Using a new N-4-donor chelate ligand having a mixture of hydrophobic phenyl and hydrogen-bond-donor appendages, a trinuclear nickel(II) complex of the doubly deprotonated form of 2-hydroxy-1,3-diphenylpropane-1,3-dione was isolated, characterized (X-ray crystallography, elemental analysis, UV-vis, H-1 NMR, FTIR, and magnetic moment measurement), and evaluated for O-2 reactivity. This complex, [(6-NA-6-Ph(2)TPANi)(2)(mu-PhC(O)C(O)C(O)Ph)(2)Ni](ClO4)(2) (4), has two terminal pseudooctahedral Ni-II centers supported by the tetradentate chelate ligand and a central square-planar Ni-II ion ligated by oxygen atoms of two bridging enediolate ligands. In CH3CN, 4 exhibits a deep orange/brown color and lambda(max) = 463 nm (epsilon = 16 000 M(-1)cm(-1)). The room temperature magnetic moment of 4, determined by Evans method, is mu(eff) = 5.3(2) mu B. This is consistent with the presence of two noninteracting high-spin Ni-II centers, a diamagnetic central Ni-II ion, and an overall quintet ground state. Exposure of a CH3CN solution of 4 to O-2 results in the rapid loss of the orange/brown color to give a green solution. The products identified from this reaction are [(kappa(3)-6-NA-6-Ph(2)TPA)Ni(O2Ph)(H2O)]ClO4 (5), benzil [PhC(O)C(O)Ph], and CO. Identification of 5 was achieved via its independent synthesis and a comparison of its H-1 NMR and mass spectral features with those of the 6-NA-6-Ph(2)TPA-containing product generated upon reaction of 4 with O-2. The independently prepared sample of 5 was characterized by X-ray crystallography, elemental analysis, UV-vis, mass spectrometry, and FTIR. The O-2 reactivity of 4 has relevance to the active-site chemistry of Ni-II-containing acireductone dioxygenase (Ni(II)ARD).