Osmium tetroxide (also osmium(VIII) oxide) is the
chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
with the
formula
In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...
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
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
In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three Vector (geometric), vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in t ...
crystals.
[ It has a characteristic acrid ]chlorine
Chlorine is a chemical element; it has Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between ...
-like odor. The element name osmium is derived from ''osme'', Greek
Greek may refer to:
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group
*Greek language, a branch of the Indo-European language family
**Proto-Greek language, the assumed last common ancestor of all kno ...
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
A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...
in character (the ionization energy
In physics and chemistry, ionization energy (IE) is the minimum energy required to remove the most loosely bound electron of an isolated gaseous atom, Ion, positive ion, or molecule. The first ionization energy is quantitatively expressed as
: ...
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
An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation st ...
ligands
In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's ...
, 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.
:
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 + 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
Stoichiometry () is the relationships between the masses of reactants and products before, during, and following chemical reactions.
Stoichiometry is based on the law of conservation of mass; the total mass of reactants must equal the total m ...
amounts. However, its reactions are made catalytic
Catalysis () is the increase in reaction rate, rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst ...
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 ( 2(OH)4)">sO2(OH)4)sup>2−, and osmium trichloride hydrate (OsCl3·''x''H2O). These species oxidise to osmium(VIII) in the presence of such oxidants.
Lewis bases such as tertiary amine
In chemistry, amines (, ) are organic compounds that contain carbon-nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of elec ...
s and pyridine
Pyridine is a basic (chemistry), basic heterocyclic compound, heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom . It is a highly flammable, weak ...
s increase the rate of dihydroxylation. This "ligand-acceleration" arises via the formation of adduct
In chemistry, an adduct (; alternatively, a contraction of "addition product") is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all components. The resultant is ...
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 ]aldehyde
In organic chemistry, an aldehyde () (lat. ''al''cohol ''dehyd''rogenatum, dehydrogenated alcohol) is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred ...
s 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
A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any ...
and a mild oxidant. It reacts with alkaline aqueous solution
An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, also known as sodium chloride (NaCl), in water ...
to give the perosmate anion . This species is easily reduced to osmate anion, .
When the Lewis base is an amine
In chemistry, amines (, ) are organic compounds that contain carbon-nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of elec ...
, 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
Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water- miscible organic liquid with low viscosity. It is ...
(THF) and diluted in an aqueous buffer solution
A buffer solution is a solution where the pH does not change significantly on dilution or if an acid or base is added at constant temperature. Its pH changes very little when a small amount of strong acid or base is added to it. 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 3">s(N)O3+ 2 H2O
The 3">s(N)O3sup>− 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
Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...
in methanol at 400 K and 200 sbar to produce the triangular cluster Os3(CO)12:
:3 OsO4 + 24 CO → Os3(CO)12 + 12 CO2
Osmate perosmate.png, Osmate and perosmate solutions
Osmiamate2.png, Solution of sodium nitrido-osmate,
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 4N">e4N at 298 K and 2 equivalents at 253 K:[
: OsO4 + 4N">e4N → 4N">e4NOsO4F]
: OsO4 + 2 4N">e4N → 4N">e4Nsub>2 4F2">'cis''-OsO4F2]
Uses
Organic synthesis
In organic synthesis OsO4 is widely used to oxidize alkene
In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins.
The Internationa ...
s 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 alkene
In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins.
The Internationa ...
s ( 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.
Biological staining
OsO4 is a widely used staining
Staining is a technique used to enhance contrast in samples, generally at the Microscope, microscopic level. Stains and dyes are frequently used in histology (microscopic study of biological tissue (biology), tissues), in cytology (microscopic ...
agent used in transmission electron microscopy
Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a g ...
(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
Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...
stain, it is also useful in scanning electron microscopy
A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that ...
(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
The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...
, osmium(VIII) oxide binds phospholipid
Phospholipids are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typ ...
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
In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are som ...
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 4(OH)2">sO4(OH)2 which is soluble. When exposed to chlorine
Chlorine is a chemical element; it has Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between ...
, this salt gives OsO4. In the final stages of refining, crude OsO4 is dissolved in alcoholic NaOH forming Na2 2(OH)4">sO2(OH)4 which, when treated with NH4Cl, to give (NH4)4 2Cl2">sO2Cl2 This salt is reduced under hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
to give osmium.[
]
Buckminsterfullerene adduct
OsO4 allowed for the confirmation of the soccer ball model of buckminsterfullerene, a 60-atom carbon
Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
allotrope. The adduct
In chemistry, an adduct (; alternatively, a contraction of "addition product") is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all components. The resultant is ...
, formed from a derivative of OsO4, was C60(OsO4)(4-''tert''- butylpyridine
Pyridine is a basic (chemistry), basic heterocyclic compound, heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom . It is a highly flammable, weak ...
)2. The adduct broke the fullerene's symmetry, allowing for crystallization and confirmation of the structure of C60 by X-ray crystallography
X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...
.
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
The cornea is the transparency (optics), transparent front part of the eyeball which covers the Iris (anatomy), iris, pupil, and Anterior chamber of eyeball, anterior chamber. Along with the anterior chamber and Lens (anatomy), lens, the 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.[
]
References
External links
International Chemical Safety Card 0528
CDC - Osmium Tetroxide - NIOSH Workplace Safety and Health Topic
BBC report on bomb plot
BBC What is Osmium tetroxide article
Chemical Reactions
{{Oxides
Osmium compounds
Transition metal oxides
Oxidizing agents
Staining dyes
Electron microscopy stains
Foul-smelling chemicals