Editing Osmium tetroxide (section)

==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>]