Osmium is a hard, brittle, blue-gray metal, and the denseststable element—about twice as dense aslead. The density of osmium is slightly greater than that ofiridium; the two are so similar (22.587 versus22.562 g/cm3 at 20 °C) that each was at one time considered to be the densest element. Only in the 1990s were measurements made accurately enough (by means ofX-ray crystallography) to be certain that osmium is the denser of the two.[11][15]
Osmium has a blue-gray tint.[12] Thereflectivity of single crystals of osmium is complex and strongly direction-dependent, with light in the red and near-infrared wavelengths being more strongly absorbed whenpolarized parallel to thec crystal axis than when polarized perpendicular to thec axis; thec-parallel polarization is also slightly more reflected in the mid-ultraviolet range. Reflectivity reaches a sharp minimum at around 1.5 eV (near-infrared) for thec-parallel polarization and at 2.0 eV (orange) for thec-perpendicular polarization, and peaks for both in the visible spectrum at around 3.0 eV (blue-violet).[16]
Osmium is a hard but brittlemetal that remainslustrous even at high temperatures. It has a very lowcompressibility. Correspondingly, itsbulk modulus is extremely high, reported between395 and462 GPa, which rivals that ofdiamond (443 GPa). The hardness of osmium is moderately high at4 GPa.[17][18][19] Because of itshardness, brittleness, lowvapor pressure (the lowest of the platinum-group metals), and very highmelting point (thefourth highest of all elements, aftercarbon,tungsten, andrhenium), solid osmium is difficult to machine, form, or work.
Osmium forms compounds withoxidation states ranging from −4 to +8. The most common oxidation states are +2, +3, +4, and +8. The +8 oxidation state is notable for being the highest attained by any chemical element aside from iridium's +9[21] and is encountered only inxenon,[22][23]ruthenium,[24]hassium,[25]iridium,[26] andplutonium.[27][28] The oxidation states −1 and −2 represented by the two reactive compoundsNa 2[Os 4(CO) 13] andNa 2[Os(CO) 4] are used in the synthesis of osmiumcluster compounds.[29][30]
Osmium tetroxide (OsO4)
The most common compound exhibiting the +8 oxidation state isosmium tetroxide (OsO4). This toxic compound is formed when powdered osmium is exposed to air. It is a very volatile, water-soluble, pale yellow, crystalline solid with a strong smell. Osmium powder has the characteristic smell of osmium tetroxide.[31] Osmium tetroxide forms red osmatesOsO 4(OH)2− 2 upon reaction with a base. Withammonia, it forms the nitrido-osmatesOsO 3N− .[32][33][34] Osmium tetroxide boils at 130 °C and is a powerfuloxidizing agent. By contrast,osmium dioxide (OsO 2) is black, non-volatile, and much less reactive and toxic.
Osmium pentafluoride (OsF 5) is known, but osmium trifluoride (OsF 3) has not yet been synthesized. The lower oxidation states are stabilized by the larger halogens, so that the trichloride, tribromide, triiodide, and even diiodide are known. The oxidation state +1 is known only for osmium monoiodide (OsI), whereas several carbonyl complexes of osmium, such astriosmium dodecacarbonyl (Os 3(CO) 12), represent oxidation state 0.[32][33][36][37]
In general, the lower oxidation states of osmium are stabilized byligands that are good σ-donors (such asamines) and π-acceptors (heterocycles containingnitrogen). The higher oxidation states are stabilized by strong σ- and π-donors, such asO2− andN3− .[38]
Despite its broad range of compounds in numerous oxidation states, osmium in bulk form at ordinary temperatures and pressures is stable in air. It resists attack by most acids and bases includingaqua regia, but is attacked byF2 andCl2 at high temperatures, and by hot concentrated nitric acid to produceOsO4. It can be dissolved by molten alkalis fused with an oxidizer such assodium peroxide (Na2O2) orpotassium chlorate (KClO3) to give osmates such asK2[OsO2(OH)4].[36]
Osmium has seven naturally occurringisotopes, five of which are stable:187 Os,188 Os,189 Os,190 Os, and (most abundant)192 Os. At least 37 artificial radioisotopes and 20nuclear isomers exist, with mass numbers ranging from 160 to 203; the most stable of these is194 Os with a half-life of 6 years.[39]
186 Os undergoesalpha decay with such a longhalf-life(2.0±1.1)×1015 years, approximately140000 times theage of the universe, that for practical purposes it can be considered stable.184 Os is also known to undergo alpha decay with a half-life of(1.12±0.23)×1013 years.[10] Alpha decay is predicted for all the other naturally occurring isotopes, but this has never been observed, presumably due to very long half-lives. It is predicted that184 Os and192 Os can undergodouble beta decay, but this radioactivity has not been observed yet.[39]
189Os has a spin of 5/2 but187Os has a nuclear spin 1/2. Its low natural abundance (1.64%) and low nuclear magnetic moment means that it is one of the most difficult natural abundance isotopes forNMR spectroscopy.[40]
187 Os is the descendant of187 Re (half-life4.56×1010 years) and is used extensively in dating terrestrial as well asmeteoricrocks (seeRhenium–osmium dating). It has also been used to measure the intensity of continental weathering over geologic time and to fix minimum ages for stabilization of themantle roots of continentalcratons. This decay is a reason why rhenium-rich minerals are abnormally rich in187 Os.[41] However, the most notable application of osmium isotopes in geology has been in conjunction with the abundance of iridium, to characterise the layer ofshocked quartz along theCretaceous–Paleogene boundary that marks the extinction of the non-aviandinosaurs 65 million years ago.[42]
Osmium was discovered in 1803 bySmithson Tennant andWilliam Hyde Wollaston inLondon, England.[43] The discovery of osmium is intertwined with that of platinum and the other metals of theplatinum group. Platinum reached Europe asplatina ("small silver"), first encountered in the late 17th century in silver mines around theChocó Department, inColombia.[44] The discovery that this metal was not an alloy, but a distinct new element, was published in 1748.[45]Chemists who studied platinum dissolved it inaqua regia (a mixture ofhydrochloric andnitric acids) to create soluble salts. They always observed a small amount of a dark, insoluble residue.[46]Joseph Louis Proust thought that the residue wasgraphite.[46]Victor Collet-Descotils,Antoine François, comte de Fourcroy, andLouis Nicolas Vauquelin also observed iridium in the black platinum residue in 1803, but did not obtain enough material for further experiments.[46] Later the two French chemists Fourcroy and Vauquelin identified a metal in a platinum residue they calledptène.[47]
In 1803,Smithson Tennant analyzed the insoluble residue and concluded that it must contain a new metal. Vauquelin treated the powder alternately with alkali and acids[48] and obtained a volatile new oxide, which he believed was of this new metal—which he namedptene, from the Greek wordπτηνος (ptènos) for winged.[49][50] However, Tennant, who had the advantage of a much larger amount of residue, continued his research and identified two previously undiscovered elements in the black residue, iridium and osmium.[46][48] He obtained a yellow solution (probably ofcis–[Os(OH)2O4]2−) by reactions withsodium hydroxide at red heat. After acidification he was able to distill the formed OsO4.[49] He named it osmium afterGreekosme meaning "a smell", because of the chlorine-like and slightly garlic-like smell of the volatileosmium tetroxide.[51] Discovery of the new elements was documented in a letter to theRoyal Society on June 21, 1804.[46][52]
Uranium and osmium were early successfulcatalysts in theHaber process, thenitrogen fixation reaction ofnitrogen andhydrogen to produceammonia, giving enough yield to make the process economically successful. At the time, a group atBASF led byCarl Bosch bought most of the world's supply of osmium to use as a catalyst. Shortly thereafter, in 1908, cheaper catalysts based on iron and iron oxides were introduced by the same group for the first pilot plants, removing the need for the expensive and rare osmium.[53]
Osmium is now obtained primarily from the processing ofplatinum andnickel ores.[54]
Osmium is found in nature as an uncombined element or in naturalalloys; especially the iridium–osmium alloys,osmiridium (iridium rich), andiridosmium (osmium rich).[48] Innickel andcopper deposits, the platinum-group metals occur assulfides (i.e.,(Pt,Pd)S),tellurides (e.g.,PtBiTe),antimonides (e.g.,PdSb), andarsenides (e.g.,PtAs2); in all these compounds platinum is exchanged by a small amount of iridium and osmium. As with all of the platinum-group metals, osmium can be found naturally in alloys with nickel orcopper.[56]
Within Earth's crust, osmium, like iridium, is found at highest concentrations in three types of geologic structure: igneous deposits (crustal intrusions from below),impact craters, and deposits reworked from one of the former structures. The largest known primary reserves are in theBushveld Igneous Complex inSouth Africa,[57] though the large copper–nickel deposits nearNorilsk inRussia, and theSudbury Basin inCanada are also significant sources of osmium. Smaller reserves can be found in the United States.[57] Thealluvial deposits used bypre-Columbian people in theChocó Department, Colombia, are still a source for platinum-group metals. The second large alluvial deposit was found in theUral Mountains, Russia, which is still mined.[54][58]
Osmium is obtained commercially as a by-product fromnickel andcopper mining and processing. Duringelectrorefining of copper and nickel, noble metals such as silver, gold and the platinum-group metals, together with non-metallic elements such asselenium andtellurium, settle to the bottom of the cell asanode mud, which forms the starting material for their extraction.[59][60] Separating the metals requires that they first be brought into solution. Several methods can achieve this, depending on the separation process and the composition of the mixture. Two representative methods are fusion withsodium peroxide followed by dissolution inaqua regia, and dissolution in a mixture ofchlorine withhydrochloric acid.[57][61] Osmium, ruthenium, rhodium, and iridium can be separated from platinum, gold, and base metals by their insolubility in aqua regia, leaving a solid residue. Rhodium can be separated from the residue by treatment with moltensodium bisulfate. The insoluble residue, containing ruthenium, osmium, and iridium, is treated withsodium oxide, in which Ir is insoluble, producing water-soluble ruthenium and osmium salts. After oxidation to the volatile oxides,RuO 4 is separated fromOsO 4 by precipitation of (NH4)3RuCl6 with ammonium chloride.
After it is dissolved, osmium is separated from the other platinum-group metals by distillation or extraction with organic solvents of the volatile osmium tetroxide.[62] The first method is similar to the procedure used by Tennant and Wollaston. Both methods are suitable for industrial-scale production. In either case, the product is reduced using hydrogen, yielding the metal as a powder orsponge that can be treated usingpowder metallurgy techniques.[63]
Estimates of annual worldwide osmium production are on the order of several hundred to a few thousand kilograms.[64][36] Production and consumption figures for osmium are not well reported because demand for the metal is limited and can be fulfilled with the byproducts of other refining processes.[36] To reflect this, statistics often report osmium with other minor platinum group metals such as iridium and ruthenium. US imports of osmium from 2014 to 2021 averaged 155 kg annually.[65][66]
Because osmium is virtually unforgeable when fully dense and very fragile whensintered, it is rarely used in its pure state, but is instead often alloyed with other metals for high-wear applications. Osmium alloys such asosmiridium are very hard and, along with other platinum-group metals, are used in the tips offountain pens, instrument pivots, and electrical contacts, as they can resist wear from frequent operation. They were also used for the tips ofphonograph styli during the late 78 rpm and early "LP" and "45" record era, circa 1945 to 1955. Osmium-alloy tips were significantly more durable than steel and chromium needle points, but wore out far more rapidly than competing, and costlier,sapphire anddiamond tips, so they were discontinued.[67]
Osmium tetroxide has been used infingerprint detection[68] and in stainingfatty tissue for optical andelectron microscopy. As a strong oxidant, it cross-links lipids mainly by reacting with unsaturated carbon–carbon bonds and thereby both fixesbiological membranes in place in tissue samples and simultaneously stains them. Because osmium atoms are extremely electron-dense, osmium staining greatly enhances image contrast intransmission electron microscopy (TEM) studies of biological materials. Those carbon materials otherwise have very weak TEM contrast.[35] Another osmium compound, osmium ferricyanide (OsFeCN), exhibits similar fixing and staining action.[69]
In 1898, the Austrian chemistAuer von Welsbach developed the Oslamp with afilament made of osmium, which he introduced commercially in 1902. After only a few years, osmium was replaced bytungsten, which is more abundant (and thus cheaper) and more stable. Tungsten has the highest melting point among all metals, and its use in light bulbs increases the luminous efficacy and life ofincandescent lamps.[49]
The light bulb manufacturerOsram (founded in 1906, when three German companies, Auer-Gesellschaft, AEG and Siemens & Halske, combined their lamp production facilities) derived its name from the elements ofosmium andWolfram (the latter is German for tungsten).[72]
Likepalladium, powdered osmium effectively absorbs hydrogen atoms. This could make osmium a potential candidate for a metal-hydride battery electrode. However, osmium is expensive and would react with potassium hydroxide, the most common battery electrolyte.[73]
Osmium has highreflectivity in theultraviolet range of theelectromagnetic spectrum; for example, at 600 Å osmium has a reflectivity twice that of gold.[74] This high reflectivity is desirable in space-basedUV spectrometers, which have reduced mirror sizes due to space limitations. Osmium-coated mirrors were flown in several space missions aboard theSpace Shuttle, but it soon became clear that the oxygen radicals inlow Earth orbit are abundant enough to significantly deteriorate the osmium layer.[75]
The Sharpless dihydroxylation: RL = largest substituent; RM = medium-sized substituent; RS = smallest substituent
Post-flight appearance of Os, Ag, and Au mirrors from the front (left images) and rear panels of the Space Shuttle. Blackening reveals oxidation due to irradiation by oxygen atoms.[76][77]
A bead of osmium, about 0.5 cm in diameter, displaying the metal's reflectivity
The primary hazard of metallic osmium is the potential formation ofosmium tetroxide (OsO4), which isvolatile and very poisonous.[78] This reaction is thermodynamically favorable at room temperature,[79] but the rate depends on temperature and the surface area of the metal.[80][81] As a result, bulk material is not considered hazardous[80][82][83][84] while powders react quickly enough that samples can sometimes smell like OsO4 if they are handled in air.[36][85]
Between 1990 and 2010, the nominal price of osmium metal was almost constant, while inflation reduced the real value from ~US$950/ounce to ~US$600/ounce.[86] Because osmium has few commercial applications, it is not heavily traded and prices are seldom reported.[86]
^The thermal expansion of Os isanisotropic: the coefficients for each crystal axis (at 20 °C) are: αa = 4.57×10−6/K, αc = 5.85×10−6/K, and αaverage = αV/3 = 4.99×10−6/K.
^Rumble, John R.; Bruno, Thomas J.; Doa, Maria J. (2022). "Section 4: Properties of the Elements and Inorganic Compounds".CRC Handbook of Chemistry and Physics: A Ready Reference Book of Chemical and Physical Data (103rd ed.). Boca Raton, FL: CRC Press. p. 40.ISBN978-1-032-12171-0.
^Rumble, John R.; Bruno, Thomas J.; Doa, Maria J. (2022). "Section 4: Properties of the Elements and Inorganic Compounds".CRC Handbook of Chemistry and Physics: A Ready Reference Book of Chemical and Physical Data (103rd ed.). Boca Raton, FL: CRC Press. p. 40.ISBN978-1-032-12171-0.
^Rumble, John R.; Bruno, Thomas J.; Doa, Maria J. (2022). "Section 4: Properties of the Elements and Inorganic Compounds".CRC Handbook of Chemistry and Physics: A Ready Reference Book of Chemical and Physical Data (103rd ed.). Boca Raton, FL: CRC Press. p. 40.ISBN978-1-032-12171-0.
^Arblaster, John W. (2018).Selected Values of the Crystallographic Properties of Elements. Materials Park, Ohio: ASM International.ISBN978-1-62708-155-9.
^abPeters, Stefan T.M.; Münker, Carsten; Becker, Harry; Schulz, Toni (April 2014). "Alpha-decay of184Os revealed by radiogenic180W in meteorites: Half life determination and viability as geochronometer".Earth and Planetary Science Letters.391:69–76.doi:10.1016/j.epsl.2014.01.030.
^Fe(−4), Ru(−4), and Os(−4) have been observed in metal-rich compounds containing octahedral complexes [MIn6−xSnx]; Pt(−3) (as a dimeric anion [Pt–Pt]6−), Cu(−2), Zn(−2), Ag(−2), Cd(−2), Au(−2), and Hg(−2) have been observed (as dimeric and monomeric anions; dimeric ions were initially reported to be [T–T]2− for Zn, Cd, Hg, but later shown to be [T–T]4− for all these elements) in La2Pt2In, La2Cu2In, Ca5Au3, Ca5Ag3, Ca5Hg3, Sr5Cd3, Ca5Zn3(structure (AE2+)5(T–T)4−T2−⋅4e−), Yb3Ag2, Ca5Au4, and Ca3Hg2; Au(–3) has been observed in ScAuSn and in other 18-electron half-Heusler compounds. SeeChanghoon Lee; Myung-Hwan Whangbo (2008). "Late transition metal anions acting as p-metal elements".Solid State Sciences.10 (4):444–449.Bibcode:2008SSSci..10..444K.doi:10.1016/j.solidstatesciences.2007.12.001. andChanghoon Lee; Myung-Hwan Whangbo; Jürgen Köhler (2010). "Analysis of Electronic Structures and Chemical Bonding of Metal-rich Compounds. 2. Presence of Dimer (T–T)4– and Isolated T2– Anions in the Polar Intermetallic Cr5B3-Type Compounds AE5T3 (AE = Ca, Sr; T = Au, Ag, Hg, Cd, Zn)".Zeitschrift für Anorganische und Allgemeine Chemie.636 (1):36–40.doi:10.1002/zaac.200900421.
^"Chemistry of Hassium"(PDF).Gesellschaft für Schwerionenforschung mbH. 2002. Archived fromthe original(PDF) on January 14, 2012. RetrievedJanuary 31, 2007.
^Gong, Yu; Zhou, Mingfei; Kaupp, Martin; Riedel, Sebastian (2009). "Formation and Characterization of the Iridium Tetroxide Molecule with Iridium in the Oxidation State +VIII".Angewandte Chemie International Edition.48 (42):7879–7883.doi:10.1002/anie.200902733.PMID19593837.[dead link]
^Kiselev, Yu. M.; Nikonov, M. V.; Dolzhenko, V. D.; Ermilov, A. Yu.; Tananaev, I. G.; Myasoedov, B. F. (January 17, 2014). "On existence and properties of plutonium(VIII) derivatives".Radiochimica Acta.102 (3):227–237.doi:10.1515/ract-2014-2146.S2CID100915090.
^Zaitsevskii, Andréi; Mosyagin, Nikolai S.; Titov, Anatoly V.; Kiselev, Yuri M. (July 21, 2013). "Relativistic density functional theory modeling of plutonium and americium higher oxide molecules".The Journal of Chemical Physics.139 (3): 034307.Bibcode:2013JChPh.139c4307Z.doi:10.1063/1.4813284.PMID23883027.
^Krause, J.; Siriwardane, Upali; Salupo, Terese A.; Wermer, Joseph R.; et al. (1993). "Preparation of [Os3(CO)11]2− and its reactions with Os3(CO)12; structures of [Et4N] [HOs3(CO)11] and H2OsS4(CO)".Journal of Organometallic Chemistry.454 (1–2):263–271.doi:10.1016/0022-328X(93)83250-Y.
^Carter, Willie J.; Kelland, John W.; Okrasinski, Stanley J.; Warner, Keith E.; et al. (1982). "Mononuclear hydrido alkyl carbonyl complexes of osmium and their polynuclear derivatives".Inorganic Chemistry.21 (11):3955–3960.doi:10.1021/ic00141a019.
^abGriffith, W. P. (1965). "Osmium and its compounds".Quarterly Reviews, Chemical Society.19 (3):254–273.doi:10.1039/QR9651900254.
^Subcommittee on Platinum-Group Metals, Committee on Medical and Biologic Effects of Environmental Pollutants, Division of Medical Sciences, Assembly of Life Sciences, National Research Council (1977).Platinum-group metals. National Academy of Sciences. p. 55.ISBN978-0-309-02640-6.
^Gulliver, D. J; Levason, W. (1982). "The chemistry of ruthenium, osmium, rhodium, iridium, palladium, and platinum in the higher oxidation states".Coordination Chemistry Reviews.46:1–127.doi:10.1016/0010-8545(82)85001-7.
^Bell, Andrew G.; Koźmiński, Wiktor; Linden, Anthony; von Philipsborn, Wolfgang (1996). "187Os NMR Study of (η6-Arene)osmium(II) Complexes: Separation of Electronic and Steric Ligand Effects".Organometallics.15 (14):3124–3135.doi:10.1021/om960053i.
^abcEmsley, J. (2003)."Osmium".Nature's Building Blocks: An A-Z Guide to the Elements. Oxford, England, UK: Oxford University Press. pp. 199–201.ISBN978-0-19-850340-8.
^George, M. W. (2008)."Platinum-group metals"(PDF).U.S. Geological Survey Mineral Commodity Summaries.Archived(PDF) from the original on January 11, 2019. RetrievedSeptember 16, 2008.
^Singerling, S.A.; Schulte, R.F. (August 2021)."2018 Minerals Yearbook: Platinum-Group Metals [Advance Release]".Platinum-Group Metals Statistics and Information. U.S. Geological Survey. Archived from the original on July 14, 2023. RetrievedSeptember 24, 2023.{{cite web}}: CS1 maint: bot: original URL status unknown (link)
^Schulte, R.F."Mineral commodity summaries 2022 - Platinum-Group Metals".Platinum-Group Metals Statistics and Information. U.S. Geological Survey. Archived from the original on July 14, 2023. RetrievedSeptember 24, 2023.{{cite web}}: CS1 maint: bot: original URL status unknown (link)
^Kolb, H. C.; Van Nieuwenhze, M. S.; Sharpless, K. B. (1994). "Catalytic Asymmetric Dihydroxylation".Chemical Reviews.94 (8):2483–2547.doi:10.1021/cr00032a009.
^Bowers, B., B. (2001). "Scanning our past from London: the filament lamp and new materials".Proceedings of the IEEE.89 (3):413–415.doi:10.1109/5.915382.S2CID28155048.
^Gull, T. R.; Herzig, H.; Osantowski, J. F.; Toft, A. R. (1985). "Low earth orbit environmental effects on osmium and related optical thin-film coatings".Applied Optics.24 (16): 2660.Bibcode:1985ApOpt..24.2660G.doi:10.1364/AO.24.002660.PMID18223936.
^Linton, Roger C.; Kamenetzky, Rachel R.; Reynolds, John M.; Burris, Charles L. (1992). "LDEF experiment A0034: Atomic oxygen stimulated outgassing".NASA Langley Research Center: 763.Bibcode:1992ldef.symp..763L.
^Lebeau, Alex (March 20, 2015). "Platinum Group Elements: Palladium, Iridium, Osmium, Rhodium, and Ruthenium".Hamilton & Hardy's Industrial Toxicology. John Wiley & Sons, Inc. pp. 187–192.ISBN978-1-118-83401-5.
^"Osmium(VIII) oxide".CRC Handbook of Chemistry and Physics, 103rd Edition (Internet Version 2022). CRC Press/Taylor & Francis Group.Archived from the original on October 28, 2023. RetrievedFebruary 6, 2023.
^Luttrell, William E.; Giles, Cory B. (September 1, 2007). "Toxic tips: Osmium tetroxide".Journal of Chemical Health & Safety.14 (5):40–41.doi:10.1016/j.jchas.2007.07.003.
^Gadaskina, I. D."Osmium".ILO Encyclopaedia of Occupational Health and Safety.Archived from the original on November 3, 2023. RetrievedFebruary 6, 2023.
_NIST_ASD_emission_spectrum.png)
![Post-flight appearance of Os, Ag, and Au mirrors from the front (left images) and rear panels of the Space Shuttle. Blackening reveals oxidation due to irradiation by oxygen atoms.[76][77]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aNASAmirroroxidation.jpg)
