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• elimination to yield [NiO(NO3)2]− and [NiO2(NO3)]−, followed by elimination of O2. 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 O2 elimination from [NiO2(NO3)]− involves oxygen atom transfer from a nitrate ligand to yield an intermediate, [NiO(O2)(NO2)]−, containing an oxygen radical anion ligand, O•–, a superoxide ligand, O2•–, 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)]− + O2 products.