Isomers of Iron(III) Oxides and Cobalt(III) Oxides and Their Redox Properties: Quantum-Chemical Insights

Iron(III) oxide and cobalt(III) oxide can form distinct spatial and spin configurations. Kite, spindle, and linear geometries have been shown to be stable for the specified electron configurations; however, these oxides generally favor higher open-shell configurations, which are ferromagnetic or antiferromagnetic. Reduction and oxidation reactions affect the geometry and spin states of these systems, sometimes leading to isomer transformations. Calculated standard reduction potentials of iron trioxides against the Standard Hydrogen Electrode (SHE) range from -0.37 V to -0.72 V, depending notably on the oxide geometry, spin, and computational method employed. For cobalt trioxides, standard reduction potentials range from -0.63 V to +0.18 V. Ionization potentials range from approximately 8 eV to 10 eV for iron oxides and from about 9 eV to 10 eV for cobalt oxides. Electron affinity values range from 2.36 eV to 2.76 eV for iron oxides and from 2.47 eV to about 2.94 eV for cobalt oxides, with these values being more sensitive to the computational method employed and the specific isomer considered. Consequently, iron(III) and cobalt(III) oxides are about three times more susceptible to one-electron reduction than oxidation. Specifically, kite-shaped Fe₂O₃ and Co₂O₃ configurations are most vulnerable to reduction. Conversely, the linear configuration of iron oxide and cobalt oxide exhibits the lowest susceptibility to oxidation, as indicated by their elevated ionization potentials. Overall, both iron(III) and cobalt(III) oxides act as relatively effective redox agents.