Fragmentation of Cr(NO3)4(-): Metal Oxidation upon O(•-) Abstraction

The decomposition of chromium nitrate anion, Cr(NO3)4(-), was investigated by tandem mass spectrometry. The major fragments correspond to sequential elimination of NO2(•) via O(•-) abstraction from each nitrate ligand to yield CrOn(NO3)(4-n)(-), n = 1-4, products. The metal is oxidized upon the first three O(•-) abstraction reactions to yield the fully oxidized Cr(VI), closed-shell, CrO3(NO3)(-) fragment. A CrO4(-) fragment was detected, but the metal is not further oxidized upon the fourth O(•-) abstraction. Experiment and theory indicate the first three O(•-) abstraction reactions are low energy processes, but the formation of CrO4(-) is considerably higher in energy. Theoretical studies show the 3d electrons in chromium are removed by O(•-) for CrOn(NO3)(4-n)(-), n = 1-3, to yield oxo, O(2-) ligands, but the electron density is replaced by donation from π bonds involving the oxygen lone pairs. Theory predicts a decrease in metal charge for each O(•-) abstraction, opposite the trend expected for oxidation, due to π electron donation from the oxygen atoms.