The chemical compoundosmium tetroxide (OsO4), also known as osmium tetraoxide, osmium(VIII) oxide, or osmic acid, is an oxide of the element osmium. The compound is noteworthy for its many uses, despite the rarity of osmium.
Pure osmium tetroxide is colorless, but it is usually contaminated by a small amount of yellow-brown osmium dioxide (OsO2), giving it a yellowish hue. OsO4 is soluble in carbon tetrachloride (CCl4) and moderately soluble in water, with which it reacts reversibly to form osmic acid (see below). OsO4 is volatile: it sublimes at room temperature. It has a characteristic odor similar to ozone. In fact, the element name osmium is derived from osme, Greek for odor.Dr. M Thomson, Osmium tetroxide(OsO4). [1]
The osmium of OsO4 has an oxidation number of 8, the highest known oxidation state for a transition element. The osmium has a total valence electron count of 8. With 2 electrons donated by each of the oxygen atoms, the total electron count is 16, as also seen for MnO4- and CrO42-. As a d0 metal, the coordination geometry follows the points-on-a-sphere motif being tetrahedral.
OsO4 is formed slowly when osmium powder reacts with O2 at 298 K. Reaction of bulk solid requires heating to 670 K.Housecroft & Sharpe. "Inorganic Chemistry". (ed 2, 2005). p 671-673
Osmium forms several oxofluorides, all of which are very sensitive to moisture.Purple cis-OsO2F4 forms at 77 K in an aqueous solution of HFChriste et al. Osmium Tetraflouride Dioxide, cis-OsO2F4. J. Am. Chem. Soc.1993, 115. 11279-11284:
OsO4 + 2 KrF2 â†' cis-OsO2F4 + 2 Kr + O2
OsO4 also reacts with F2 to form yellow OsOF2:
2 OsO4 + 2 F2 â†' 2 OsO3F2 + O2
OsO4 reacts with one equivalent of [Me4N]F at 298 K and 2 equivalents at 253 KHousecroft & Sharpe. "Inorganic Chemistry". (ed 2, 2005). p 671-673:
OsO4 + [Me4N]F â†' [Me4N][OsO4F]
OsO4 + 2 [Me4N]F â†' [Me4N]2[cis-OsO4F2]
Oxidation of alkenes
OsO4 will react with alkenes to deliver two hydroxyl groups on the same side of the alkene (syn-dihydroxylation) forming a vicinaldiol:
Oso4catrxn.gif
The following is the mechanism of the above reaction:
Oso4mechanism.gif
The osmium catalyst can be regained by being reoxidized by another reagent, such as H2O2.
Since OsO4 is highly toxic, several catalytic procedures has been developed where substoichiometric amounts of OsO4 are used with another oxidizing agent such as H2O2, NaIO4, N-methylmorpholine N-oxide (NMO), and K3Fe(CN)6.
A tertiary amine, such as pyridine, increases the reaction rate by coordinating to the OsO4.
Miscellaneous reactions
OsO4 dissolves in alkaline aqueous solution to give the osmate anion::OsO4 + 2 NaOH â†' Na2[cis-OsO4(OH)2] + O2
OsO4 is Lewis acidic, and when the Lewis bases are amines, the oxides can undergo substitution. Thus with NH3 one obtains the nitrido-oxide::OsO4 + NH3 + KOH â†' K[Os(N)O3] + 2 H2OThe [Os(N)O3]- anion is isoelectronic and isostructural with OsO4. Using primary amine tert-BuNH2 one obtains the corresponding imido derivative::OsO4 + 4 (Me3CNH2 â†' Os(NCMe3)4 + 4 H2O
OsO4 undergoes "reductive carbonylation" in methanol at 400 K and 200 bar of pressure to produce the triangular cluster Os3(CO)12::3 OsO4 + 24 CO â†' Os3(CO)12 + 12 CO2Housecroft & Sharpe. "Inorganic Chemistry". (ed 2, 2005). p 672,710In this reaction osmium changes oxidation state by eight units.
In organic synthesis OsO4 is widely used to oxidise alkenes to the dialcohols, adding two hydroxyl groups at the same side (syn addition). See reaction and mechanism above. One catalytic reaction with osmium tetroxide is the Sharpless bishydroxylation named after Nobelist K. Barry Sharpless. In the presence of chiral ligands, OsO4 will catalyze the cis-dihydroxylation of pro-chiral alkenes to give chiral 1,2-diols.Sharpless, K. B.; "Searching for New Reactivity (Nobel Lecture)".Angew. Chem. Int. Ed.2002, 41, 2024-2032
Biological staining
OsO4 is a widely used staining agent used in transmission electron microscopy (TEM) to provide contrast to the image. As a lipid stain, it is also useful in scanning electron microscopy 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. Osmium tetroxide is also used as a stain for lipids in optical microscopy. OsO4 also stains the human cornea (see safety considerations).
Osmeth
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 to make a dilute (0.25%) working solution of OsO4.Kiernan, J.A. Department of Anatomy & Cell Biology, The University of Western Ontario. [2]
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(NH3)4. This is ignited under hydrogen (H2) gas leaving behind pure osmium (Os).Dr. M Thomson, Osmium tetroxide(OsO4). [3]
Buckminsterfullerene adduct
OsO4 allowed for the confirmation of the soccer ball model of buckminsterfullerene, a 60 atom carbonallotrope. 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 crystallographyHawkins et al."Crystal Structure of Osmylated C60 : Confirmation of the Soccer Ball Framework". Science, New Series, Vol. 252, No. 5003. (Apr. 12, 1991), pp. 312-313. [4]
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.
On the 6th of April 2004 the American news organization ABC News reported that British intelligence sources believed they had foiled a plot to detonate a bomb involving OsO4 due to its poisonous properties.
*Cotton, S. A. "Chemistry of Precious Metals," Chapman and Hall (London): 1997. ISBN 0751404136. *Berrisford, D. J.; Bolm, C.; Sharpless, K. B., "Ligand Accelerated Catalysis", Angewandte Chemie, International Edition English, 1995, volume 34, pp. 1059-1070.
