The superoxide dismutase (SOD) activity of mononuclear Ni-II complexes, whose structures are inspired by the NiSOD, has been investigated. They have been designed with a sulfur-rich pseudopeptide ligand, derived from nitrilotriacetic acid (NTA), where the three acid functions are grafted with cysteines (L-3S). Two mononuclear complexes, which exist in pH-dependent proportions, have been fully characterized by a combination of spectroscopic techniques including H-1 NMR, UV-vis, circular dichroism, and X-ray absorption spectroscopy, together with theoretical calculations. They display similar square-planar S30 coordination, with the three thiolates of the three cysteine moieties from L-3S coordinated to the N-II ion, together with either a water molecule at physiological pH, as [NiL3S(OH2)](-), or a hydroxo ion in more basic conditions, as [NiL3S(OH)](2)(-). The H-1 NMR study has revealed that contrary to the hydroxo ligand, the bound water molecule is labile. The cyclic voltammogram of both complexes displays an irreversible one-electron oxidation process assigned to the Ni-II/Ni-II redox system with E-pa = 0 . 48 and 0.31 V versus SCE for NiL3S(OH2) and NiL3S(OH), respectively. The SOD activity of both complexes has been tested. On the basis of the xanthine oxidase assay, an IC50 of about 1 mu M has been measured at pH 7.4, where NiL3S(OH2) is mainly present (93% of the Ni-II species), while the IC50 is larger than 100 mu M at pH 9.6, where NiL3S(OH) is the major species (92% of the Ni-II species). Interestingly, only NiL3S(OH2) displays SOD activity, suggesting that the presence of a labile ligand is required. The SOD activity has been also evaluated under catalytic conditions at pH 7.75, where the ratio between NiL3S(OH2)/NiL3S(OH) is about (86:14), and a rate constant, k(cat) = 1.8 X 10(5) M-1 s(-1),has been measured. NiL3S(OH2) is thus the first low-molecular weight, synthetic, bioinspired Ni complex that displays catalytic SOD activity in water at physiological pH, although it does not contain any N-donor ligand in its first coordination sphere, as in the NiSOD. Overall, the data show that a key structural feature is the presence of a labile ligand in the coordination sphere of the Ni-II ion.