Mononuclear inorganic diradical species are scarce. Here, we confirm, via X-ray absorption spectroscopy in the gas phase combined with computational studies, the oxygen-centered diradical character of the tetraoxidorhenium(VII) cation. A dioxido-superoxido isomer, close in energy to the diradical, is also found, where rhenium appears in its rare oxidation state of +6. Addition of one or two hydrogen atoms to [Re,O4]+ forms hydroxido ligands, and strongly disfavors isomers with any oxygen-oxygen bond. This adds spectroscopic characterization of the rhenium oxidation state and the nature of ligands to the known ability of [Re,O4]+ to perform two consecutive hydrogen-atom abstraction reactions from methane, and demonstrates that pentaatomic [Re,O4]+ combines a metal center in its highest oxidation state with two oxygen-centered radical ligands in a highly reactive species.
