Osmiumtetroxide

Osmium tetroxide, OsO4, is a volatile, highly toxic inorganic compound and a powerful oxidizing agent. It forms orange-red crystals at room temperature and readily sublimates. Its molecular weight is 254.23 g/mol. It has a very strong, distinctive odor, though exposure to osmium compounds is dangerous even at low concentrations.

In organic synthesis, OsO4 is best known for catalyzing the syn-dihydroxylation of alkenes to give vicinal diols, a reaction often referred to as the Upjohn dihydroxylation. The process proceeds via a cyclic osmate ester, and OsO4 is typically used in catalytic amounts with a stoichiometric co-oxidant to regenerate the active catalyst. Common co-oxidants include N-methylmorpholine N-oxide (NMO), hydrogen peroxide, or other oxidants that reoxidize osmium(VI) to OsO4. This method is valued for its high stereospecificity and mild conditions.

OsO4 also serves as a stain and fixative in electron microscopy, where its high electron density enhances contrast of lipids and membranes in biological specimens. In histology and EM sample preparation, it helps preserve and visualize subcellular structures.

Safety and handling require strong caution. OsO4 is highly toxic by inhalation, ingestion, and skin contact and poses serious health risks, including respiratory and ocular irritation and potential long-term effects. Work with OsO4 in a well-ventilated fume hood, using appropriate personal protective equipment, and follow strict hazardous-w waste disposal procedures.