The dianionic NiN2S2 complex, Ni(ema)(2-), ema = N,N'-ethylenebis-2-mercaptoacetamide, known as a reasonable model of the tripeptide complex Ni(CGC)(2-) (C = cysteine; G = glycine) with respect to the two carboxyamido nitrogens and cis-dithiolates in a (N2S2)(4-) ligand scaffold as found in acetyl CoA synthase, has been explored for S-based reactivity toward oxygenation and alkylation. The isolation and structural characterization of a sulfinato species, [Et4N](2)[Ni(ema)center dot O-2], prepared through a unique direct reaction of molecular O-2 with crystalline [Et4N](2)[Ni(ema)] is described. Reaction of [Et4N](2)[Ni(ema)] with Br(CH2)(3)Br yields a neutral N2S2 macrocyclic complex shown by DFT computations and electrostatic-potential mapping to be opposite in electron distribution from the neutral NiN2S2 complexes in which the anionic charge is localized on sulfur.