The geometries, vibrational frequencies, and metal-ligand bond dissociation energies of 18 different structures of the Cu(NO2)(2) complex have been studied. Mg(NO2)(2) and Cu(NO3)(2) have also been studied for comparison. The most stable structure of Cu(NO2)(2) and CuO(NO3)(2) corresponds to a D-2h one with a coplanar eta(2)-O,O coordination for the two NO2 ligands. For Mg(NO2)(2) the most stable structure is a D-2d one. The bonding in the D-2h and D-2d structures of Cu(NO2)(2) is analyzed. For the MNO2 systems the binding energy is very similar with both metals, while for the M(NO2)(2) complexes the difference when changing the metal is very important. This behavior is related to the first and second ionization potentials of Cu and Mg. The computed vibrational frequencies are in good agreement with the available experimental data.