Copper nitrate anion clusters Cu(NO3)(3-) and Cu(NO3)(2-) were generated by electrospray ionization and studied with collision-induced dissociation and energy-resolved mass spectrometry. Collision-induced dissociation resulted in three different fragmentation reactions-loss of NO3-, NO3 center dot, and NO2 center dot. The type of fragmentation reaction depends on the oxidation state of the metal. The Cu(NO3)(3-) cluster showed loss of NO3 center dot but no loss of NO2 center dot, whereas the Cu(NO3)(2-) cluster showed loss of NO2 center dot but no loss of NO3 center dot. The fragmentation reactions were studied by theoretical methods. These studies show loss of NO3 center dot corresponds to reduction of the metal charge by electron transfer, whereas loss of NO2 center dot and metal-oxide bond formation by O- abstraction in Cu(NO3)(2-) does not necessarily result in the expected oxidation of the metal.