The ligand tris(2-hydroxyiminopropyl)amine (Ox(3)H(3)) binds to nickel(II) in multiple protonation states. In the neutral state, the X-ray crystal structure of the monomeric complex [Ni(Ox(3)H(3))(NO(3))(H(2)O)](NO(3)).(H(2)O), 1, has six-coordinate pseudo-octahedral geometry, with binding of the amine and three oxime nitrogens, a nitrate, and a water. In the mono-deprotonated form, the X-ray crystal structure shows a dimer, [Ni(Ox(3)H(2))(CH(3)CN)](2)(ClO(4))(2), 2, which has bridging oximate groups and a Ni-Ni distance of 3.575 A. The fully deprotonated complex, 3, shows significantly low Ni(II) oxidation potentials at -390 and +165 mV (versus Fc(+)/Fc). Complex 3 shows reactivity when exposed to O(2), consuming multiple O(2) equivalents and turning from the purple 3 to a dark brown complex, 4. Complex 4 has an EPR spectrum consistent with Ni(III), but spin quantitation accounts for only about 10% of the total Ni, consistent with turnover of the Ni oxidation states. This Ni(II)/O(2) system oxidizes triphenylphosphine to its oxide, with incorporation of the isotopic label from O(2).