Editing Osmium (section)

== Characteristics ==
=== Physical properties ===
[[File:Osmium 1-crop.jpg|thumb|left|upright|Osmium, remelted pellet]]

Osmium is a hard, brittle, blue-gray metal, and the densest [[stable element]]—about twice as dense as [[lead]]. The density of osmium is slightly greater than that of [[iridium]]; the two are so similar (22.587  versus {{val|22.562|ul=g/cm3}} at 20&nbsp;°C) that each was at one time considered to be the densest element. Only in the 1990s were measurements made accurately enough (by means of [[X-ray crystallography]]) to be certain that osmium is the denser of the two.<ref name="Arblaster1995">{{cite journal|title=Osmium, the Densest Metal Known|author=Arblaster, J. W.|journal=[[Platinum Metals Review]]|volume=39|issue=4|year=1995|page=164|doi=10.1595/003214095X394164164 |url=https://technology.matthey.com/article/39/4/164-164/|access-date=November 11, 2023|archive-url=https://web.archive.org/web/20230506024553/https://technology.matthey.com/article/39/4/164-164|archive-date=May 6, 2023|url-status=live}}</ref><ref>{{cite journal |last1=Girolami |first1=Gregory |title=Osmium weighs in |journal=Nature Chemistry |date=November 2012 |volume=4 |issue=11 |pages=954 |doi=10.1038/nchem.1479|doi-access=free |pmid=23089872 |bibcode=2012NatCh...4..954G }}</ref>

Osmium has a blue-gray tint.{{sfn|Haynes|2011|p=4.25}} The [[reflectivity]] of single crystals of osmium is complex and strongly direction-dependent, with light in the red and near-infrared wavelengths being more strongly absorbed when [[polarization (waves)|polarized]] parallel to the ''c'' crystal axis than when polarized perpendicular to the ''c'' axis; the ''c''-parallel polarization is also slightly more reflected in the mid-ultraviolet range. Reflectivity reaches a sharp minimum at around 1.5&nbsp;eV (near-infrared) for the ''c''-parallel polarization and at 2.0&nbsp;eV (orange) for the ''c''-perpendicular polarization, and peaks for both in the visible spectrum at around 3.0&nbsp;eV (blue-violet).<ref>{{cite journal |last1=Nemoshkalenko |first1=V. V. |last2=Antonov |first2=V. N. |last3=Kirillova |first3=M. M. |last4=Krasovskii |first4=A. E. |last5=Nomerovannaya |first5=L. V. |title=The structure of the energy bands and optical absorption in osmium |journal=Sov. Phys. JETP |date=January 1986 |volume=63 |issue=I |page=115 |bibcode=1986JETP...63..115N |url=http://www.jetp.ras.ru/cgi-bin/dn/e_063_01_0115.pdf |access-date=28 December 2022 |archive-date=March 11, 2023 |archive-url=https://web.archive.org/web/20230311062236/http://www.jetp.ras.ru/cgi-bin/dn/e_063_01_0115.pdf |url-status=live }}</ref>

Osmium is a hard but brittle [[metal]] that remains [[lustrous]] even at high temperatures. It has a very low [[compressibility]]. Correspondingly, its [[bulk modulus]] is extremely high, reported between {{val|395}} and {{val|462|ul=GPa}}, which rivals that of [[diamond]] ({{val|443|u=GPa}}). The hardness of osmium is moderately high at {{val|4|u=GPa}}.<ref>{{cite journal|title=Osmium Metal Studied under High Pressure and Nonhydrostatic Stress|journal=Phys. Rev. Lett.|volume=100|issue=4|page=045506|date=2008|doi=10.1103/PhysRevLett.100.045506|pmid=18352299|bibcode=2008PhRvL.100d5506W|last1=Weinberger|first1=Michelle|last2=Tolbert|first2=Sarah|last3=Kavner|first3=Abby|s2cid=29146762}}</ref><ref>{{cite journal|first=Hyunchae|last=Cynn|author2=Klepeis, J. E.|author3=Yeo, C. S.|author4=Young, D. A.|title=Osmium has the Lowest Experimentally Determined Compressibility|journal=Physical Review Letters|volume=88|issue=13|date=2002|doi=10.1103/PhysRevLett.88.135701|page=135701|pmid=11955108|bibcode=2002PhRvL..88m5701C|url=https://zenodo.org/record/1233939|access-date=August 27, 2019|archive-date=September 28, 2023|archive-url=https://web.archive.org/web/20230928093417/https://zenodo.org/record/1233939|url-status=live}}</ref><ref>{{cite journal|first=B. R.|last=Sahu|author2=Kleinman, L.|title=Osmium Is Not Harder Than Diamond|journal=Physical Review B|volume=72|date=2005|issue=11|doi=10.1103/PhysRevB.72.113106|page=113106|bibcode=2005PhRvB..72k3106S }}</ref> Because of its [[hardness]], brittleness, low [[vapor pressure]] (the lowest of the platinum-group metals), and very high [[melting point]] (the [[List of elements by melting point|fourth highest]] of all elements, after [[carbon]], [[tungsten]], and [[rhenium]]), solid osmium is difficult to machine, form, or work.

=== Chemical properties ===
{{Main article|Osmium compounds}}
<div style="float:right; margin:5px;">
{|class="wikitable"
|-
! colspan=2|Oxidation states of osmium
|-
| −4 || [OsIn<sub>6−''x''</sub>Sn<sub>''x''</sub>]<ref name="MetalAnions">Fe(−4), Ru(−4), and Os(−4) have been observed in metal-rich compounds containing octahedral complexes [MIn<sub>6−''x''</sub>Sn<sub>''x''</sub>]; Pt(−3) (as a dimeric anion [Pt–Pt]<sup>6−</sup>), 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]<sup>2−</sup> for Zn, Cd, Hg, but later shown to be [T–T]<sup>4−</sup> for all these elements) in La<sub>2</sub>Pt<sub>2</sub>In, La<sub>2</sub>Cu<sub>2</sub>In, Ca<sub>5</sub>Au<sub>3</sub>, Ca<sub>5</sub>Ag<sub>3</sub>, Ca<sub>5</sub>Hg<sub>3</sub>, Sr<sub>5</sub>Cd<sub>3</sub>, Ca<sub>5</sub>Zn<sub>3</sub>(structure (AE<sup>2+</sup>)<sub>5</sub>(T–T)<sup>4−</sup>T<sup>2−</sup>⋅4e<sup>−</sup>), Yb<sub>3</sub>Ag<sub>2</sub>, Ca<sub>5</sub>Au<sub>4</sub>, and Ca<sub>3</sub>Hg<sub>2</sub>; Au(–3) has been observed in ScAuSn and in other 18-electron half-Heusler compounds. See {{cite journal|title=Late transition metal anions acting as p-metal elements|year=2008|author1=Changhoon Lee|author2=Myung-Hwan Whangbo|volume=10|issue=4|pages=444–449|journal=Solid State Sciences|doi=10.1016/j.solidstatesciences.2007.12.001|bibcode=2008SSSci..10..444K}} and {{cite journal|doi=10.1002/zaac.200900421|title=Analysis of Electronic Structures and Chemical Bonding of Metal-rich Compounds. 2. Presence of Dimer (T–T)<sup>4–</sup> and Isolated T<sup>2–</sup> Anions in the Polar Intermetallic Cr<sub>5</sub>B<sub>3</sub>-Type Compounds AE<sub>5</sub>T<sub>3</sub> (AE = Ca, Sr; T = Au, Ag, Hg, Cd, Zn)|year=2010|author1=Changhoon Lee|author2=Myung-Hwan Whangbo|author3=Jürgen Köhler|volume=636|issue=1|pages=36–40|journal=Zeitschrift für Anorganische und Allgemeine Chemie}}</ref>
|-
| −2 || {{chem|Na|2|[Os(CO)|4|]}}
|-
| −1 || {{chem|Na|2|[Os|4|(CO)|13|]}}
|-
| 0 ||[[Triosmium dodecacarbonyl|{{chem|Os|3|(CO)|12|}}]]
|-
| +1 ||{{chem|OsI}}
|-
| '''+2''' ||{{chem|OsI|2}}
|-
| '''+3''' || {{chem| OsBr|3|}}
|-
| '''+4''' || [[Osmium dioxide|{{chem|OsO|2}}]], [[Osmium(IV) chloride|{{chem|OsCl|4}}]]
|-
| +5 ||{{chem|OsF|5}}
|-
| +6 ||[[Osmium hexafluoride|{{chem|OsF|6}}]]
|-
| +7 ||{{chem|OsOF|5}}
|-
| '''+8''' ||[[Osmium tetroxide|{{chem|OsO|4}}]], {{chem|Os|(|NCH|3|)|4}}
|}</div>

Osmium forms compounds with [[oxidation state]]s 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<ref>{{cite web|url=http://www.rsc.org/chemistryworld/2014/10/iridium-oxide-cation-oxidation-state-9|title=Iridium forms compound in +9 oxidation state|author=Stoye, Emma|work=Chemistry World|date=23 October 2014|publisher=[[Royal Society of Chemistry]]|access-date=December 19, 2014|archive-date=August 9, 2016|archive-url=https://web.archive.org/web/20160809143724/http://www.rsc.org/chemistryworld/2014/10/iridium-oxide-cation-oxidation-state-9|url-status=live}}</ref> and is encountered only in [[xenon]],<ref name="selig">{{cite journal|title=Xenon tetroxide – Preparation + Some Properties|journal=Science| date=1964 |volume=143|pages=1322–1323| doi=10.1126/science.143.3612.1322|pmid=17799234|issue=3612|jstor=1713238|bibcode=1964Sci...143.1322S|last1=Selig|first1=H.|display-authors=4|last2=Claassen|first2=H. H.|last3=Chernick|first3=C. L.|last4=Malm|first4=J. G.|last5=Huston|first5=J. L.|s2cid=29205117}}</ref><ref>{{cite journal|title=Xenon tetroxide – Mass Spectrum|journal=Science|date=1964|volume=143|pages=1162–1163|doi=10.1126/science.143.3611.1161-a|pmid=17833897|issue=3611|jstor=1712675|bibcode=1964Sci...143.1161H|last1=Huston|first1=J. L.|last2=Studier|first2=M. H.|last3=Sloth|first3=E. N.|s2cid=28547895}}</ref> [[ruthenium]],<ref>{{cite journal|doi=10.1595/147106704X10801|title=Oxidation States of Ruthenium and Osmium|date=2004|author=Barnard, C. F. J.|journal=Platinum Metals Review|volume=48|issue=4|page=157|doi-access=free}}</ref> [[hassium]],<ref>{{cite web|url=http://www.gsi.de/documents/DOC-2003-Jun-29-2.pdf|archive-url=https://web.archive.org/web/20120114084650/http://www.gsi.de/documents/DOC-2003-Jun-29-2.pdf|url-status=dead|archive-date=2012-01-14|title=Chemistry of Hassium|access-date=2007-01-31|date=2002|work=Gesellschaft für Schwerionenforschung mbH}}</ref> [[iridium]],<ref>{{cite journal|doi=10.1002/anie.200902733|pmid=19593837|title=Formation and Characterization of the Iridium Tetroxide Molecule with Iridium in the Oxidation State +VIII|date=2009|last1=Gong|first1=Yu|last2=Zhou|first2=Mingfei|last3=Kaupp|first3=Martin|last4=Riedel|first4=Sebastian|journal=Angewandte Chemie International Edition|volume=48|issue=42|pages=7879–7883}}{{dead link|date=June 2019}}</ref> and [[plutonium]].<ref>{{cite journal |last1=Kiselev |first1=Yu. M. |last2=Nikonov |first2=M. V. |last3=Dolzhenko |first3=V. D. |last4=Ermilov |first4=A. Yu. |last5=Tananaev |first5=I. G. |last6=Myasoedov |first6=B. F. |title=On existence and properties of plutonium(VIII) derivatives |journal=Radiochimica Acta |date=17 January 2014 |volume=102 |issue=3 |pages=227–237 |doi=10.1515/ract-2014-2146|s2cid=100915090 }}</ref><ref>{{cite journal |last1=Zaitsevskii |first1=Andréi |last2=Mosyagin |first2=Nikolai S. |last3=Titov |first3=Anatoly V. |last4=Kiselev |first4=Yuri M. |title=Relativistic density functional theory modeling of plutonium and americium higher oxide molecules |journal=The Journal of Chemical Physics |date=21 July 2013 |volume=139 |issue=3 |pages=034307 |doi=10.1063/1.4813284 |pmid=23883027 |bibcode=2013JChPh.139c4307Z }}</ref> The oxidation states −1 and −2 represented by the two reactive compounds {{chem|Na|2|[Os|4|(CO)|13|]}} and {{chem|Na|2|[Os(CO)|4|]}} are used in the synthesis of osmium [[Cluster chemistry#Transition metal carbonyl clusters|cluster compounds]].<ref>{{cite journal|doi=10.1016/0022-328X(93)83250-Y|title=Preparation of [Os<sub>3</sub>(CO)<sub>11</sub>]<sup>2−</sup> and its reactions with Os<sub>3</sub>(CO)<sub>12</sub>; structures of [Et<sub>4</sub>N] [HOs<sub>3</sub>(CO)<sub>11</sub>] and H<sub>2</sub>OsS<sub>4</sub>(CO)|date=1993|last1 =Krause|first1=J.|journal=Journal of Organometallic Chemistry|volume=454|issue=1–2|pages=263–271|display-authors=4|last2=Siriwardane|first2=Upali|last3=Salupo|first3=Terese A.|last4=Wermer|first4=Joseph R.|last5=Knoeppel|first5=David W.|last6=Shore|first6=Sheldon G.}}</ref><ref>{{cite journal|doi=10.1021/ic00141a019|title=Mononuclear hydrido alkyl carbonyl complexes of osmium and their polynuclear derivatives|date=1982|first=Willie J.|last=Carter|display-authors=4|author2=Kelland, John W. |author3=Okrasinski, Stanley J. |author4=Warner, Keith E. |author5= Norton, Jack R. | journal=Inorganic Chemistry|volume=21|issue=11|pages=3955–3960}}</ref>

[[File:Osmiumtetroxide1.jpg|thumb|left|Osmium tetroxide ({{chem2|OsO4}})]]

The most common compound exhibiting the +8 oxidation state is [[osmium tetroxide]] ({{chem2|OsO4}}). It is a very volatile, water-soluble, pale yellow, crystalline solid. This toxic compound is formed when powdered osmium is exposed to air, so osmium powder has the "pronounced and nauseating" smell of osmium tetroxide.<ref name="mager">{{cite book| last = Mager Stellman| first = J.| title = Encyclopaedia of Occupational Health and Safety| chapter-url = https://books.google.com/books?id=nDhpLa1rl44C| date = 1998| publisher = International Labour Organization| isbn = 978-92-2-109816-4| oclc = 35279504| pages = [https://archive.org/details/encyclopaediaofo0003unse/page/63 63.34]| chapter = Osmium| url = https://archive.org/details/encyclopaediaofo0003unse/page/63}}</ref> Osmium tetroxide forms red osmates {{chem|OsO|4|(OH)|2|2-}} upon reaction with a base. With [[ammonia]], it forms the nitrido-osmates {{chem|OsO|3|N|-}}.<ref name="Holle">{{cite book| author2 = Wiberg, E.| author3 = Wiberg, N.| last = Holleman| first = A. F.| title = Inorganic Chemistry| edition = 1st| date = 2001| publisher = Academic Press| isbn = 978-0-12-352651-9| oclc = 47901436 }}</ref><ref name="Griffith">{{cite journal|journal=Quarterly Reviews, Chemical Society|date=1965|volume=19|issue=3|pages=254–273|doi=10.1039/QR9651900254|title=Osmium and its compounds|first=W. P.|last=Griffith}}</ref><ref>{{cite book| author = ((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)) | title = Platinum-group metals| url = https://books.google.com/books?id=yEcrAAAAYAAJ| date = 1977| publisher = National Academy of Sciences| isbn = 978-0-309-02640-6| page = 55 }}</ref> Osmium tetroxide boils at 130&nbsp;°[[Celsius|C]] and is a powerful [[Oxidizer|oxidizing]] agent. By contrast, [[osmium dioxide]] ({{chem|Os|O|2}}) is black, non-volatile, and much less reactive and toxic.

Only two osmium compounds have major applications: osmium tetroxide for [[staining]] tissue in [[electron microscopy]] and for the oxidation of [[alkenes]] in [[organic synthesis]], and the non-volatile osmates for [[Sharpless asymmetric dihydroxylation|organic oxidation reactions]].<ref name="Bozzola" />

Osmium pentafluoride ({{chem|Os|F|5}}) is known, but osmium trifluoride ({{chem|Os|F|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 as [[triosmium dodecacarbonyl]] ({{chem|Os|3|(CO)|12}}), represent oxidation state 0.<ref name="Holle" /><ref name="Griffith" /><ref name="greenwood">{{cite book |editor1-last=Greenwood |editor1-first=N.N. |editor2-last=Earnshaw |editor2-first=A. |title=Chemistry of the Elements |date=1997 |publisher=Butterworth-Heinemann |isbn=9780750633659 |pages=1070–1112 |edition=2 |url=https://doi.org/10.1016/B978-0-7506-3365-9.50031-6 |chapter=25 - Iron, Ruthenium and Osmium|doi=10.1016/B978-0-7506-3365-9.50031-6 }}</ref><ref>{{cite journal|title=The chemistry of ruthenium, osmium, rhodium, iridium, palladium, and platinum in the higher oxidation states|journal=Coordination Chemistry Reviews|volume=46|date=1982|pages=1–127|author=Gulliver, D. J|author2=Levason, W.|doi=10.1016/0010-8545(82)85001-7}}</ref>

In general, the lower oxidation states of osmium are stabilized by [[ligand]]s that are good σ-donors (such as [[amines]]) and π-acceptors ([[heterocyclic compound|heterocycles]] containing [[nitrogen]]). The higher oxidation states are stabilized by strong σ- and π-donors, such as {{chem|O|2-}} and {{chem|N|3-}}.<ref>{{cite book| author = Sykes, A. G. | title = Advances in Inorganic Chemistry| url = https://archive.org/details/advancesinorgani39syke | url-access = limited | date = 1992| publisher = Academic Press| isbn = 978-0-12-023637-4| page = [https://archive.org/details/advancesinorgani39syke/page/n227 221]}}</ref>

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 including [[aqua regia]], but is attacked by {{chem2|F2}} and {{chem2|Cl2}} at high temperatures, and by hot concentrated nitric acid to produce {{chem2|OsO4}}. It can be dissolved by molten alkalis fused with an oxidizer such as [[sodium peroxide]] ({{chem2|Na2O2}}) or [[potassium chlorate]] ({{chem2|KClO3}}) to give osmates such as [[potassium osmate|{{chem2|K2[OsO2(OH)4]}}]].<ref name="greenwood" />

=== Isotopes ===
{{Main|Isotopes of osmium}}
Osmium has seven naturally occurring [[isotope]]s, five of which are stable: {{chem|187|Os}}, {{chem|188|Os}}, {{chem|189|Os}}, {{chem|190|Os}}, and (most abundant) {{chem|192|Os}}. At least 37 artificial radioisotopes and 20 [[nuclear isomer]]s exist, with mass numbers ranging from 160 to 203; the most stable of these is {{chem|194|Os}} with a half-life of 6 years.<ref name="nubase">{{NUBASE2020}}</ref>

{{chem|186|Os}} undergoes [[alpha decay]] with such a long [[half-life]] {{val|2.0e15|1.1}} years, approximately {{val|140000}} times the [[age of the universe]], that for practical purposes it can be considered stable. {{chem|184|Os}} is also known to undergo alpha decay with a half-life of {{val|1.12e13|0.23}} years.<ref name="184Os"/> 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 that {{chem|184|Os}} and {{chem|192|Os}} can undergo [[double beta decay]], but this radioactivity has not been observed yet.<ref name="nubase"/>

<sup>189</sup>Os has a spin of 5/2 but <sup>187</sup>Os 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 for [[NMR spectroscopy]].<ref>{{cite journal|doi=10.1021/om960053i |title=<sup>187</sup>Os NMR Study of (η<sup>6</sup>-Arene)osmium(II) Complexes: Separation of Electronic and Steric Ligand Effects |date=1996 |last1=Bell |first1=Andrew G. |last2=Koźmiński |first2=Wiktor |last3=Linden |first3=Anthony |last4=von Philipsborn |first4=Wolfgang |journal=Organometallics |volume=15 |issue=14 |pages=3124–3135 }}</ref>

{{chem|187|Os}} is the descendant of {{chem|187|[[rhenium|Re]]}} (half-life {{val|4.56|e=10|u=years}}) and is used extensively in dating terrestrial as well as [[meteorite|meteoric]] [[rock (geology)|rocks]] (see ''[[Rhenium–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 the [[Earth's mantle|mantle]] roots of continental [[craton]]s. This decay is a reason why rhenium-rich minerals are abnormally rich in {{chem|187|Os}}.<ref>{{cite journal|first=Józef|last=Dąbek|author2=Halas, Stanislaw|title=Physical Foundations of Rhenium-Osmium Method – A Review|journal=Geochronometria|volume=27|date=2007|issue=1 |doi=10.2478/v10003-007-0011-4|pages=23–26|bibcode=2007Gchrm..27...23D |doi-access=free}}</ref> However, the most notable application of osmium isotopes in geology has been in conjunction with the abundance of iridium, to characterise the layer of [[shocked quartz]] along the [[Cretaceous–Paleogene boundary]] that marks the extinction of the non-avian [[dinosaur]]s 65 million years ago.<ref name="Alvarez">{{cite journal|title=Extraterrestrial cause for the Cretaceous–Tertiary extinction|author=Alvarez, L. W.|author-link=Luis Walter Alvarez|author2=Alvarez, W.|author3=Asaro, F.|author4=Michel, H. V.|date=1980|journal=Science|volume=208|issue=4448|pages=1095–1108|doi=10.1126/science.208.4448.1095|pmid=17783054|bibcode=1980Sci...208.1095A|url=http://earthscience.rice.edu/wp-content/uploads/2015/11/Alvarez_K-Timpact_Science80.pdf|citeseerx=10.1.1.126.8496|s2cid=16017767|access-date=November 2, 2017|archive-date=May 21, 2023|archive-url=https://web.archive.org/web/20230521231012/https://earthscience.rice.edu/wp-content/uploads/2015/11/Alvarez_K-Timpact_Science80.pdf|url-status=live}}</ref>