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 osmium dioxide (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.
Physical properties
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.
Structure and electron configuration 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. OsO4 is isoelectronic with permanganate and chromate ions.
Synthesis OsO4 is formed slowly when osmium powder reacts with O2 at ambient temperature. Reaction of bulk solid requires heating to 400 °C.
Os + 2 O2 → OsO4
Reactions
Oxidation of alkenes Alkenes add to OsO4 to give diolate species that hydrolyze to cis-diols. The net process is called dihydroxylation. This proceeds via a [3 + 2] cycloaddition reaction between the OsO4 and alkene to form an intermediate osmate ester that rapidly hydrolyses to yield the vicinal diol. As the oxygen atoms are added in a concerted step, the resulting stereochemistry is cis.
OsO4 is expensive and highly toxic, making it an unappealing reagent to use in stoichiometric amounts. However, its reactions are made catalytic by adding reoxidants to reoxidise the Os(VI) by-product back to Os(VIII). Typical reagents include H2O2 (Milas hydroxylation), N-methylmorpholine N-oxide (Upjohn dihydroxylation) and K3Fe(CN)6/water. These reoxidants do not react with the alkenes on their own. Other osmium compounds can be used as catalysts, including osmate(VI) salts ([OsO2(OH)4)]2−, and osmium trichloride hydrate (OsCl3·xH2O). These species oxidise to osmium(VIII) in the presence of such oxidants. Lewis bases such as tertiary amines and pyridines increase the rate of dihydroxylation. This "ligand-acceleration" arises via the formation of adduct OsO4L, which adds more rapidly to the alkene. If the amine is chiral, then the dihydroxylation can proceed with enantioselectivity (see Sharpless asymmetric dihydroxylation). OsO4 does not react with most carbohydrates. The process can be extended to give two aldehydes in the Lemieux–Johnson oxidation, which uses periodate to achieve diol cleavage and to regenerate the catalytic loading of OsO4. This process is equivalent to that of ozonolysis.
Coordination chemistry OsO4 is a Lewis acid and a mild oxidant. It reacts with alkaline aqueous solution to give the perosmate anion OsO4(OH)2−2. This species is easily reduced to osmate anion, OsO2(OH)2−4. When the Lewis base is an amine, adducts are also formed. Thus OsO4 can be stored in the form of osmeth, in which OsO4 is complexed with hexamine. Osmeth can be dissolved in tetrahydrofuran (THF) and diluted in an aqueous buffer solution to make a dilute (0.25%) working solution of OsO4. With tert-BuNH2, the imido derivative is produced:
OsO4 + Me3CNH2 → OsO3(NCMe3) + H2O Similarly, with NH3 one obtains the nitrido complex:
OsO4 + NH3 + KOH → K[OsO3N] + 2 H2O The [OsO3N]− anion is isoelectronic and isostructural with OsO4. OsO4 is very soluble in tert-butyl alcohol. In solution, it is readily reduced by hydrogen to osmium metal. The suspended osmium metal can be used to catalytically hydrogenate a wide variety of organic chemicals containing double or triple bonds.
OsO4 + 4 H2 → Os + 4 H2O OsO4 undergoes reductive carbonylation with carbon monoxide in methanol at 400 K and 200 bar to produce the triangular cluster Os3(CO)12:
3 OsO4 + 24 CO → Os3(CO)12 + 12 CO2
Oxofluorides Osmium forms several oxofluorides, all of which are very sensitive to moisture. Purple cis-OsO2F4 forms at 77 K in an anhydrous HF solution:
OsO4 + 2 KrF2 → cis-OsO2F4 + 2 Kr + O2 OsO4 also reacts with F2 to form yellow OsO3F2:
2 OsO4 + 2 F2 → 2 OsO3F2 + O2 OsO4 reacts with one equivalent of [Me4N]F at 298 K and 2 equivalents at 253 K:
OsO4 + [Me4N]F → [Me4N][OsO4F] OsO4 + 2 [Me4N]F → [Me4N]2[cis-OsO4F2]
Uses
Organic synthesis In organic synthesis OsO4 is widely used to oxidize alkenes to the vicinal diols, adding two hydroxyl groups at the same side (syn addition). See reaction and mechanism above. This reaction has been made both catalytic (Upjohn dihydroxylation) and asymmetric (Sharpless asymmetric dihydroxylation). Osmium(VIII) oxide is also used in catalytic amounts in the Sharpless oxyamination to give vicinal amino-alcohols. In combination with sodium periodate, OsO4 is used for the oxidative cleavage of alkenes (Lemieux-Johnson oxidation) when the periodate serves both to cleave the diol formed by dihydroxylation, and to regenerate OsO4. The net transformation is identical to that produced by ozonolysis. Below an example from the total synthesis of Isosteviol.
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