Editing Osmium tetroxide

{{Chembox
| Verifiedfields = changed
| Watchedfields = changed
| verifiedrevid = 451133444
| ImageFileL1 = Osmium-tetroxide-2D-dimensions.svg
| ImageClassL1 = skin-invert 
| ImageSizeL1 = 130px
| ImageNameL1 = Stick model osmium tetroxide
| ImageFileR1 = Osmium-tetroxide-ED-3D-balls-A.png
| ImageNameR1 = Ball and stick model of osmium tetroxide
| ImageClassR1 = bg-transparent 
| ImageFile2 = Osmium tetroxide.jpg
| ImageSize2 = 270px
| PIN = Osmium tetraoxide
| SystematicName = Tetraoxoosmium
| OtherNames = Osmium(VIII) oxide
|Section1={{Chembox Identifiers
| InChI1 = 1/4O.Os/rO4Os/c1-5(2,3)4
| InChIKey1 = VUVGYHUDAICLFK-TYHKRQCIAE
| CASNo = 20816-12-0
| CASNo_Ref = {{cascite|correct|CAS}}
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = P40W033BGM
| PubChem = 30318
| ChemSpiderID = 28158
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}
| ChEBI_Ref = {{ebicite|correct|EBI}} 
| ChEBI = 88215
| EINECS = 244-058-7
| UNNumber = UN 2471
| MeSHName = Osmium+tetroxide
| RTECS = RN1140000
| StdInChI_Ref = {{stdinchicite|correct|chemspider}}
| StdInChI = 1S/4O.Os
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}
| StdInChIKey = VUVGYHUDAICLFK-UHFFFAOYSA-N
| SMILES = O=[Os](=O)(=O)=O
| InChI = 1S/4O.Os
| InChIKey = VUVGYHUDAICLFK-UHFFFAOYSA-N}}
|Section2={{Chembox Properties
| Formula = OsO<sub>4</sub>
| MolarMass = 254.23{{nbsp}}g/mol
| Appearance = White volatile solid
| Odor = Acrid, chlorine-like
| Density = 4.9{{nbsp}}g/cm<sup>3</sup><ref>{{ cite web | url = http://www.inchem.org/documents/icsc/icsc/eics0528.htm | publisher = InChem | title = Osmium tetroxide ICSC: 0528 }}</ref>
| MeltingPtC = 40.25
| BoilingPtC = 129.7<ref>{{cite journal |last1=Koda |first1=Yoshio |date=1986 |title=Boiling Points and Ideal Solutions of Ruthenium and Osmium Tetraoxides |url= |journal=Journal of the Chemical Society, Chemical Communications |volume=1986 |issue=17 |pages=1347–1348 |doi=10.1039/C39860001347 |access-date=}}</ref>
| Solubility = 5.70{{nbsp}}g/100{{nbsp}}mL (10&nbsp;°C) <br />6.23{{nbsp}}g/100{{nbsp}}mL (25&nbsp;°C)
| Solvent1 = carbon tetrachloride{{!}}CCl<sub>4</sub>
| Solubility1 = 375{{nbsp}}g/100{{nbsp}}mL
| SolubleOther = Soluble in most organic solvents, [[ammonium hydroxide]], [[phosphorus oxychloride]]
| pKa =
| VaporPressure = 7{{nbsp}}mmHg (20&nbsp;°C)<ref name=PGCH/>
}}
|Section3={{Chembox Structure
| Structure_ref = <ref name=s1>{{ cite journal |author1=Krebs, B. |author2=Hasse, K. D. | title = Refinements of the Crystal Structures of KTcO<sub>4</sub>, KReO<sub>4</sub> and OsO<sub>4</sub>. The Bond Lengths in Tetrahedral Oxo-Anions and Oxides of d<sup>0</sup> Transition Metals | journal = Acta Crystallographica B | year = 1976 | volume = 32 | issue = 5 | pages = 1334–1337 | doi = 10.1107/S056774087600530X |bibcode=1976AcCrB..32.1334K }}</ref>
| CrystalStruct = Monoclinic, [[Pearson symbol|mS20]]
| SpaceGroup = C2/c
| LattConst_a = 9.379&nbsp;[[Angstrom|Å]]
| LattConst_b = 4.515&nbsp;[[Angstrom|Å]]
| LattConst_c = 8.630&nbsp;[[Angstrom|Å]]
| LattConst_beta = 116.58
| UnitCellVolume = 326.8&nbsp;Å<sup>3</sup>
| UnitCellFormulas = 4
| MolShape = tetrahedral
 }}
|Section7={{Chembox Hazards
| ExternalSDS = [http://www.inchem.org/documents/icsc/icsc/eics0528.htm ICSC 0528]
| GHSPictograms = {{GHS05}}{{GHS06}}
| GHSSignalWord = Danger
| HPhrases = {{H-phrases|300|310|314|330}}
| PPhrases = {{P-phrases|260|262|264|270|271|280|284|301+310|301+330+331|302+350|303+361+353|304+340|305+351+338|310|320|321|322|330|361|363|403+233|405|501}}
| NFPA-H = 3
| NFPA-F = 0
| NFPA-R = 1
| NFPA-S = OX
| IDLH = 1 mg/m<sup>3</sup><ref name=PGCH>{{PGCH|0473}}</ref>
| REL = TWA 0.002{{nbsp}}mg/m<sup>3</sup> (0.0002{{nbsp}}ppm) ST 0.006{{nbsp}}mg/m<sup>3</sup> (0.0006{{nbsp}}ppm)<ref name=PGCH/>
| PEL = TWA 0.002{{nbsp}}mg/m<sup>3</sup><ref name=PGCH/>
| LCLo = 1316{{nbsp}}mg/m<sup>3</sup> (rabbit, 30{{nbsp}}min)<br/>423{{nbsp}}mg/m<sup>3</sup> (rat, 4{{nbsp}}hr)<br/>423{{nbsp}}mg/m<sup>3</sup> (mouse, 4{{nbsp}}hr)<ref>{{IDLH|20816120|Osmium tetroxide (as Os)}}</ref>
 }}
|Section8={{Chembox Related
| OtherAnions =
| OtherCations = [[Ruthenium tetroxide]]<br />[[Hassium tetroxide]]
| OtherFunction = [[Osmium(IV) oxide]]
| OtherFunction_label = [[osmium]] [[oxide]]s
| OtherCompounds =
 }}
}}

'''Osmium tetroxide''' (also '''osmium(VIII) oxide''') is the [[chemical compound]] with the [[Chemical formula|formula]] OsO<sub>4</sub>. 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 [[Volatility (chemistry)|volatile]].  The compound is colourless, but most samples appear yellow.<ref>{{cite journal | last1 = Girolami | first1 = Gregory | year = 2012 | title = Osmium weighs | journal = Nature Chemistry | volume = 4 | issue = 11| page = 954 | doi = 10.1038/nchem.1479 | pmid = 23089872 | bibcode = 2012NatCh...4..954G | doi-access = free }}</ref> This is most likely due to the presence of the impurity [[Osmium dioxide|osmium dioxide (OsO<sub>2</sub>)]], which is yellow-brown in colour.<ref>Cotton and Wilkinson, Advanced Inorganic Chemistry, p.1002</ref> In biology, its property of binding to lipids has made it a widely used stain in electron microscopy.

==Physical properties==
[[File:OsO4structure.png|thumb|left|Crystal structure of OsO<sub>4</sub><ref name=s1/>]]

Osmium(VIII) oxide forms [[monoclinic]] crystals.<ref name=s1/><ref name=niosh/> It has a characteristic acrid [[chlorine]]-like odor.  The [[chemical element|element]] name osmium is derived from ''osme'', [[Greek language|Greek]] for ''odor''. OsO<sub>4</sub> is volatile: it [[sublimation (chemistry)|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).<ref name=thompson>{{ cite web | author = Thompson, M. | publisher = [[Bristol University]] | title = Osmium tetroxide (OsO<sub>4</sub>) | url = http://www.chm.bris.ac.uk/motm/oso4/oso4h.htm | access-date = 2012-04-07 }}</ref> ''Pure'' osmium(VIII) oxide is probably colourless;<ref>{{ cite book |author1=Butler, I. S. |author2=Harrod, J. F. | title = Inorganic Chemistry: Principles and Applications | year = 1989 | publisher = Benjamin / Cummings | isbn = 978-0-8053-0247-9 | page = 343 | url = https://books.google.com/books?id=Nd3vAAAAMAAJ | access-date= 2012-04-07 }}</ref> it has been suggested that its yellow hue is attributable due to osmium dioxide (OsO<sub>2</sub>) impurities.<ref>{{ cite book | author = Cotton, F. A. | title = Advanced Inorganic Chemistry | edition = 6th | url = https://books.google.com/books?id=U3MWRONWAmMC | year = 2007 | publisher = J. Wiley | location = New Delhi, India | isbn = 978-81-265-1338-3 | page = 1002 }}</ref> The osmium tetroxide molecule is tetrahedral and therefore nonpolar. This nonpolarity helps OsO<sub>4</sub> penetrate charged cell membranes.

==Structure and electron configuration==
The [[osmium]] of OsO<sub>4</sub> 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 Complexes#16e complexes|16 electron complex]]. OsO<sub>4</sub> is isoelectronic with [[permanganate]] and [[Chromate ion|chromate]] ions.

==Synthesis==
OsO<sub>4</sub> is formed slowly when osmium powder reacts with O<sub>2</sub> at ambient temperature. Reaction of bulk solid requires heating to 400&nbsp;°C.<ref name = h&s>{{Housecroft2nd|pages=671–673, 710}}</ref>

:{{chem2 | Os + 2 O2 -> OsO4 }}

==Reactions==

===Oxidation of alkenes===
Alkenes add to OsO<sub>4</sub> to give [[alkoxide|diolate]] species that hydrolyze to ''cis''-diols. The net process is called dihydroxylation. This proceeds via a [3 + 2] [[cycloaddition]] reaction between the OsO<sub>4</sub> and alkene to form an intermediate osmate ester that rapidly hydrolyses to yield the [[Diol#Vicinal diols|vicinal diol]]. As the oxygen atoms are added in a concerted step, the resulting stereochemistry is ''[[Cis-trans isomerism|cis]]''.

:[[File:Dihydroxylation with OsO4.png|thumb|center|600px|Idealized depiction of the cis-dihydroxylation of alkenes.]]

OsO<sub>4</sub> is expensive and highly toxic, making it an unappealing reagent to use in [[stoichiometric]] amounts. However, its reactions are made  [[Catalysis|catalytic]] by adding [[reoxidant]]s to reoxidise the Os(VI) by-product back to Os(VIII). Typical reagents include [[hydrogen peroxide|H<sub>2</sub>O<sub>2</sub>]] ([[Milas hydroxylation]]), [[N-Methylmorpholine N-oxide|N-methylmorpholine N-oxide]] ([[Upjohn dihydroxylation]]) and [[potassium ferricyanide|K<sub>3</sub>Fe(CN)<sub>6</sub>]]/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<sub>2</sub>(OH)<sub>4</sub>)]<sup>2−</sup>, and osmium trichloride hydrate (OsCl<sub>3</sub>·''x''H<sub>2</sub>O). These species oxidise to osmium(VIII) in the presence of such oxidants.<ref>{{ cite journal | title = On the timing of hydrolysis / reoxidation in the osmium-catalyzed asymmetric dihydroxylation of olefins using potassium ferricyanide as the reoxidant |author1=Ogino, Y. |author2=Chen, H. |author3=Kwong, H.-L. |author4=Sharpless, K. B. | journal = [[Tetrahedron Letters]] | year = 1991 | volume = 32 | issue = 32 | pages = 3965–3968 | doi = 10.1016/0040-4039(91)80601-2 }}</ref>

Lewis bases such as tertiary [[amine]]s and [[pyridine]]s increase the rate of dihydroxylation. This "ligand-acceleration" arises via the formation of [[adduct]] OsO<sub>4</sub>L, which adds more rapidly to the alkene. If the amine is chiral, then the dihydroxylation can proceed with enantioselectivity (see [[Sharpless asymmetric dihydroxylation]]).<ref name=catalysis>{{ cite journal |author1=Berrisford, D. J. |author2=Bolm, C. |author3=Sharpless, K. B. | title = Ligand-Accelerated Catalysis | year = 1995 | journal = [[Angewandte Chemie International Edition]] | volume = 34 | issue = 10 | pages = 1059–1070 | doi = 10.1002/anie.199510591 }}</ref> OsO<sub>4</sub> does not react with most carbohydrates.<ref name=stain/>

The process can be extended to give two [[aldehyde]]s in the [[Lemieux–Johnson oxidation]], which uses [[periodate]] to achieve [[diol cleavage]] and to regenerate the catalytic loading of OsO<sub>4</sub>. This process is equivalent to that of [[ozonolysis]].

:[[File:Lemieux–Johnson oxidation.svg|600px]]

===Coordination chemistry===
[[File:CSD CIF KEWMEE.png|thumb|144px|left|Structure of OsO<sub>3</sub>(N-t-Bu) (multiple bonds are not drawn explicitly), illustrating the type of osmium(VIII)-oxo-imide that adds alkenes en route to the amino alcohol.<ref>{{cite journal|title=Reactions of Dimesityldioxo-Osmium(VI) with Donor Ligands; Reactions of MO<sub>2</sub>(2,4,6-Me<sub>3</sub>C<sub>6</sub>H<sub>2</sub>)<sub>2</sub>, M = Os or Re, with Nitrogen Oxides. X-Ray Crystal Structures of [2,4,6-Me<sub>3</sub>C<sub>6</sub>H<sub>2</sub>N<sub>2</sub>]<sup>+</sup>[OsO<sub>2</sub>(ONO<sub>2</sub>)<sub>2</sub>(2,4,6-Me<sub>3</sub>C<sub>6</sub>H<sub>2</sub>)]<sup>–</sup>, OsO(NBu<sup>t</sup>)(2,4,6-Me<sub>3</sub>C<sub>6</sub>H<sub>2</sub>)<sub>2</sub>, OsO<sub>3</sub>(NBu<sup>t</sup>), and ReO<sub>3</sub>[N(2,4,6-Me<sub>3</sub>C<sub>6</sub>H<sub>2</sub>)<sub>2</sub>]|author=Brian S. McGilligan |author2=John Arnold |author3=Geoffrey Wilkinson |author4=Bilquis Hussain-Bates |author5=Michael B. Hursthouse |journal=J. Chem. Soc., Dalton Trans.|year=1990|issue=8|pages=2465–2475 |doi=10.1039/DT9900002465}}</ref>]]OsO<sub>4</sub> is a [[Lewis acid]] and a mild oxidant. It reacts with alkaline [[aqueous solution]] to give the perosmate anion {{chem|OsO|4|(OH)|2|2−}}.<ref>{{Greenwood&Earnshaw2nd}}</ref> This species is easily reduced to [[potassium osmate|osmate]] anion, {{chem|OsO|2|(OH)|4|2-}}.

When the [[Lewis base]] is an [[amine]], adducts are also formed.  Thus OsO<sub>4</sub> can be stored in the form of '''osmeth''', in which OsO<sub>4</sub> is [[complexation|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 (chemistry)|solution]] of OsO<sub>4</sub>.<ref>{{ cite web | author = Kiernan, J. A. | publisher = Department of Anatomy & Cell Biology, The University of Western Ontario | url = http://www.histosearch.com/histonet/Nov00A/Re.quotDisposalquotofOsmi.html | title = Re: "Disposal" of Osmium Tetroxide "Waste" }}</ref>

With [[tert-Butylamine|tert-BuNH<sub>2</sub>]], the [[imido]] derivative is produced:
:OsO<sub>4</sub>  +  Me<sub>3</sub>CNH<sub>2</sub>   →  OsO<sub>3</sub>(NCMe<sub>3</sub>) + H<sub>2</sub>O
Similarly, with [[ammonia|NH<sub>3</sub>]] one obtains the [[nitrido complex]]:
:OsO<sub>4</sub> + NH<sub>3</sub> + KOH → K[Os(N)O<sub>3</sub>] + 2 H<sub>2</sub>O
The [Os(N)O<sub>3</sub>]<sup>−</sup> anion is isoelectronic and isostructural with OsO<sub>4</sub>.

OsO<sub>4</sub> 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 [[catalysis|catalytically]] [[hydrogenation|hydrogenate]] a wide variety of organic chemicals containing double or triple bonds.
:OsO<sub>4</sub> + 4 H<sub>2</sub> → Os  + 4 H<sub>2</sub>O

OsO<sub>4</sub> undergoes "reductive carbonylation" with [[carbon monoxide]] in methanol at 400&nbsp;K and 200&nbsp;sbar to produce the triangular cluster [[Triosmium dodecacarbonyl|Os<sub>3</sub>(CO)<sub>12</sub>]]:

:3 OsO<sub>4</sub>  +  24 CO   →   Os<sub>3</sub>(CO)<sub>12</sub>  +  12 CO<sub>2</sub><ref name = "h&s"/>

====Oxofluorides====
Osmium forms several oxofluorides, all of which are very sensitive to moisture.
Purple ''cis''-OsO<sub>2</sub>F<sub>4</sub> forms at 77&nbsp;K in an anhydrous [[hydrogen fluoride|HF]] solution:<ref>{{ cite journal |author1=Christe, K. O. |author2=Dixon, D. A. |author3=Mack, H. G. |author4=Oberhammer, H. |author5=Pagelot, A. |author6=Sanders, J. C. P. |author7=Schrobilgen, G. J. | title = Osmium tetrafluoride dioxide, ''cis''-OsO<sub>2</sub>F<sub>4</sub> | year = 1993 | journal = [[Journal of the American Chemical Society]] | volume = 115 | issue = 24 | pages = 11279–11284 | doi = 10.1021/ja00077a029 |bibcode=1993JAChS.11511279C }}</ref>
: OsO<sub>4</sub> + 2 KrF<sub>2</sub> → ''cis''-OsO<sub>2</sub>F<sub>4</sub> + 2 Kr + O<sub>2</sub>

OsO<sub>4</sub> also reacts with F<sub>2</sub> to form yellow OsO<sub>3</sub>F<sub>2</sub>:<ref name=chem>{{ cite book | author = Cotton, S. A. | title = Chemistry of Precious Metals | publisher = Chapman and Hall | location = London | year = 1997 | isbn = 0-7514-0413-6 }}</ref>
: 2 OsO<sub>4</sub> + 2 F<sub>2</sub> → 2 OsO<sub>3</sub>F<sub>2</sub> + O<sub>2</sub>

OsO<sub>4</sub> reacts with one equivalent of [Me<sub>4</sub>N]F at 298&nbsp;K and 2 equivalents at 253&nbsp;K:<ref name = h&s/>
: OsO<sub>4</sub> + [Me<sub>4</sub>N]F → [Me<sub>4</sub>N][OsO<sub>4</sub>F]

: OsO<sub>4</sub> + 2 [Me<sub>4</sub>N]F → [Me<sub>4</sub>N]<sub>2</sub>[''cis''-OsO<sub>4</sub>F<sub>2</sub>]

==Uses==

===Organic synthesis===
In organic synthesis OsO<sub>4</sub> is widely used to oxidize [[alkene]]s to the [[Vicinal (chemistry)|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 (chemistry)|vicinal]] amino-alcohols.

In combination with [[sodium periodate]], OsO<sub>4</sub> is used for the oxidative cleavage of [[alkene]]s ([[Lemieux-Johnson oxidation]]) when the periodate serves both to cleave the diol formed by dihydroxylation, and to reoxidize the OsO<sub>3</sub> back to OsO<sub>4</sub>. The net transformation is identical to that produced by [[ozonolysis]]. Below an example from the total synthesis of Isosteviol.<ref>{{ cite journal |author1=Snider, B. B. |author2=Kiselgof, J. Y. |author3=Foxman, B. M. | title = Total Syntheses of (±)-Isosteviol and (±)-Beyer-15-ene-3β,19-diol by Manganese(III)-Based Oxidative Quadruple Free-Radical Cyclization | year = 1998 | journal = [[Journal of Organic Chemistry]] | volume = 63 | issue = 22 | pages = 7945–7952 | doi = 10.1021/jo981238x }}</ref>

[[File:Isosteviol-OsO4.svg|700px]]

===Biological staining===


OsO<sub>4</sub> is a widely used [[staining]] agent used in [[transmission electron microscopy]] (TEM) to provide contrast to the image.<ref name="Bozzola">{{ cite book |author1=Bozzola, J. J. |author2=Russell, L. D. | chapter = Specimen Preparation for Transmission Electron Microscopy | title = Electron Microscopy: Principles and Techniques for Biologists | year = 1999 | publisher = Jones and Bartlett | location = Sudbury, MA | pages = 21–31 | isbn = 978-0-7637-0192-5 | chapter-url = https://books.google.com/books?id=RqSMzR-IXk0C&pg=PA21 }}</ref> This staining method may also be known in the literature as the OTO<ref>{{Cite journal|last1=Seligman|first1=Arnold M.|last2=Wasserkrug|first2=Hannah L.|last3=Hanker|first3=Jacob S.|title=A new staining method (OTO) for enhancing contrast of lipid--containing membranes and droplets in osmium tetroxide--fixed tissue with osmiophilic thiocarbohydrazide(TCH)|date=1966-08-01|journal=The Journal of Cell Biology|volume=30|issue=2|pages=424–432|doi=10.1083/jcb.30.2.424|issn=0021-9525|pmc=2106998|pmid=4165523}}</ref><ref>{{Citation |last1=Unger |first1=Ann-Katrin |date=2020 |work=Volume Microscopy : Multiscale Imaging with Photons, Electrons, and Ions |pages=165–178 |editor-last=Wacker |editor-first=Irene |place=New York, NY |publisher=Springer US |language=en |doi=10.1007/978-1-0716-0691-9_9 |isbn=978-1-0716-0691-9 |last2=Neujahr |first2=Ralph |last3=Hawes |first3=Chris |last4=Hummel |first4=Eric |title=Improving Serial Block Face SEM by Focal Charge Compensation |series=Neuromethods |volume=155 |s2cid=226563386 |editor2-last=Hummel |editor2-first=Eric |editor3-last=Burgold |editor3-first=Steffen |editor4-last=Schröder |editor4-first=Rasmus}}</ref> (osmium-thiocarbohydrazide-osmium) method, or osmium impregnation<ref>{{Cite journal|last1=Tapia|first1=Juan C.|last2=Kasthuri|first2=Narayanan|last3=Hayworth|first3=Kenneth|last4=Schalek|first4=Richard|last5=Lichtman|first5=Jeff W.|last6=Smith|first6=Stephen J|last7=Buchanan|first7=JoAnn|date=2012-01-12|title=High contrast en bloc staining of neuronal tissue for field emission scanning electron microscopy|journal=Nature Protocols|volume=7|issue=2|pages=193–206|doi=10.1038/nprot.2011.439|issn=1754-2189|pmc=3701260|pmid=22240582}}</ref> 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<sub>2</sub>. Its rapid killing abilities are used to quickly kill live specimens such as protozoa. OsO<sub>4</sub> stabilizes many proteins by transforming them into gels without destroying structural features. Tissue proteins that are stabilized by OsO<sub>4</sub> are not coagulated by alcohols during dehydration.<ref name=stain>{{ cite book | author = Hayat, M. A. | title = Principles and Techniques of Electron Microscopy: Biological Applications | publisher = Cambridge University Press | year = 2000 | pages = 45–61 | isbn = 0-521-63287-0 | url = https://books.google.com/books?id=nfsVMH8it1kC }}</ref> Osmium(VIII) oxide is also used as a stain for lipids in optical microscopy.<ref>{{ cite journal | title = A simple protocol for paraffin-embedded myelin sheath staining with osmium(VIII) oxide for light microscope observation |author1=Di Scipio, F. |author2=Raimondo, S. |author3=Tos, P. |author4=Geuna, S. | journal = Microscopy Research and Technique | year = 2008 | volume = 71 | issue = 7 | pages = 497–502 | doi = 10.1002/jemt.20577 | pmid = 18320578 |s2cid=9404999 }}</ref> OsO<sub>4</sub> also stains the human cornea (see [[Osmium tetroxide#Safety considerations|safety considerations]]).

[[File:Resin-Embedded Transmission Electron Microscope Sample.jpg|thumb|left|A sample of cells fixed/stained with osmium tetroxide (black) embedded in epoxy resin (amber). The cells are black as a result of the effects of osmium tetroxide.]]

===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<sub>4</sub>, 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 [[Transmission electron microscopy|TEM]].

===Osmium ore refining===
OsO<sub>4</sub> is an intermediate in the extraction of osmium from its ores. Osmium-containing residues are treated with sodium peroxide (Na<sub>2</sub>O<sub>2</sub>) forming Na<sub>2</sub>[OsO<sub>4</sub>(OH)<sub>2</sub>], which is soluble.  When exposed to [[chlorine]], this salt gives OsO<sub>4</sub>. In the final stages of refining, crude OsO<sub>4</sub> is dissolved in alcoholic [[NaOH]] forming Na<sub>2</sub>[OsO<sub>2</sub>(OH)<sub>4</sub>], which, when treated with [[ammonium chloride|NH<sub>4</sub>Cl]], to give (NH<sub>4</sub>)<sub>4</sub>[OsO<sub>2</sub>Cl<sub>2</sub>]. This salt is reduced under [[hydrogen]] to give osmium.<ref name=thompson/>

===Buckminsterfullerene adduct===
OsO<sub>4</sub> allowed for the confirmation of the soccer ball model of [[buckminsterfullerene]], a 60-atom [[carbon]] [[allotrope]]. The [[adduct]], formed from a derivative of OsO<sub>4</sub>, was C<sub>60</sub>(OsO<sub>4</sub>)(4-''tert''-[[butyl]][[pyridine]])<sub>2</sub>. The adduct broke the fullerene's symmetry, allowing for crystallization and confirmation of the structure of C<sub>60</sub> by [[X-ray crystallography]].<ref>{{ cite journal |author1=Hawkins, J. M. |author2=Meyer, A. |author3=Lewis, T. A. |author4=Loren, S. |author5=Hollander, F. J. | title = Crystal Structure of Osmylated C<sub>60</sub>: Confirmation of the Soccer Ball Framework | year = 1991 | journal = [[Science (journal)|Science]] | volume = 252 | issue = 5003 | pages = 312–313 | doi = 10.1126/science.252.5003.312 | pmid = 17769278 |bibcode=1991Sci...252..312H |s2cid=36255748 }}</ref>

===Medicine===
The only known clinical use of osmium tetroxide is for the treatment of arthritis.<ref>{{cite journal|last=Sheppeard|first=H.|author2=D. J. Ward|journal=Rheumatology|date=1980|volume=19|pages=25–29|doi=10.1093/rheumatology/19.1.25|pmid=7361025|title=Intra-articular osmic acid in rheumatoid arthritis: five years' experience|issue=1}}</ref> The lack of reports of long-term side effects from the local administration of osmium tetroxide (OsO<sub>4</sub>) suggest that osmium itself can be [[biocompatible]], though this depends on the osmium compound administered.

==Safety considerations==
[[File:Label for ampoules of OsO4.jpg|thumb|right|300px|Label with poison warning]]
OsO<sub>4</sub> 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&nbsp;μg/m<sup>3</sup>.<ref name=niosh>{{ cite web | url = https://www.cdc.gov/niosh/idlh/20816120.html | title = Osmium tetroxide (as Os) | work = Documentation for Immediately Dangerous to Life or Health Concentrations (IDLHs)| date = 2 November 2018 | publisher = Centers for Disease Control }}</ref> Osmium(VIII) oxide can penetrate plastics and food packaging, and therefore must be stored in glass under refrigeration.<ref name=stain/>

==References==
{{reflist}}

==External links==
*[http://www.inchem.org/documents/icsc/icsc/eics0528.htm International Chemical Safety Card 0528]
*[https://www.cdc.gov/niosh/npg/npgd0473.html NIOSH Pocket Guide to Chemical Hazards]
*[https://www.cdc.gov/niosh/topics/osmium-tetroxide/ CDC - Osmium Tetroxide - NIOSH Workplace Safety and Health Topic]
*[http://news.bbc.co.uk/1/hi/uk/3603961.stm BBC report on bomb plot]
*[http://news.bbc.co.uk/1/hi/uk/3604857.stm BBC What is Osmium tetroxide article]
*[http://www.chm.bris.ac.uk/motm/oso4/oso4h.htm Osmium Tetroxide: Molecule of the Month]
*[https://www.organic-chemistry.org/chemicals/oxidations/osmiumtetroxide.shtm Chemical Reactions]

{{Osmium compounds}}
{{Oxides}}

[[Category:Osmium compounds]]
[[Category:Transition metal oxides]]
[[Category:Oxidizing agents]]
[[Category:Staining dyes]]
[[Category:Electron microscopy stains]]
[[Category:Foul-smelling chemicals]]