Gas-phase nickel nitrate anions are known to produce nickel oxide nitrate anions, [NiOx(NO3)(y))](-) upon fragmentation. The goal of this study was to investigate the properties of nickel oxide nitrate complexes generated by electrospray ionization using a tandem quadrupole mass spectrometer and theoretical calculations. The [Ni(NO3)(3)](-) ion undergoes sequential NO2 center dot. elimination to yield [NiO(NO3)(2)](-) and [NiO2(NO3)](-), followed by elimination of O-2. The electronic structure of the nickel oxide core influences decomposition. Calculations indicate electron density from oxygen is delocalized onto the metal, yielding a partially oxidized oxygen in [NiO(NO3)(2)](-). Theoretical studies suggest the mechanism for O-2 elimination from [NiO2(NO3)](-) involves oxygen atom transfer from a nitrate ligand to yield an intermediate, [NiO(O-2)(NO2)](-), containing an oxygen radical anion ligand, O center dot-, a superoxide ligand, O-2(center dot-), and a nitrite ligand bound to Ni2+. Electron transfer from superoxide partially reduces both the metal and oxygen and yields the energetically favored [NiO(NO2)](-) + O-2 products.