Osmium tetroxide

Osmium tetroxide
Osmium tetroxide
CAS number 20816-12-0 YesY
PubChem 30318 YesY, 56370778 (monopotassiate) YesY, 75811001 (monoquinuclidiniate) YesY, 53113021 (monotemediate)
ChemSpider 28158 YesY
EC number 244-058-7
UN number UN 2471
MeSH Osmium+tetroxide
RTECS number RN1140000
Jmol-3D images Image 1
Molecular formula OsO4
Molar mass 254.23 g/mol
Appearance pale yellow solid
Density 4.91 g/cm3[1]
Melting point

40.25 °C

Boiling point

129.7 °C

Solubility in water 65 g/L[1]
Solubility soluble in most organic solvents
Crystal structure Monoclinic, mS20
Space group C2/c; a = 0.4515 nm, b = 0.52046 nm, c = 0.80838 nm, α = 77.677°, β = 73.784°, γ = 64.294°[2]
EU Index 076-001-00-5
EU classification Very toxic (T+)
Corrosive (C)
R-phrases R26/27/28, R34
S-phrases (S1/2), S7/9, S26, S45
NFPA 704
NFPA 704.svg
Related compounds
Other cations Ruthenium tetroxide
Related osmium oxides Osmium(IV) oxide
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Osmium tetroxide (also called osmium tetraoxide) is the chemical compound with the formula OsO4. The compound is noteworthy for its many uses, despite the rarity of osmium. It also has a number of interesting properties, one being that the solid is volatile.


Physical properties

Crystal structure of OsO4[2]

Osmium tetroxide exists as a pale yellow-brown crystalline solid (monoclinic crystal symmetry[2]) with a characteristic acrid chlorine-like odor.[3] 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, and moderately soluble in water, with which it reacts reversibly to form osmic acid (see below).[4] Pure osmium tetroxide is probably colourless[5] and it has been suggested that its yellow hue is due to osmium dioxide (OsO2) impurities[6] although osmium dioxide normally exists as a black powder.[7] Osmium tetroxide molecule is tetrahedral and therefore non-polar. This nonpolarity helps OsO4 penetrate charged cell membranes. OsO4 is 518 times more soluble in CCl4 than in water.

Structure and electron configuration

Osmium tetroxide.png

With a d0 configuration, Os(VIII) is expected to form tetrahedral complexes when bound to four ligands. Tetrahedral structures are seen for the electronically related oxides MnO4 and CrO42–.

The osmium of OsO4 has a formal oxidation state of +8, the highest oxidation state known for a transition metal. The osmium atom has eight valence electrons. If one assumes that two electrons are donated by each of the four oxide ligands, the total electron count for the complex is 16, as also seen for the isoelectronic species permanganate and chromate.

The high oxidation state of osmium in this compound can be rationalized by comparison of main-group and transition-metal chemistry. Just as the elements in groups 3 through 7 form compounds analogous to those formed by elements in groups 13 through 17 (e.g. TiCl4 and GeCl4, VF5 and AsF5, CrO42− and SeO42−, etc.), we might expect the elements in group 8 to form compounds analogous to those formed by the noble gases. This is the case, as demonstrated by the existence of compounds like OsO4 and XeO4.


OsO4 is formed slowly when osmium powder reacts with O2 at ambient temperature. Reaction of bulk solid requires heating to 400 °C.[8]

Os + 2 O2 → OsO4



Osmium forms several oxofluorides, all of which are very sensitive to moisture. Purple cis-OsO2F4 forms at 77 K in an anhydrous HF solution:[9]

OsO4 + 2 KrF2cis-OsO2F4 + 2 Kr + O2

OsO4 also reacts with F2 to form yellow OsO3F2:[10]

2 OsO4 + 2 F2 → 2 OsO3F2 + O2

OsO4 reacts with one equivalent of [Me4N]F at 298 K and 2 equivalents at 253 K:[8]

OsO4 + [Me4N]F → [Me4N][OsO4F]
OsO4 + 2 [Me4N]F → [Me4N]2[cis-OsO4F2]

Oxidation of alkenes

OsO4 catalyzes the cis-dihydroxylation of alkenes by hydrogen peroxide or related sources of oxygen atoms in the presence of water. The reaction that is catalyzed is[11]

R2C=CR2 + H2O2 → R2C(OH)-C(OH)R2.

In terms of mechanism, OsVIIIO4 adds to alkenes R2C=CR2 to afford cyclic "esters" R4C2O2OsVIO2, which undergo hydrolysis to give the vicinal diol and release a reduced osmium oxide (OsVI):


Lewis bases such as tertiary amines and pyridines increase the reaction rate. 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).[11]

OsO4 is used in catalytic amounts due to its toxicity and high cost. The osmium catalyst is regenerated by oxidizing agents, such as H2O2, N-methylmorpholine N-oxide (NMO, see Upjohn dihydroxylation), and K3Fe(CN)6. These oxidizing reagents do not react with the alkenes on their own. Other sources of osmium tetroxide include potassium osmate(VI) dihydrate (K2OsO4·2H2O) and osmium(III) chloride hydrate (OsCl3·xH2O) which oxidise to osmium(VIII) in the presence of such oxidants.[12]

Miscellaneous reactions

OsO4 does not react with most carbohydrates.[13] It dissolves in alkaline aqueous solution to give the osmate anion OsO2(OH)42−.[14] OsO4 is a Lewis acid, and when the Lewis bases are amines, the oxides can undergo substitution. Thus with NH3 one obtains the nitrido-oxide:

OsO4 + NH3 + KOH → K[Os(N)O3] + 2 H2O

The [Os(N)O3]- anion is isoelectronic and isostructural with OsO4. Using primary amine tert-BuNH2 one obtains the corresponding imido derivative:

OsO4 + 4 Me3CNH2 → Os(NCMe3)4 + 4 H2O

OsO4 is very soluble in tert-butanol and in solution is readily reduced by molecular hydrogen to osmium metal. The suspended osmium metal can be used to catalyze hydrogenation of a wide variety of organic chemicals containing double or triple bonds.

OsO4 + 4 H2 (g) → Os (s) + 4 H2O

OsO4 undergoes "reductive carbonylation" with carbon monoxide in methanol at 400 K and 200 bar of pressure to produce the triangular cluster Os3(CO)12:

3 OsO4 + 24 CO → Os3(CO)12 + 12 CO2[8]

In this reaction osmium changes oxidation state by eight units.


Organic synthesis

In organic synthesis OsO4 is widely used to oxidise 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 tetroxide is also used in catalytic amount 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 reoxidize the OsO3 back to OsO4. The net transformation is identical to that produced by ozonolysis. Below an example from the total synthesis of Isosteviol.[15]


Biological staining

Electron micrograph of (organic) plant tissue without (top) and with OsO4 staining

OsO4 is a widely used staining agent used in transmission electron microscopy (TEM) to provide contrast to the image.[16] 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 secondary electron emission 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 tetroxide binds phospholipid head regions, thus creating contrast with the neighbouring protoplasm (cytoplasm). Additionally, osmium tetroxide is also used for fixing biological samples in conjunction with HgCl2. Its rapid killing abilities are used to quickly kill specimen like 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.[13] Osmium tetroxide is also used as a stain for lipids in optical microscopy.[17] 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.


OsO4 can be recycled and stored in the form of osmeth, a golden crystalline solid. Osmeth is OsO4 complexed with hexamine and does not emit toxic fumes as opposed to pure OsO4. It can be dissolved in tetrahydrofuran (THF) and diluted in an aqueous buffer solution to make a dilute (0.25%) working solution of OsO4.[18]

Osmium ore refining

OsO4 is an intermediate in osmium ore refining. Osmium residues are reacted with Na2O2 forming [OsO4(OH)2]2− anions, which, when reacted with chlorine (Cl2) gas and heated, form OsO4. The oxide is dissolved in alcoholic NaOH forming [OsO2(OH)4]2− anions, which, when reacted with NH4Cl, forms OsO2Cl2(NH4)4. This is ignited under hydrogen (H2) gas leaving behind pure osmium (Os).[4]

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.[19]

Safety considerations

Label with poison warning

OsO4 is highly poisonous, even at low exposure levels, and must be handled with appropriate precautions. In particular, inhalation at concentrations well below those at which a smell can be perceived can lead to pulmonary edema, and subsequent death. Noticeable symptoms can take hours to appear after exposure.

OsO4 also stains the human cornea, which can lead to blindness if proper safety precautions are not observed. The permissible exposure limit for osmium tetroxide (8 hour time-weighted average) is 2 µg/m3.[3] Osmium tetroxide can penetrate plastics and therefore is stored in glass in a cold place.[13]

On April 6, 2004 British intelligence sources believed they had foiled a plot to detonate a bomb involving OsO4.[20] Experts interviewed by New Scientist affirmed osmium tetroxide's toxicity, though some highlighted the difficulties of using it in a weapon: osmium tetroxide is very expensive. The osmium tetroxide may be destroyed by the blast; what remaining toxic fumes may also be dispersed by the blast as well.[21]


  1. ^ a b Sicherheitsdatenblatt (Merck) (at 20 °C)
  2. ^ a b c Krebs B., Hasse K.D. (1976). "Refinements of the Crystal Structures of KTcO4, KReO4 and OsO4. The Bond Lengths in Tetrahedral Oxo-Anions and Oxides of d0 Transition Metals". Acta Crystallographica B 32 (5): 1334–1337. doi:10.1107/S056774087600530X. 
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Look at other dictionaries:

  • osmium tetroxide — n a crystalline compound OsO4 that is an oxide of osmium, has a poisonous irritating vapor, and is used chiefly as a catalyst, as an oxidizing and hydroxylating agent (as in the conversion of alkenes to glycols), and as a biological fixative and… …   Medical dictionary

  • osmium tetroxide — noun Etymology: International Scientific Vocabulary osmium + tetroxide : a crystalline compound OsO4 that has a poisonous irritating pungent vapor, that is usually made by oxidizing osmium, and that is used chiefly as a catalyst, as an oxidizing… …   Useful english dictionary

  • osmium tetroxide — (= OsO4) Used as a post fixative/stain in electron microscopy. Membranes in particular are osmiophilic, ie. bind osmium tetroxide …   Dictionary of molecular biology

  • osmium tetroxide — osmic acid a colourless or faintly yellowish compound used to stain fats or as a fixative in the preparation of tissues for microscopical study. Osmium tetroxide evaporates readily, the vapour having a toxic action on the eyes, skin, and… …   The new mediacal dictionary

  • osmium tetroxide — noun Date: 1869 a crystalline compound OsO4 that is an oxide of osmium, has a poisonous irritating vapor, and is used as a catalyst, oxidizing agent, and biological fixative and stain …   New Collegiate Dictionary

  • osmium tetroxide — noun an oxide of the metal osmium, OsO; it is very poisonous, has a pungent smell and can cause blindness; it is a powerful oxidizing agent, and is used to stain biological materials …   Wiktionary

  • osmium tetroxide — Chem. a crystalline or amorphous, colorless, poisonous compound, OsO4, soluble in water, alcohol, and ether: used for microscopic staining, in photography, and as a catalyst in organic synthesis. Also called osmic acid. [1875 80] * * * …   Universalium

  • Осмия Тетраоксид (Osmium Tetroxide), Кислота Осмиевая (Osmic Acid) — бесцветное или желтоватое соединение, применяющееся для окрашивания жиров или в качестве фиксирующего вещества в процессе подготовки тканей для микроскопического изучения. Тетраоксид осмия является летучим веществом; его пары оказывают… …   Медицинские термины

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