Osmium tetroxide

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The chemical compound osmium tetroxide (OsO4), also known as osmium tetraoxide, osmium(VIII) oxide, or osmic acid, is an oxide of the element osmium.

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Physical properties

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. It is very volatile as 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.<ref>Dr. M Thomson, Osmium tetroxide(OsO4). [1]</ref>

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Structure

The osmium of OsO4 has an oxidation number of 8, the highest possible oxidation state for a transition element. The osmium has a total electron count of 16 - 8 from its valence shell and 2 donated by each of the oxygens - and is a d0 metal. OsO4 is tetrahedral, similar to permanganate (MnO4-), with which it is isoelectronic.

Image:Oso4.gif
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Synthesis

OsO4 is formed slowly when osmium powder reacts with O2 at 298 K. Reaction of bulk solid requires heating to 670 K.<ref>Housecroft & Sharpe. "Inorganic Chemistry". (ed 2, 2005). p 671-673</ref>

Os + 2 O2 → OsO4
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Reactions

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Oxofluorides

Osmium forms several oxofluorides, all of which are very sensitive to moisture. Purple cis-OsO2F4 forms at 77 K in an aqueous solution of HF<ref>Christe et al. Osmium Tetraflouride Dioxide, cis-OsO2F4. J. Am. Chem. Soc. 1993, 115. 11279-11284</ref>:

OsO4 + 2 KrF2 → cis-OsO2F4 + 2 Kr + O2

OsO4 also reacts with F2 to form yellow OsOF2:

2 OsO4 + 2 F2 → 2 OsO3F2 + O2

OsO4 reacts with one equivalent of [Me4N]F at 298 K and 2 equivalents at 253 K<ref>Housecroft & Sharpe. "Inorganic Chemistry". (ed 2, 2005). p 671-673</ref>:

OsO4 + [Me4N]F → [Me4N][OsO4F]
OsO4 + 2 [Me4N]F → [Me4N]2[cis-OsO4F2]
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Oxidation of alkenes

OsO4 will react with alkenes to deliver two hydroxyl groups on the same side of the alkene (syn-dihydroxylation) forming a dialcohol: Image:Oso4rxn.gif

The following is the mechanism of the above reaction: Image:Osvicsyndih.gif

Since OsO4 is highly toxic, a catalytic procedure has been developed where substoichiometric amounts of OsO4 are used with another oxidizing agent such as H2O2, which reoxidizes the OsO3 to OsO4.

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Miscellaneous reactions

OsO4 may be dissolved in aqueous alkalis such as NaOH:

OsO4 + 2 NaOH → Na2[cis-OsO4(OH)2] + O2

OsO4 reacts with NH3 and KOH:

OsO4 + NH3 + KOH → K[Os(N)O3] + 2 H2O

The [Os(N)O3]- anion is isoelectronic and isostructural with OsO4.

OsO4 + 4 (Me3)NHtBu → Os(NtBu)4 + 4 (Me3)OH

OsO4 will react in methanol with CO at 400 K and up to 200 bar of pressure to produce Os3(CO)12:

3 OsO4 + 24 CO → Os3(CO)12 + 12 CO2<ref>Housecroft & Sharpe. "Inorganic Chemistry". (ed 2, 2005). p 672,710</ref>
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Uses

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Organic synthesis

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.

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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.

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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).<ref>Dr. M Thomson, Osmium tetroxide(OsO4). [2]</ref>

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Safety considerations

OsO4 is highly poisonous, even at very 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 April 2004 the American news organisation ABC News reported that British intelligence sources believed they had foiled a plot to detonate a bomb involving OsO4 due to its poisonous properties.

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External links

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References

  • 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.de:Osmiumtetroxid

fr:Tétroxyde d'osmium ga:Teatrocsaíd oismiam he:אוסמיום ארבע-חמצני

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