Reaction of dimeric bis{[mu-(2-mercaptoethyl)(2-mercaptoethyl)methylaminato(2-)] nickel(II)}, 1, with either O-2 or H2O2 leads to the formation of a dimeric product, bis{[mu-(2-mercaptoethyl)(2-sulfinatoethyl)methylaminato (2-)]nickel(II)}, 2. The reaction is first-order in [Ni] and proceeds at a very slow rate at 40 degrees C (t(1/2) = 7.8 days; k = 1.0(1) x 10(-6) s(-1)) under 1 atm of O-2. Crystals of 2 form in the orthorhombic space group Pbca with cell dimensions a = 14.271(2) Angstrom, b = 20.424(3) Angstrom, c = 11.766(2) Angstrom, V = 3429.4(9) Angstrom(3), and Z = 8. A single-crystal X-ray diffraction study of these crystals reveals a dimeric structure that is quite similar to 1, where the terminal thiolates have been oxidized to sulfinate ligands in 2. Compound 1 has several structural features in common with the dimeric mu-dithiolato Ni,Fe site found in the active site of Desulfovibrio gigas hydrogenase. The structural similarities and the potential relevance of the oxidation reaction are discussed.