Ab initio computations at the B3LYP/ECP level have been performed on a series of charged and neutral rhodium species containing carbon monoxide and dinitrogen ligands. The computations have been employed to predict possible oxidation states of supported Rh for new surface species detected by infrared spectroscopy during the interaction of the metal with CO and N-2 in the presence of ultraviolet radiation. Specifically, it has been concluded that Rh species containing N-2 ligands giving rise to low-frequency N-N stretching modes most likely contain Rh with a -1 oxidation state, the source of electrons being due to photoinduced electron transfer between supported Rh metal centers in the presence of UV irradiation. Although the computations refer to gas-phase Rh species, it is believed that computations, such as those reported herein, can be useful in predicting trends in vibrational frequencies for similar surface species and thus aid in vibrational band assignments for specific structures.