FeO2- anions were produced by co-condensation of laser-ablated iron atoms and electrons with dioxygen in excess argon at 6 K. A photosensitive absorption at 870.6 cm(-1) is assigned to the antisymmetric OFeO stretching vibration (nu(3)) of the inserted FeO2-anion trapped in solid argon. On the basis of the observed nu(3) vibrational frequencies for (FeO2)-O-16, and (FeO2)-O-18, the anion is estimated to be linear. Due to the severe symmetry-breaking problems of the reference wave function, calculations with single-reference methods, including various DFT and post-HF methods, are unreliable for this molecule. However, the state-averaged multireference MRC1 method, which incorporates both dynamical and nondynamical correlations, predicted that the anion has a linear doublet ground state, consistent with the experimental observations.