Osmium tetroxide

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 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 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.

Biological staining

OsO₄ 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 HgCl₂. Its rapid killing abilities are used to quickly kill live specimens such as protozoa. OsO₄ stabilizes many proteins by transforming them into gels without destroying structural features. Tissue proteins that are stabilized by OsO₄ are not coagulated by alcohols during dehydration. Osmium(VIII) oxide is also used as a stain for lipids in optical microscopy. OsO₄ 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 OsO₄, 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

OsO₄ is an intermediate in the extraction of osmium from its ores. Osmium-containing residues are treated with sodium peroxide (Na₂O₂) forming Na₂[OsO₄(OH)₂], which is soluble. When exposed to chlorine, this salt gives OsO₄. In the final stages of refining, crude OsO₄ is dissolved in alcoholic NaOH forming Na₂[OsO₂(OH)₄], which, when treated with NH₄Cl, to give [OsO₂(NH₄)₃]Cl₂. This salt is reduced under hydrogen to give osmium.

Buckminsterfullerene adduct

OsO₄ allowed for the confirmation of the soccer ball model of buckminsterfullerene, a 60-atom carbon allotrope. The adduct, formed from a derivative of OsO₄, was C₆₀(OsO₄)(4-tert-butylpyridine)₂. The adduct broke the fullerene's symmetry, allowing for crystallization and confirmation of the structure of C₆₀ 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 (OsO₄) suggest that osmium itself can be biocompatible, though this depends on the osmium compound administered.

Safety considerations

OsO₄ 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/m³. Osmium(VIII) oxide can penetrate plastics and food packaging, and therefore must be stored in glass under refrigeration.