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| Names | |||
|---|---|---|---|
| Preferred IUPAC name Osmium tetraoxide | |||
| Systematic IUPAC name Tetraoxoosmium | |||
| Other names Osmium(VIII) oxide | |||
| Identifiers | |||
3D model (JSmol) | |||
| ChEBI | |||
| ChemSpider |
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| ECHA InfoCard | 100.040.038 | ||
| EC Number |
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| MeSH | Osmium+tetroxide | ||
| RTECS number |
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| UNII | |||
| UN number | UN 2471 | ||
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| Properties | |||
| OsO4 | |||
| Molar mass | 254.23 g/mol | ||
| Appearance | White volatile solid | ||
| Odor | Acrid, chlorine-like | ||
| Density | 4.9 g/cm3[1] | ||
| Melting point | 40.25 °C (104.45 °F; 313.40 K) | ||
| Boiling point | 129.7[2] °C (265.5 °F; 402.8 K) | ||
| 5.70 g/100 mL (10 °C) 6.23 g/100 mL (25 °C) | |||
| Solubility | Soluble in most organic solvents,ammonium hydroxide,phosphorus oxychloride | ||
| Solubility inCCl4 | 375 g/100 mL | ||
| Vapor pressure | 7 mmHg (20 °C)[3] | ||
| Structure[4] | |||
| Monoclinic,mS20 | |||
| C2/c | |||
α = 90°, β = 116.58°, γ = 90° | |||
Lattice volume (V) | 326.8 Å3 | ||
Formula units (Z) | 4 | ||
| tetrahedral | |||
| Hazards | |||
| GHS labelling: | |||
  | |||
| Danger | |||
| H300,H310,H314,H330 | |||
| P260,P262,P264,P270,P271,P280,P284,P301+P310,P301+P330+P331,P302+P350,P303+P361+P353,P304+P340,P305+P351+P338,P310,P320,P321,P322,P330,P361,P363,P403+P233,P405,P501 | |||
| NFPA 704 (fire diamond) | |||
| Lethal dose or concentration (LD, LC): | |||
LCLo (lowest published) | 1316 mg/m3 (rabbit, 30 min) 423 mg/m3 (rat, 4 hr) 423 mg/m3 (mouse, 4 hr)[5] | ||
| NIOSH (US health exposure limits): | |||
PEL (Permissible) | TWA 0.002 mg/m3[3] | ||
REL (Recommended) | TWA 0.002 mg/m3 (0.0002 ppm) ST 0.006 mg/m3 (0.0006 ppm)[3] | ||
IDLH (Immediate danger) | 1 mg/m3[3] | ||
| Safety data sheet (SDS) | ICSC 0528 | ||
| Related compounds | |||
Othercations | Ruthenium tetroxide Hassium tetroxide | ||
| Osmium(IV) oxide | |||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
Osmium tetroxide (alsoosmium(VIII) oxide) is thechemical compound with theformula OsO4. The compound is noteworthy for its many uses, despite its toxicity and the rarity ofosmium. It also has a number of unusual properties, one being that the solid isvolatile. The compound is colourless, but most samples appear yellow.[6] This is most likely due to the presence of the impurityosmium dioxide (OsO2), which is yellow-brown in colour.[7] In biology, its property of binding to lipids has made it a widely used stain in electron microscopy.
Osmium(VIII) oxide formsmonoclinic crystals.[4][8] It has a characteristic acridchlorine-like odor. Theelement name osmium is derived fromosme,Greek forodor. OsO4 is volatile: itsublimes atroom 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).[9]Pure osmium(VIII) oxide is probably colourless;[10] it has been suggested that its yellow hue is attributable due to osmium dioxide (OsO2) impurities.[11] The osmium tetroxide molecule is tetrahedral and therefore nonpolar. This nonpolarity helps OsO4 penetrate charged cell membranes.
Theosmium of OsO4 has anoxidation number of VIII; however, the metal does not possess a corresponding 8+ charge as the bonding in the compound is largelycovalent in character (theionization 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 fouroxideligands, resulting in a16 electron complex. OsO4 is isoelectronic withpermanganate andchromate ions.
OsO4 is formed slowly when osmium powder reacts with O2 at ambient temperature. Reaction of bulk solid requires heating to 400 °C.[12]
Alkenes add to OsO4 to givediolate species that hydrolyze tocis-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 thevicinal diol. As the oxygen atoms are added in a concerted step, the resulting stereochemistry iscis.
OsO4 is expensive and highly toxic, making it an unappealing reagent to use instoichiometric amounts. However, its reactions are madecatalytic by addingreoxidants to reoxidise the Os(VI) by-product back to Os(VIII). Typical reagents includeH2O2 (Milas hydroxylation),N-methylmorpholine N-oxide (Upjohn dihydroxylation) andK3Fe(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.[13]
Lewis bases such as tertiaryamines andpyridines increase the rate of dihydroxylation. This "ligand-acceleration" arises via the formation ofadduct OsO4L, which adds more rapidly to the alkene. If the amine is chiral, then the dihydroxylation can proceed with enantioselectivity (seeSharpless asymmetric dihydroxylation).[14] OsO4 does not react with most carbohydrates.[15]
The process can be extended to give twoaldehydes in theLemieux–Johnson oxidation, which usesperiodate to achievediol cleavage and to regenerate the catalytic loading of OsO4. This process is equivalent to that ofozonolysis.
OsO4 is aLewis acid and a mild oxidant. It reacts with alkalineaqueous solution to give the perosmate anionOsO
4(OH)2−
2.[17] This species is easily reduced toosmate anion,OsO
2(OH)2−
4.
When theLewis base is anamine, adducts are also formed. Thus OsO4 can be stored in the form ofosmeth, in which OsO4 iscomplexed withhexamine. Osmeth can be dissolved intetrahydrofuran (THF) and diluted in an aqueousbuffer solution to make a dilute (0.25%) workingsolution of OsO4.[18]
Withtert-BuNH2, theimido derivative is produced:
Similarly, withNH3 one obtains thenitrido complex:
The[OsO3N]− anion is isoelectronic and isostructural with OsO4.
OsO4 is very soluble intert-butyl alcohol. In solution, it is readily reduced by hydrogen to osmium metal. The suspended osmium metal can be used tocatalyticallyhydrogenate a wide variety of organic chemicals containing double or triple bonds.
OsO4 undergoes "reductive carbonylation" withcarbon monoxide in methanol at 400 K and 200 bar to produce the triangular clusterOs3(CO)12:
Osmium forms several oxofluorides, all of which are very sensitive to moisture.Purplecis-OsO2F4 forms at 77 K in an anhydrousHF solution:[19]
OsO4 also reacts with F2 to form yellow OsO3F2:[20]
OsO4 reacts with one equivalent of [Me4N]F at 298 K and 2 equivalents at 253 K:[12]
In organic synthesis OsO4 is widely used to oxidizealkenes to thevicinal diols, adding twohydroxyl 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 theSharpless oxyamination to givevicinal amino-alcohols.
In combination withsodium periodate, OsO4 is used for the oxidative cleavage ofalkenes (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 byozonolysis. Below an example from the total synthesis of Isosteviol.[21]
OsO4 is a widely usedstaining agent used intransmission electron microscopy (TEM) to provide contrast to the image.[22] This staining method may also be known in the literature as the OTO[23][24] (osmium-thiocarbohydrazide-osmium) method, or osmium impregnation[25] technique or simply as osmium staining. As alipid stain, it is also useful inscanning electron microscopy (SEM) as an alternative tosputter 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 theplasma membrane, osmium(VIII) oxide bindsphospholipid head regions, thus creating contrast with the neighbouringprotoplasm (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.[15] Osmium(VIII) oxide is also used as a stain for lipids in optical microscopy.[26] OsO4 also stains the human cornea (seesafety considerations).
It is also used to staincopolymers preferentially, the best known example being block copolymers where one phase can be stained so as to show themicrostructure of the material. For example, styrene-butadiene block copolymers have a centralpolybutadiene 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 inTEM.
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 tochlorine, this salt gives OsO4. In the final stages of refining, crude OsO4 is dissolved in alcoholicNaOH forming Na2[OsO2(OH)4], which, when treated withNH4Cl, to give[OsO2(NH4)3]Cl2. This salt is reduced underhydrogen to give osmium.[9]
OsO4 allowed for the confirmation of the soccer ball model ofbuckminsterfullerene, a 60-atomcarbonallotrope. Theadduct, 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 byX-ray crystallography.[27]
The only known clinical use of osmium tetroxide is for the treatment of arthritis.[28] The lack of reports of long-term side effects from the local administration of osmium tetroxide (OsO4) suggest that osmium itself can bebiocompatible, though this depends on the osmium compound administered.
OsO4 will irreversibly stain the humancornea, which can lead to blindness. The permissible exposure limit for osmium(VIII) oxide (8 hour time-weighted average) is 2 μg/m3.[8] Osmium(VIII) oxide can penetrate plastics and food packaging, and therefore must be stored in glass under refrigeration.[15]
