Density functional theory calculations on a series of six square-planar NiN₂S₂ complexes have been performed. The nitrogen donor type was varied from diamino in Ni(bme-dmed), 1, to amino-amido in [Ni(mama)]⁻, 2, to diamido in [Ni(ema)]²⁻, 3. The sulfur-oxygenated derivative Ni(bme-O₂-dmed), 4, and hydrogen-bonded derivatives (5 and 6) of 2 and 3 were also studied. Full geometric optimization and subsequent population analyses were performed using the 6–311g(d,p) basis set. The frontier molecular orbitals for all complexes contain significant nickel and sulfur character. Molecular electrostatic potentials show that amido nitrogen donors increase electron density at nickel relative to sulfur. Sulfur modification further shifts electron density away from the ligand towards the metal. It is proposed that the nitrogen donor type and sulfur modification may regulate sulfur-site reactivity in nickel-containing superoxide dismutase.