Osmium tetroxide is an oxide of osmium.
It has many uses, despite the fact that the abundance of Os in the earth's crust is only 1.5 ppb by mass.
OsO₄ is colourless and has a chlorine-like odour. Most samples appear yellowish because of contamination by yellow-brown OsO₂.
OsO₄ is tetrahedral and nonpolar.
Some of its physical properties are:
- melting point = 40°C
- sublimes at room temperature
- boiling point = 130°C
- solubility in water = 6.2 g/100 mL
- solubility in CCl₄ = 375 g/100 mL
The most common use of OsO₄ in organic chemistry is to convert alkenes to vic-diols.
The mechanism involves a concerted cis addition to form a cyclic osmate ester, which then hydrolyzes to form the diol.
OsO₄ is usually used in small amounts as a catalyst. Reactants such as H₂O₂ are added to regenerate the OsO₄.
OsO₄ is expensive (over $200/g) and highly toxic. The permissible exposure limit is only 2 µg/m³ over 8 h.
OsO₄ can even diffuse through plastic, so it must be kept in glass containers and in a fume cupboard.