Two interconvertible iron dioxide-dioxygen complexes were prepared and characterized by matrix isolation infrared absorption spectroscopy as well as theoretical calculations. Iron atoms react with O-2 to form the inserted FeO2 molecule in solid argon only upon UV-visible light irradiation. Annealing allows the dioxygen molecules to diffuse and to react with FeO2 and form the side-on and end-on bonded dioxygen-iron dioxide complexes, (eta(2)-O-2)FeO2 and (eta'-O-2)FeO2. The side-on bonded structure is a peroxide complex having a singlet ground state with a nonplanar C-2 nu symmetry. The end-on bonded isomer is characterized to be a superoxide complex with a planar (3)A '' ground state. These two isomers are photoreversible, that is, near-infrared light (lambda > 850 nm) induces the conversion of the side-on bonded (eta(2)-O-2)FeO2 complex to the end-on bonded (eta'-O-2)FeO2 isomer and vice versa with red light irradiation (lambda > 600 nm).