Both the cations and anions of the elemental composition [FeO2] were generated by chemical ionization of a mixture of Fe(CO)(5) and O-2, and the connectivities of the ions were probed by various mass spectrometric methods. The neutral [FeO2] molecule was generated from either the anion and cation by means of neutralization reionization (NR) mass spectrometry. Collisional activation, charge reversal, and NR studies, as well as results of ion-molecule reactions of thermalized [FeO2](+) and [FeO2](-) ions with different substrates under the conditions of Fourier transform ion cyclotron resonance mass spectrometry, were employed to explore the potential energy hypersurfaces for both ions. The connectivities of the cations are best described as a side-on peroxo complex of the triplet dioxygen molecule with the metal cation, Fe(O-2)(+) (I), which easily isomerizes to the high-valent iron(V) dioxide, OFeO+ (III). For the anion we suggest the structure of the iron(III) dioxide anion, OFeO-. With respect to the neutral [FeO2] molecules, generated from the corresponding cation and anion by vertical electron transfer in the NR experiments, both connectivities seem to be stable on the mass spectrometric timescale. In addition, the energetics and binding situations in side-on and end-on Fe(O-2)(+) complexes as well as in the inserted iron dioxide cation, OFeO+, were examined by means of ab initio MO calculations at the CASPT2D level of theory. The calculations predict a bond dissociation energy of 24 kcal/mol for the (6)A(1) State of the C-2v-symmetrical side-on complex Fe(O2)(+) (I) and of 29 kcal/mol for the (6)A(1) state of bent OFeO+ (III) with respect to isolated Fe+(D-6) and O-2((3) Sigma(g)(-)). These numbers agree well with the bracketed BDE of 25 +/- 6 kcal. The interconversion of side-on Fe(O-2)(+) (I) to the energetically more stable OFeO+ (III) and its implications for the interpretation of the mass spectrometric experiments are discussed.