Is rhodium tetroxide in the formal oxidation state VIII stable? a quantum chemical and matrix isolation investigation of rhodium oxides

The structures and reactions of different rhodium oxides and dioxygen complexes with RhO4 stoichiometry were investigated by matrix isolation infrared spectroscopy and quantum chemical calculations. The inserted RhO2 molecule reacted with dioxygen upon sample annealing to form the [(eta(1)-O-2)RhO2] complex, which can further isomerize to the known [(eta(2)-O-2)RhO2] complex via infrared irradiation. Both experimental and theoretical studies suggest that the [(eta(1)-O-2)RhO2] complex has a doublet ground state with non-planar C (s) symmetry in which the O-2 ligand is end-on bonded to the rhodium centre. Although rhodium tetroxide is predicted to be a stable molecule with D (2d) symmetry at different level of theory, no evidence is found for the formation of this Rh(VIII) species in noble gas matrices. Our experiments also suggest the formation of a new peroxo [Rh(eta(2)-O-2)(2)] complex, which is calculated to have a doublet ground state with D (2d) symmetry. This peroxo complex undergoes isomerization to the known superoxo [Rh(eta(2)-O-2)(2)] complex via the rotation of the dioxygen ligand under infrared irradiation.