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Concept
Osmium tetroxide
Summary
Osmium tetroxide (also osmium(VIII) oxide) is the chemical compound with the formula OsO4. The compound is noteworthy for its many uses, despite its toxicity and the rarity of osmium. It also has a number of unusual properties, one being that the solid is volatile. The compound is colourless, but most samples appear yellow. This is most likely due to the presence of the impurity OsO2, which is yellow-brown in colour. In biology, its property of binding to lipids has made it a widely-used stain in electron microscopy.
Osmium(VIII) oxide forms monoclinic crystals. It has a characteristic acrid chlorine-like odor. The element name osmium is derived from osme, Greek for odor. OsO4 is volatile: it sublimes at room temperature. It is soluble in a wide range of organic solvents. It is moderately soluble in water, with which it reacts reversibly to form osmic acid (see below). Pure osmium(VIII) oxide is probably colourless; it has been suggested that its yellow hue is attributable due to osmium dioxide (OsO2) impurities. The osmium tetroxide molecule is tetrahedral and therefore nonpolar. This nonpolarity helps OsO4 penetrate charged cell membranes. OsO4 is 518 times more soluble in carbon tetrachloride than in water.
The osmium of OsO4 has an oxidation number of VIII; however, the metal does not possess a corresponding 8+ charge as the bonding in the compound is largely covalent in character (the ionization energy required to produce a formal 8+ charge also far exceeds the energies available in normal chemical reactions). The osmium atom exhibits double bonds to the four oxide ligands, resulting in a 16 electron complex. This is isoelectronic with permanganate and chromate ions.
OsO4 is formed slowly when osmium powder reacts with O2 at ambient temperature. Reaction of bulk solid requires heating to 400 °C.
Os + 2O2 ->[\Delta T] OsO4
Alkenes add to OsO4 to give diolate species that hydrolyze to cis-diols. The net process is called dihydroxylation.
Official source
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