Osmium tetroxide

Osmium tetroxide, also called osmium(VIII) oxide or (non-systematically) osmic acid, is a chemical compound made of osmium and oxygen. It has the chemical formula OsO₄. It is a Lewis acid, oxidising agent, and biological stain.

Oxidiser

A simplified mechanism for the oxidation of alkenes by OsO₄.

Osmium tetroxide is a useful oxidising agent in a name reaction called the Upjohn dihydroxylation.^[1] It reacts with alkenes to become cyclic osmate diesters. Water breaks apart the osmate, leaving a hydrated osmium(VI) compound and a diol.^[2] The diagram above shows osmium(VI) dioxide dihydroxide, but the actual chemistry of osmium(VI) in water is much more complicated.

Usually, a cooxidant like NMMO is used to regenerate osmium tetroxide, allowing it to act as a catalyst.