A mononuclear Ni(II) complex ([(6-Ph(2)TPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO4 (1)), supported by the 6-Ph(2)TPA chelate ligand (6-Ph(2)TPA = N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine) and containing a cis-beta-keto-enolate ligand having a C2 hydroxyl substituent, undergoes reaction with O-2 to produce a Ni(II) monobenzoate complex ([(6-Ph(2)TPA)Ni(O2CPh)]ClO4 (3)), CO, benzil (PhC(O)C(O)Ph), benzoic acid, and other minor unidentified phenyl-containing products. Complex 3 has been identified through independent synthesis and was characterized by X-ray crystallography, H-1 NMR, FAB-MS, FTIR, and elemental analysis. A series of cis-beta-keto-enolate Ni(II) complexes supported by the 6-Ph(2)TPA ligand ([(6-Ph(2)TPA)Ni(PhC(O)CHC(O)Ph)]ClO4 (4), [(6-Ph(2)TPA)Ni(CH3C(O)CHC(O)CH3)]ClO4 (5), and [(6-Ph(2)TPA)Ni(PhC(O)CHC(O)C(O)Ph) (6)) have been prepared and characterized. While these complexes exhibit structural and/or spectroscopic similarity to 1, all are unreactive with O-2. The results of this study are discussed in terms of relevance to Ni(II)-containing acireductone dioxygenase enzymes, as well as in the context of recently reported cofactor-free, quercetin, and beta-diketone dioxygenases.