Monomeric five-coordinate nickel−cysteine complexes were prepared using anionic tris(3,5-disubstituted pyrazolyl)borates (Tp* - and TpPhMe-) and l-cysteine (ethyl ester and amino acid forms). Tp*NiCysEt crystallizes with a single methanol of solvation in the monoclinic space group P21:  a = 7.8145(18), b = 24.201(6), c = 7.9925(14) Å; β = 117.991(16)°. [Tp*NiCys-][K+] and TpPhMeNiCysEt show magnetic and electronic characteristics similar to Tp*NiCysEt, so that the trigonal bipyramidal coordination geometry confirmed for Tp*NiCysEt in the solid state likely applies to all three. All three complexes have high spin magnetic ground states at room temperature (μeff = 2.9−3.2 μB, S =1). Their electronic spectra are dominated by sulfur to nickel charge-transfer bands (388−430 nm in chloroform) with energies that correlate to respective thiolate basicities and TpX- donor strengths. The Tp* derivatives undergo a rapid reaction with molecular oxygen. Stoichiometric, infrared, and electronic spectroscopy measurements are consistent with formation of a sulfinate as a result of reaction with dioxygen. Kinetics measurements for the reaction of Tp*NiCysEt and O2 fit the following composite rate law:  rate = k 1[Tp*NiCysEt] + k 2[O2][Tp*NiCysEt] with k 1 = 0.013(1) min-1 and k 2 = 4.8(1) M-1·min-1 at 22 °C. Increased nucleophilicity of the nickel−sulfur center enhanced by electron donation from Tp*- (vs TpPhMe-) and encouraged by a trigonal bipyramidal geometry (vs square planar Ni(CysEt)2) is hypothesized as the reason for the susceptibility of Tp*NiCys complexes to oxygen.