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Osmium tetroxide

Osmium(VIII) oxide

General
Systematic name	Osmium tetraoxide Osmium(VIII) oxide Osmic acid
Molecular formula	OsO₄
Molar mass	254.23 g·mol⁻¹
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 (pK_a)	?
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, ε_r, 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

Osmium tetroxide is the chemical compound with the formula OsO₄. The compound is noteworthy for its many uses, despite the rarity of osmium. It also has a number of interesting properties, one being that the solid is volatile.

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1 Physical properties
2 Structure and electron configuration
3 Synthesis
4 Reactions
5 Uses
6 Safety considerations
7 General references
8 References

Physical properties

Osmium tetroxide exists as a pale yellow-brown crysalline solid with a characteristic acrid odor^[1] similar to ozone. In fact, the element name osmium is derived from osme, Greek for odor. OsO₄ is volatile: it sublimes at room temperature. It is soluble in a wide range of organic solvents, and moderately soluble in water, with which it reacts reversibly to form osmic acid (see below).^[2] Pure osmium tetraoxide is probably colourless^[3] and it has been suggested that it's yellow hue is due to osmium dioxide (OsO₂)impurities ^[4] although osmium (IV) oxide normally exists as a black powder^[5] so this seems unlikely.

Structure and electron configuration

With a d⁰ configuration, Os(VIII) is expected to form tetrahedral complexes when bound to four ligands. Tetrahedral structures are seen for the electronically related oxides MnO₄^- and CrO₄^2-.

The osmium of OsO₄ has a formal oxidation state of 8+, the highest oxidation state known 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 the isoelectronic species permanganate and chromate.

Synthesis

OsO₄ is formed slowly when osmium powder reacts with O₂ at 298 K. Reaction of bulk solid requires heating to 670 K.^[6]

Os + 2 O₂ → OsO₄

Reactions

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:^[7]

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:^[6]

OsO₄ + [Me₄N]F → [Me₄N][OsO₄F]

OsO₄ + 2 [Me₄N]F → [Me₄N]₂[cis-OsO₄F₂]

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, Os^VIIIO₄ adds to alkenes R₂C=CR₂ to afford cyclic "esters" R₄C₂O₂Os^VIO₂, which undergo hydrolysis to give the vicinal diol and release a reduced osmium oxide (Os^VI):

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. Other sources of osmium tetroxide include potassium osmate(VI) dihydrate (K₂OsO₄·2H₂O) and osmium (III) chloride hydrate (OsCl₃.xH₂O) which oxidise to osmium (VIII) in the presence of such oxidants.^[8]

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:

3 OsO₄ + 24 CO → Os₃(CO)₁₂ + 12 CO₂^[6]

In this reaction osmium changes oxidation state by eight units.

Uses

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 reaction has been made both catalytic (Upjohn dihydroxylation) and asymmetric (Sharpless asymmetric dihydroxylation, named after Nobel Laureate K. Barry Sharpless).^[9]

Osmium tetroxide is also used in catalytic amount in the Sharpless oxyamination to give vicinal amino-alcohols.

In combination with sodium periodate, OsO₄ is used for the oxidative cleavage of alkenes (the Lemieux-Johnson oxidation). Here the periodate serves both to cleave the diol formed by dihydroxylation, and to reoxidize the OsO₃ back to OsO₄. The net transformation is identical to that produced by ozonolysis. Below an example from the total synthesis of Isosteviol.^[10]

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).

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.^[11]

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).^[2]

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-butyl pyridine)₂. The adduct broke the fullerene's symmetry allowing for crystallization and confirmation of the structure of C₆₀ by x-ray crystallography.^[12]

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.

General references

Cotton, S. A. "Chemistry of Precious Metals," Chapman and Hall (London): 1997. ISBN 0-7514-0413-6.
D. J. Berrisford, C. Bolm and K. B. Sharpless (1995). "Ligand-Accelerated Catalysis". Angewandte Chemie International Edition in English 34 (10): 1059-1070. doi:10.1002/anie.199510591.

References

^ http://dohs.ors.od.nih.gov/pdf/Osmium%20Tetroxide%20REVISED.pdf
^ ^a ^b Mike Thompson. Osmium tetroxide (OSO₄). Bristol University. Retrieved on 2007-08-24.
^ Butler and Harrod, Inorganic Chemistry: Principles and Applications, p.343
^ Cotton and Wilkinson, Advanced Inorganic Chemistry, p.1002
^ http://www.avocadochem.com/daten_msds/GB/39497_-_Osmium_(IV)_oxide_(GB).pdf AlfarAesar MSDS
^ ^a ^b ^c Catherine E. Housecroft, Alan Sharpe (2005). Inorganic Chemistry, 2nd, 671-673, 710.
^ K. O. Christe, D. A. Dixon, H. G. Mack, H. Oberhammer, A. Pagelot, J. C. P. Sanders and G. J. Schrobilgen (1993). "Osmium tetrafluoride dioxide, cis-OsO₂F₄". J. Am. Chem. Soc. 115 (24): 11279-11284. doi:10.1021/ja00077a029.
^ Yasukazu Ogino, Hou Chen, Hoi-Lun Kwong and K. Barry Sharpless (1991). "On the timing of hydrolysis / reoxidation in the osmium-catalyzed asymmetric dihydroxylation of olefins using potassium ferricyanide as the reoxidant". Tetrahedron Letters 32 (32): 3965. doi:10.1016/0040-4039(91)80601-2.
^ K. B. Sharpless (2002). "Searching for New Reactivity (Nobel Lecture)" (reprint at nobelprize.org). Angewandte Chemie International Edition 41 (12): 2024-2032. doi:10.1002/1521-3773(20020617)41:12%3C2024::AID-ANIE2024%3E3.0.CO;2-O.
^ B. B. Snider, J. Y. Kiselgof and B. M. Foxman (1998). "Total Syntheses of (±)-Isosteviol and (±)-Beyer-15-ene-3β,19-diol by Manganese(III)-Based Oxidative Quadruple Free-Radical Cyclization". J. Org. Chem. 63 (22): 7945-7952. doi:10.1021/jo981238x.
^ Kiernan, J.A. Department of Anatomy & Cell Biology, The University of Western Ontario. [1]
^ J. M. Hawkins, A. Meyer, T. A. Lewis, S. Loren and F. J. Hollander (1991). "Crystal Structure of Osmylated C60: Confirmation of the Soccer Ball Framework". Science 252 (5003): 312-313. doi:10.1126/science.252.5003.312.

Categories: Osmium compounds | Oxides | Oxidizing agents | Staining dyes

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