Osmium tetroxide

Osmium tetroxide
Preferred IUPAC name Osmium tetraoxide
Systematic name Tetraoxoosmium
Other names Osmium(VIII) oxide
Identifiers
CAS number	20816-12-0 Y
PubChem	30318 Y, 56370778 (monopotassiate) Y, 75811001 (monoquinuclidiniate) Y, 53113021 (monotemediate)
ChemSpider	28158 Y
EC number	244-058-7
UN number	UN 2471
MeSH	Osmium+tetroxide
RTECS number	RN1140000
Jmol-3D images	Image 1
SMILES O=[Os](=O)(=O)=O
InChI InChI=1S/4O.Os Y Key: VUVGYHUDAICLFK-UHFFFAOYSA-N Y InChI=1S/4O.Os Key: VUVGYHUDAICLFK-UHFFFAOYSA-N InChI=1/4O.Os/rO4Os/c1-5(2,3)4 Key: VUVGYHUDAICLFK-TYHKRQCIAE
Properties
Molecular formula	OsO4
Molar mass	254.23 g/mol
Appearance	pale yellow solid
Density	4.91 g/cm3[1]
Melting point	40.25 °C
Boiling point	129.7 °C
Solubility in water	65 g/L[1]
Solubility	soluble in most organic solvents
Structure
Crystal structure	Monoclinic, mS20
Space group	C2/c; a = 0.4515 nm, b = 0.52046 nm, c = 0.80838 nm, α = 77.677°, β = 73.784°, γ = 64.294°[2]
Hazards
MSDS	ICSC 0528
EU Index	076-001-00-5
EU classification	Very toxic (T+) Corrosive (C)
R-phrases	R26/27/28, R34
S-phrases	(S1/2), S7/9, S26, S45
NFPA 704	0 4 1 OX
Related compounds
Other cations	Ruthenium tetroxide
Related osmium oxides	Osmium(IV) oxide
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Osmium tetroxide (also called osmium tetraoxide) 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.

Physical properties

Crystal structure of OsO₄^[2]

Osmium tetroxide exists as a pale yellow-brown crystalline solid (monoclinic crystal symmetry^[2]) with a characteristic acrid chlorine-like odor.^[3] 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).^[4] Pure osmium tetroxide is probably colourless^[5] and it has been suggested that its yellow hue is due to osmium dioxide (OsO₂) impurities^[6] although osmium dioxide normally exists as a black powder.^[7] Osmium tetroxide molecule is tetrahedral and therefore non-polar. This nonpolarity helps OsO₄ penetrate charged cell membranes. OsO₄ is 518 times more soluble in CCl₄ than in water.

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.

The high oxidation state of osmium in this compound can be rationalized by comparison of main-group and transition-metal chemistry. Just as the elements in groups 3 through 7 form compounds analogous to those formed by elements in groups 13 through 17 (e.g. TiCl₄ and GeCl₄, VF₅ and AsF₅, CrO₄²⁻ and SeO₄²⁻, etc.), we might expect the elements in group 8 to form compounds analogous to those formed by the noble gases. This is the case, as demonstrated by the existence of compounds like OsO₄ and XeO₄.

Synthesis

OsO₄ is formed slowly when osmium powder reacts with O₂ at ambient temperature. Reaction of bulk solid requires heating to 400 °C.^[8]

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 anhydrous HF solution:^[9]

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

OsO₄ also reacts with F₂ to form yellow OsO₃F₂:^[10]

2 OsO₄ + 2 F₂ → 2 OsO₃F₂ + O₂

OsO₄ reacts with one equivalent of [Me₄N]F at 298 K and 2 equivalents at 253 K:^[8]

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^[11]

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).^[11]

OsO₄ is used in catalytic amounts due to its toxicity and high cost. 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.^[12]

Miscellaneous reactions

OsO₄ does not react with most carbohydrates.^[13] It dissolves in alkaline aqueous solution to give the osmate anion OsO₂(OH)₄²⁻.^[14] OsO₄ is a Lewis acid, 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 is very soluble in tert-butanol and in solution is readily reduced by molecular hydrogen to osmium metal. The suspended osmium metal can be used to catalyze hydrogenation of a wide variety of organic chemicals containing double or triple bonds.

OsO₄ + 4 H₂ (g) → Os (s) + 4 H₂O

OsO₄ undergoes "reductive carbonylation" with carbon monoxide 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^[8]

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).

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 (Lemieux-Johnson oxidation) when 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.^[15]

Biological staining

Electron micrograph of (organic) plant tissue without (top) and with OsO₄ staining

OsO₄ is a widely used staining agent used in transmission electron microscopy (TEM) to provide contrast to the image.^[16] 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. In the staining of the plasma membrane, osmium tetroxide binds phospholipid head regions, thus creating contrast with the neighbouring protoplasm (cytoplasm). 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. OsO₄ stabilizes many proteins by transforming them into gels without destroying structural features. Tissue proteins that are stabilized by OsO₄ are not coagulated by alcohols during dehydration.^[13] Osmium tetroxide is also used as a stain for lipids in optical microscopy.^[17] OsO₄ also stains the human cornea (see safety considerations).

Polymer staining

It is also used to stain copolymers preferentially, the best known example being block copolymers where one phase can be stained so as to show the microstructure of the material. For example, styrene-butadiene block copolymers have a central polybutadiene chain with polystyrene end caps. When treated with OsO₄, the butadiene matrix reacts preferentially and so absorbs the oxide. The presence of a heavy metal is sufficient to block the electron beam, so the polystyrene domains are seen clearly in thin films in TEM.

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.^[18]

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).^[4]

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.^[19]

Safety considerations

Label with poison warning

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. The permissible exposure limit for osmium tetroxide (8 hour time-weighted average) is 2 µg/m³.^[3] Osmium tetroxide can penetrate plastics and therefore is stored in glass in a cold place.^[13]

On April 6, 2004 British intelligence sources believed they had foiled a plot to detonate a bomb involving OsO₄.^[20] Experts interviewed by New Scientist affirmed osmium tetroxide's toxicity, though some highlighted the difficulties of using it in a weapon: osmium tetroxide is very expensive. The osmium tetroxide may be destroyed by the blast; what remaining toxic fumes may also be dispersed by the blast as well.^[21]

References

1. ^ ^{a b} Sicherheitsdatenblatt (Merck) (at 20 °C)
2. ^ ^{a b c} Krebs B., Hasse K.D. (1976). "Refinements of the Crystal Structures of KTcO4, KReO4 and OsO4. The Bond Lengths in Tetrahedral Oxo-Anions and Oxides of d0 Transition Metals". Acta Crystallographica B 32 (5): 1334–1337. doi:10.1107/S056774087600530X. 
3. ^ ^{a b} "Documentation for Immediately Dangerous to Life or Health Concentrations (IDLHs)". Centers for Disease Control. http://www.cdc.gov/niosh/idlh/20816120.html. Retrieved 2010-10-25. 
4. ^ ^{a b} Mike Thompson. "Osmium tetroxide (OSO₄)". Bristol University. http://www.chm.bris.ac.uk/motm/oso4/oso4h.htm. Retrieved 2007-08-24. 
5. ^ Ian S. Butler; John Frank Harrod (1989). Inorganic chemistry: principles and applications. Benjamin/Cummings. p. 343. ISBN 9780805302479. http://books.google.com/books?id=Nd3vAAAAMAAJ. Retrieved 21 June 2011. 
6. ^ Cotton (31 August 2007). Advanced Inorganic Chemistry, 6Th Ed. Wiley India Pvt. Ltd.. p. 1002. ISBN 9788126513383. http://books.google.com/books?id=U3MWRONWAmMC. Retrieved 21 June 2011. 
7. ^ "Alfa Aesar MSDS". http://www.alfa.com/content/msds/USA/39497.pdf. Retrieved 2010-10-25. 
8. ^ ^{a b c} Housecroft, C. E.; Sharpe, A. G. (2004). Inorganic Chemistry (2nd ed.). Prentice Hall. pp. 671–673, 710. ISBN 978-0130399137. 
9. ^ 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. 
10. ^ Cotton, S. A. (1997). Chemistry of Precious Metals. London: Chapman and Hall. ISBN 0-7514-0413-6. 
11. ^ ^{a b} 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. 
12. ^ 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. 
13. ^ ^{a b c} M. A. Hayat (2000). Principles and techniques of electron microscopy: biological applications. Cambridge University Press. pp. 45–61. ISBN 0521632870. http://books.google.com/?id=nfsVMH8it1kC. 
14. ^ Thomas R. Dulski A manual for the chemical analysis of metals, ASTM International, 1996, ISBN 0803120664 p. 130
15. ^ 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. 
16. ^ Bozzola, John J.; Russell, Lonnie D. (1999). "Specimen Preparation for Transmission Electron Microscopy". Electron microscopy : principles and techniques for biologists. Sudbury, Mass.: Jones and Bartlett. pp. 21–31. ISBN 9780763701925. http://books.google.com/?id=RqSMzR-IXk0C&pg=PA21. 
17. ^ F. Di Scipio et al. (2008). "A simple protocol for paraffin-embedded myelin sheath staining with osmium tetroxide for light microscope observation". Microscopy Research and Technique 71 (7): 497–502. doi:10.1002/jemt.20577. PMID 18320578. 
18. ^ Kiernan, J.A. Department of Anatomy & Cell Biology, The University of Western Ontario. Re: "Disposal" of Osmium Tetroxide "Waste"
19. ^ 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. PMID 17769278. 
20. ^ "Chemical 'bomb plot' in UK foiled". BBC News. 2004-04-06. http://news.bbc.co.uk/1/hi/uk/3603961.stm. 
21. ^ Shaoni Bhattacharya (2004-04-07). "Experts divided over poison bomb claim". New Scientist. http://technology.newscientist.com/article/dn4863-experts-divided-over-poison-bomb-claim.html. 

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