol-2-ylidene)ethyl]amine) reacts with CO, one-electron oxidizers such as CH₂Cl₂, and O₂ to yield the cobalt complexes [(TIMEN^xyl)Co(CO)]Cl (2), [(TIMEN^xyl)Co(Cl)]Cl (3), and peroxo species [(TIMEN^xyl)Co(O₂)](BPh₄) (5). All new complexes were fully characterized by ¹H 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 η²-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.