Recent progress in oxidation chemistry of high-valent ruthenium-oxo and osmium-oxo complexes and related species

•Bio-inspired oxidation reactions performed by ruthenium and osmium complexes.•High-valent ruthenium-oxo and osmium-oxo complexes as well as related rutheniumoxyl complexes showing unique reactivity as responsible intermediates.•Structural and spectroscopic characterization of the reactive intermediates.•Mechanistic insights into oxidation reactions. Ruthenium and osmium complexes are playing important roles in stoichiometric and catalytic oxidation reactions to achieve unique chemical conversion of inorganic and organic compounds. Bio-inspired oxidation reactions have been developed using ruthenium and osmium complexes on the basis of peroxide activation and proton-coupled electron transfer (PCET) to generate high-valent Ru-oxo (Ru=O) and Os-oxo (Os=O) complexes, which can be characterized spectroscopic and crystallographic methods, as reactive species. The fairly high stability and also reactivity of high-valent Ru=O and Os=O complexes allow us to gain detailed mechanistic insights into oxidation reactions, including hydrogen atom transfer from organic compounds to those complexes. As for high-valent Ru-oxo complexes, which can be formed through PCET oxidation of lower-valent Ru-aqua complexes, we describe their reactivity in oxidation of water and organic compounds such as alcohols and alkanes. As related species, Ru-oxyl (Ru-O•) complexes that bear radical character at the oxo ligands are also included to discuss on their unique reactivity including oxidative cracking of aromatic compounds and radical coupling. Concerning high-valent Os-oxo complexes which are formed through peroxide activation or oxygen atom transfer (OAT), unique reactivity has been demonstrated to perform cis-selective dihydroxylation of alkenes as well as C–H activation.