# Osmium tetroxide

Osmium tetroxide (also osmium(VIII) oxide) is the chemical compound with the formula OsO$_4$. 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 OsO$_2$, 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. OsO$_4$ 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 (OsO$_2$) impurities. The osmium tetroxide molecule is tetrahedral and therefore nonpolar. This nonpolarity helps OsO$_4$ penetrate charged cell membranes.

## Structure and electron configuration

The osmium of OsO$_4$ 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. OsO$_4$ is isoelectronic with permanganate and chromate ions.

## Synthesis

OsO$_4$ is formed slowly when osmium powder reacts with O$_2$ at ambient temperature. Reaction of bulk solid requires heating to 400 °C.
    Os + 2 O$_2$ → OsO$_4$

## Reactions
### Oxidation of alkenes

Alkenes add to OsO$_4$ 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 OsO$_4$ 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.
    Idealized depiction of the cis-dihydroxylation of alkenes.
OsO$_4$ 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 H$_2$O$_2$ (Milas hydroxylation), N-methylmorpholine N-oxide (Upjohn dihydroxylation) and K$_3$Fe(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 ([OsO$_2$(OH)$_4$)]$^{2−}$, and osmium trichloride hydrate (OsCl$_3$·xH$_2$O). 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 OsO$_4$L, which adds more rapidly to the alkene. If the amine is chiral, then the dihydroxylation can proceed with enantioselectivity (see Sharpless asymmetric dihydroxylation). OsO$_4$ 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 OsO$_4$. This process is equivalent to that of ozonolysis.

### Coordination chemistry

OsO$_4$ is a Lewis acid and a mild oxidant. It reacts with alkaline aqueous solution to give the perosmate anion OsO $_4$(OH)$^{2−}$ $_2$. This species is easily reduced to osmate anion, OsO $_2$(OH)$^{2−}$ $_4$.
When the Lewis base is an amine, adducts are also formed. Thus OsO$_4$ can be stored in the form of osmeth, in which OsO$_4$ 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 OsO$_4$.
With tert-BuNH$_2$, the imido derivative is produced:
    OsO$_4$ + Me$_3$CNH$_2$ → OsO$_3$(NCMe$_3$) + H$_2$O
Similarly, with NH$_3$ one obtains the nitrido complex:
    OsO$_4$ + NH$_3$ + KOH → K[Os(N)O$_3$] + 2 H$_2$O
The [Os(N)O$_3$]$^−$ anion is isoelectronic and isostructural with OsO$_4$.
OsO$_4$ 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.
    OsO$_4$ + 4 H$_2$ → Os + 4 H$_2$O
OsO$_4$ undergoes "reductive carbonylation" with carbon monoxide in methanol at 400 K and 200 sbar to produce the triangular cluster Os$_3$(CO)$_{12}$:
    3 OsO$_4$ + 24 CO → Os$_3$(CO)$_{12}$ + 12 CO$_2$

#### Oxofluorides

Osmium forms several oxofluorides, all of which are very sensitive to moisture. Purple cis-OsO$_2$F$_4$ forms at 77 K in an anhydrous HF solution:
    OsO$_4$ + 2 KrF$_2$ → cis-OsO$_2$F$_4$ + 2 Kr + O$_2$
OsO$_4$ also reacts with F$_2$ to form yellow OsO$_3$F$_2$:
    2 OsO$_4$ + 2 F$_2$ → 2 OsO$_3$F$_2$ + O$_2$
OsO$_4$ reacts with one equivalent of [Me$_4$N]F at 298 K and 2 equivalents at 253 K:
    OsO$_4$ + [Me$_4$N]F → [Me$_4$N][OsO$_4$F]
    OsO$_4$ + 2 [Me$_4$N]F → [Me$_4$N]$_2$[cis-OsO$_4$F$_2$]

## Uses
### Organic synthesis

In organic synthesis OsO$_4$ 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, OsO$_4$ 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 reoxidize the OsO$_3$ back to OsO$_4$. The net transformation is identical to that produced by ozonolysis. Below an example from the total synthesis of Isosteviol.

### Biological staining

OsO$_4$ is a widely used staining agent used in transmission electron microscopy (TEM) to provide contrast to the image. This staining method may also be known in the literature as the OTO (osmium-thiocarbohydrazide-osmium) method, or osmium impregnation technique or simply as osmium staining. As a lipid stain, it is also useful in scanning electron microscopy (SEM) as an alternative to sputter coating. It embeds a heavy metal directly into cell membranes, creating a high electron scattering rate without the need for coating the membrane with a layer of metal, which can obscure details of the cell membrane. In the staining of the plasma membrane, osmium(VIII) oxide binds phospholipid head regions, thus creating contrast with the neighbouring protoplasm (cytoplasm). Additionally, osmium(VIII) oxide is also used for fixing biological samples in conjunction with HgCl$_2$. Its rapid killing abilities are used to quickly kill live specimens such as protozoa. OsO$_4$ stabilizes many proteins by transforming them into gels without destroying structural features. Tissue proteins that are stabilized by OsO$_4$ are not coagulated by alcohols during dehydration. Osmium(VIII) oxide is also used as a stain for lipids in optical microscopy. OsO$_4$ also stains the human cornea (see safety considerations).

### Polymer staining

It is also used to stain copolymers preferentially, the best known example being block copolymers where one phase can be stained so as to show the microstructure of the material. For example, styrene-butadiene block copolymers have a central polybutadiene chain with polystyrene end caps. When treated with OsO$_4$, the butadiene matrix reacts preferentially and so absorbs the oxide. The presence of a heavy metal is sufficient to block the electron beam, so the polystyrene domains are seen clearly in thin films in TEM.

### Osmium ore refining

OsO$_4$ is an intermediate in the extraction of osmium from its ores. Osmium-containing residues are treated with sodium peroxide (Na$_2$O$_2$) forming Na$_2$[OsO$_4$(OH)$_2$], which is soluble. When exposed to chlorine, this salt gives OsO$_4$. In the final stages of refining, crude OsO$_4$ is dissolved in alcoholic NaOH forming Na$_2$[OsO$_2$(OH)$_4$], which, when treated with NH$_4$Cl, to give (NH$_4$)$_4$[OsO$_2$Cl$_2$]. This salt is reduced under hydrogen to give osmium.

### Buckminsterfullerene adduct

OsO$_4$ allowed for the confirmation of the soccer ball model of buckminsterfullerene, a 60-atom carbon allotrope. The adduct, formed from a derivative of OsO$_4$, was C$_{60}$(OsO$_4$)(4-tert-butylpyridine)$_2$. The adduct broke the fullerene's symmetry, allowing for crystallization and confirmation of the structure of C$_{60}$ by X-ray crystallography.

### Medicine

The only known clinical use of osmium tetroxide is for the treatment of arthritis. The lack of reports of long-term side effects from the local administration of osmium tetroxide (OsO$_4$) suggest that osmium itself can be biocompatible, though this depends on the osmium compound administered.

## Safety considerations

OsO$_4$ will irreversibly stain the human cornea, which can lead to blindness. The permissible exposure limit for osmium(VIII) oxide (8 hour time-weighted average) is 2 μg/m$^3$. Osmium(VIII) oxide can penetrate plastics and food packaging, and therefore must be stored in glass under refrigeration.