Exposing the Oxygen-Centered Radical Character of the Tetraoxido Ruthenium(VIII) Cation [RuO₄ ]

The tetraoxido ruthenium(VIII) radical cation, [RuO₄ ]⁺ , should be a strong oxidizing agent, but has been difficult to produce and investigate so far. In our X-ray absorption spectroscopy study, in combination with quantum-chemical calculations, we show that [RuO₄ ]⁺ , produced via oxidation of ruthenium cations by ozone in the gas phase, forms the oxygen-centered radical ground state. The oxygen-centered radical character of [RuO₄ ]⁺ is identified by the chemical shift at the ruthenium M₃ edge, indicative of ruthenium(VIII), and by the presence of a characteristic low-energy transition at the oxygen K edge, involving an oxygen-centered singly-occupied molecular orbital, which is suppressed when the oxygen-centered radical is quenched by hydrogenation of [RuO₄ ]⁺ to the closed-shell [RuO₄ H]⁺ ion. Hydrogen-atom abstraction from methane is calculated to be only slightly less exothermic for [RuO₄ ]⁺ than for [OsO₄ ]⁺ .