Osmium(VIII) oxide
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| Osmium(VIII) oxide | |
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| General | |
| Systematic name | Osmium tetraoxide Osmium(VIII) oxide Osmic acid |
| Molecular formula | OsO4 |
| Molar mass | 254.23 g/mol |
| Appearance | clear or pale yellow translucent solid |
| CAS number | 20816-12-0 |
| Properties | |
| Density and phase | 4.9 g/cm³, solid |
| Solubility in water | 6 g/100 ml (25 °C) |
| Melting point | 40.25 °C |
| Boiling point | 130 °C |
| Acidity (pKa) | ? |
| Hazards | |
| MSDS | External MSDS |
| EU classification | Highly toxic (T+) |
| R-phrases | R26/27/28, R34 |
| S-phrases | S1/2, S7/9, S26, S45 |
| Supplementary data page | |
| Structure and properties |
n, εr, etc. |
| Thermodynamic data |
Phase behaviour Solid, liquid, gas |
| Spectral data | UV, IR, NMR, MS |
| Related compounds | |
| Other anions | ? |
| Other cations | Ruthenium tetroxide Osmium(IV) oxide |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references |
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Osmium tetroxide is the chemical compound with the formula OsO4. The compound is noteworthy for its many uses, despite the rarity of osmium. It has a number of interesting properties, one being that the solid is volatile.
Contents |
[edit] Physical properties
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.[1]
[edit] Structure and electron configuration
As a d0 metal, Os(VIII) is expected to adopt a tetrahedral geometry when bound to four ligands. Tetrahedral structures are seen for MnO4- and CrO42-.
The osmium of OsO4 has a formal oxidation state of 8+, the highest oxidation state for a transition metal. The osmium atom has eight valence electrons. If one assumes that two electrons are donated by each of the four oxide ligands, the total electron count for the complex is 16, as also seen for permanganate and chromate.
[edit] Synthesis
OsO4 is formed slowly when osmium powder reacts with O2 at 298 K. Reaction of bulk solid requires heating to 670 K.[2]
- Os + 2 O2 → OsO4
[edit] Reactions
[edit] 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[3]:
- OsO4 + 2 KrF2 → cis-OsO2F4 + 2 Kr + O2
OsO4 also reacts with F2 to form yellow OsO3F2:
- 2 OsO4 + 2 F2 → 2 OsO3F2 + O2
OsO4 reacts with one equivalent of [Me4N]F at 298 K and 2 equivalents at 253 K[4]:
- OsO4 + [Me4N]F → [Me4N][OsO4F]
- OsO4 + 2 [Me4N]F → [Me4N]2[cis-OsO4F2]
[edit] Oxidation of alkenes
OsO4 catalyzes the cis-dihydroxylation of alkenes by hydrogen peroxide or related sources of oxygen atoms in the presence of water. The reaction that is catalyzed is:
- R2C=CR2 + H2O2 → R2C(OH)-C(OH)R2
In terms of mechanism, OsVIIIO4 adds to alkenes R2C=CR2 to afford cyclic "esters" R4C2O2OsVIO2, which undergo hydrolysis to give the vicinal diol and release a reduced osmium oxide (OsVI):
Lewis bases such as tertiary amines and pyridines increase the reaction rate. This "ligand-acceleration" arises via the formation of adduct OsO4L, which adds more rapidly to the alkene. If the amine is chiral, then the dihydroxylation can proceed with enantioselectivity (see Sharpless asymmetric dihydroxylation.
Since OsO4 is toxic and expensive, it is used in catalytic amounts. The osmium catalyst is regenerated by oxidizing agents, such as H2O2, N-methylmorpholine N-oxide (NMO, see Upjohn dihydroxylation), and K3Fe(CN)6. These oxidizing reagents do not react with the alkenes on their own.
[edit] Miscellaneous reactions
OsO4 dissolves in alkaline aqueous solution to give the osmate anion:
- OsO4 + 2 NaOH → 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 → K[Os(N)O3] + 2 H2O
The [Os(N)O3]- anion is isoelectronic and isostructural with OsO4. Using primary amine tert-BuNH2 one obtains the corresponding imido derivative:
- OsO4 + 4 (Me3CNH2 → 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:
In this reaction osmium changes oxidation state by eight units.
[edit] Uses
[edit] Organic synthesis
In organic synthesis OsO4 is widely used to oxidise alkenes to the vicinal diols, adding two hydroxyl groups at the same side (syn addition). See reaction and mechanism above. This mechanism has been made both catalytic (Upjohn dihydroxylation) and asymmetric (Sharpless asymmetric dihydroxylation, named after Nobel Laureate K. Barry Sharpless).[6]
Osmium tetroxide is also used in catalytic amount in the Sharpless oxyamination to give vicinal amino-alcohols.
[edit] 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 (SEM) 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. Additionally, Osmium tetroxide is also used for fixing biological samples in conjunction with HgCl2. Its rapid killing abilities are used to quickly kill specimen like protozoa. Osmium tetroxide is also used as a stain for lipids in optical microscopy. OsO4 also stains the human cornea (see safety considerations).
[edit] 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 solution to make a dilute (0.25%) working solution of OsO4.[7]
[edit] 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(NH4)4. This is ignited under hydrogen (H2) gas leaving behind pure osmium (Os).[8]
[edit] Buckminsterfullerene adduct
OsO4 allowed for the confirmation of the soccer ball model of buckminsterfullerene, a 60 atom carbon allotrope. 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 crystallography[9]
[edit] Safety considerations
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.
[edit] External links
- International Chemical Safety Card 0528
- NIOSH Pocket Guide to Chemical Hazards
- BBC report on bomb plot
- BBC What is Osmium tetroxide article
- Osmium Tetroxide: Molecule of the Month
- Chemical Reactions
[edit] References
- Cotton, S. A. "Chemistry of Precious Metals," Chapman and Hall (London): 1997. ISBN 0-7514-0413-6.
- Berrisford, D. J.; Bolm, C.; Sharpless, K. B., "Ligand Accelerated Catalysis", Angewandte Chemie, International Edition English, 1995, volume 34, pp. 1059-1070.
- ^ Dr. M Thomson, Osmium tetroxide(OsO4). [1]
- ^ Housecroft & Sharpe. "Inorganic Chemistry". (ed 2, 2005). p 671-673
- ^ Christe et al. Osmium Tetrafluoride Dioxide, cis-OsO2F4. J. Am. Chem. Soc. 1993, 115. 11279-11284
- ^ Housecroft & Sharpe. "Inorganic Chemistry". (ed 2, 2005). p 671-673
- ^ Housecroft & Sharpe. "Inorganic Chemistry". (ed 2, 2005). p 672,710
- ^ Sharpless, K. B.; "Searching for New Reactivity (Nobel Lecture)".Angew. Chem. Int. Ed. 2002, 41, 2024-2032
- ^ Kiernan, J.A. Department of Anatomy & Cell Biology, The University of Western Ontario. [2]
- ^ Dr. M Thomson, Osmium tetroxide(OsO4). [3]
- ^ Hawkins 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]
