Osmium

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Osmium, 76Os
Osmium
Pronunciation/ˈɒzmiəm/ ​(OZ-mee-əm)
Appearancesilvery, blue cast
Standard atomic weight Ar°(Os)
Osmium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Ru
↑
Os
↓
Hs
rhenium ← osmium → iridium
kJ/mol
Heat of vaporization378 kJ/mol
Molar heat capacity24.7 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 3160 3423 3751 4148 4638 5256
Atomic properties
Discovery and first isolation
Smithson Tennant (1803)
Isotopes of osmium
Main isotopes[7] Decay
abun­dance half-life (t1/2) mode pro­duct
184Os 0.02% 1.12×1013 y[8] α
180W
185Os synth 92.95 d ε
185Re
186Os 1.59% 2.0×1015 y α
182W
187Os 1.96%
stable
188Os 13.2% stable
189Os 16.1% stable
190Os 26.3% stable
191Os synth 14.99 d
β−
191Ir
192Os 40.8% stable
193Os synth 29.83 h β−
193Ir
194Os synth 6 y β−
194Ir
 Category: Osmium
| references

Osmium (from

electrical contacts, and in other applications that require extreme durability and hardness.[10]

Osmium is among the rarest elements in the Earth's crust, making up only 50 parts per trillion (ppt).[11][12]

Characteristics

Physical properties

Osmium, remelted pellet

Osmium is the densest

stable element. Reports conflicted on which of osmium or iridium is denser;[10] as of 1995, calculations of density from the X-ray crystallography data gives a value of 22.587±0.009 g/cm3 for osmium, slightly denser than the 22.562±0.009 g/cm3 of iridium.[9] Both metals are nearly 23 times as dense as water, twice as lead, and 1+1⁄6 times as dense as gold
.

Osmium has a blue-gray tint.

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 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 eV (near-infrared) for the c-parallel polarization and at 2.0 eV (orange) for the c-perpendicular polarization, and peaks for both in the visible spectrum at around 3.0 eV (blue-violet).[13]

Osmium is a hard but brittle

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
Oxidation states of osmium
−4 [OsIn6−xSnx][17]
−2 Na
2[Os(CO)
4]
−1 Na
2[Os
4(CO)
13]
0 Os
3(CO)
12
+1 OsI
+2 OsI
2
+3 OsBr
3
+4 OsO
2, OsCl
4
+5 OsF
5
+6 OsF
6
+7 OsOF
5
+8 OsO
4, Os(NCH
3)
4

Osmium forms compounds with

cluster compounds.[26][27]

Osmium tetroxide (OsO4)

The most common compound exhibiting the +8 oxidation state is

oxidizing agent. By contrast, osmium dioxide
(OsO
2) is black, non-volatile, and much less reactive and toxic.

Only two osmium compounds have major applications: osmium tetroxide for

alkenes in organic synthesis, and the non-volatile osmates for organic oxidation reactions.[32]

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 as triosmium dodecacarbonyl (Os
3(CO)
12), represent oxidation state 0.[29][30][33][34]

In general, the lower oxidation states of osmium are stabilized by

amines) and π-acceptors (heterocycles containing nitrogen). The higher oxidation states are stabilized by strong σ- and π-donors, such as O2−
 and N3−
.[35]

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 F2 and Cl2 at high temperatures, and by hot concentrated nitric acid to produce OsO4. It can be dissolved by molten alkalis fused with an oxidizer such as sodium peroxide (Na2O2) or potassium chlorate (KClO3) to give osmates such as K2[OsO2(OH)4].[33]

Isotopes

Main article: Isotopes of osmium

Osmium has seven naturally occurring isotopes, five of which are stable: 187
Os, 188
Os, 189
Os, 190
Os, and (most abundant) 192
Os. At least 37 artificial radioisotopes and 20 nuclear isomers exist, with mass numbers ranging from 160 to 203; the most stable of these is 194
Os with a half-life of 6 years.[36]

186
Os undergoes alpha decay with such a long half-life (2.0±1.1)×1015 years, approximately 140000 times the age 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.[8] 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 184
Os and 192
Os can undergo double beta decay, but this radioactivity has not been observed yet.[36]

189Os has a spin of 5/2 but 187Os 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.[37]

187
Os is the descendant of 187

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 mantle roots of continental cratons. This decay is a reason why rhenium-rich minerals are abnormally rich in 187
Os.[38] 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 dinosaurs 65 million years ago.[39]

History

Osmium was discovered in 1803 by Smithson Tennant and William Hyde Wollaston in London, England.[40] The discovery of osmium is intertwined with that of platinum and the other metals of the platinum group. Platinum reached Europe as platina ("small silver"), first encountered in the late 17th century in silver mines around the Chocó Department, in Colombia.[41] The discovery that this metal was not an alloy, but a distinct new element, was published in 1748.[42] Chemists who studied platinum dissolved it in

Antoine François, comte de Fourcroy, and Louis Nicolas Vauquelin also observed iridium in the black platinum residue in 1803, but did not obtain enough material for further experiments.[43] Later the two French chemists Fourcroy and Vauquelin identified a metal in a platinum residue they called ptène.[44]

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[45] and obtained a volatile new oxide, which he believed was of this new metal—which he named ptene, from the Greek word πτηνος (ptènos) for winged.[46][47] 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.[43][45] He obtained a yellow solution (probably of cis–[Os(OH)2O4]2−) by reactions with sodium hydroxide at red heat. After acidification he was able to distill the formed OsO4.[46] He named it osmium after Greek osme meaning "a smell", because of the chlorine-like and slightly garlic-like smell of the volatile osmium tetroxide.[48] Discovery of the new elements was documented in a letter to the Royal Society on June 21, 1804.[43][49]

catalysts in the Haber process, the nitrogen fixation reaction of nitrogen and hydrogen to produce ammonia, giving enough yield to make the process economically successful. At the time, a group at BASF led by Carl 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.[50]

Osmium is now obtained primarily from the processing of platinum and nickel ores.[51]

Occurrence

Native platinum containing traces of the other platinum group metals

Osmium is one of the least abundant stable elements in Earth's crust, with an average mass fraction of 50 parts per trillion in the continental crust.[52]

Osmium is found in nature as an uncombined element or in natural

iridosmium (osmium rich).[45] In nickel and copper deposits, the platinum-group metals occur as sulfides (i.e., (Pt,Pd)S), tellurides (e.g., PtBiTe), antimonides (e.g., PdSb), and arsenides (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 or copper.[53]

Within Earth's crust, osmium, like iridium, is found at highest concentrations in three types of geologic structure: igneous deposits (crustal intrusions from below),

pre-Columbian people in the Chocó Department, Colombia, are still a source for platinum-group metals. The second large alluvial deposit was found in the Ural Mountains, Russia, which is still mined.[51][55]

Production

Osmium crystals, grown by chemical vapor transport

Osmium is obtained commercially as a by-product from

electrorefining of copper and nickel, noble metals such as silver, gold and the platinum-group metals, together with non-metallic elements such as selenium and tellurium, settle to the bottom of the cell as anode mud, which forms the starting material for their extraction.[56][57] 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 with sodium peroxide followed by dissolution in aqua regia, and dissolution in a mixture of chlorine with hydrochloric acid.[54][58] 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 molten sodium bisulfate. The insoluble residue, containing ruthenium, osmium, and iridium, is treated with sodium oxide
, in which Ir is insoluble, producing water-soluble ruthenium and osmium salts. After oxidation to the volatile oxides, RuO
4 is separated from OsO
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.

sponge that can be treated using powder metallurgy techniques.[60]

Estimates of annual worldwide osmium production are on the order of several hundred to a few thousand kilograms.[61][33] 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.[33] 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.[62][63]

Applications

Because osmium is virtually unforgeable when fully dense and very fragile when sintered, it is rarely used in its pure state, but is instead often alloyed with other metals for high-wear applications. Osmium alloys such as osmiridium are very hard and, along with other platinum-group metals, are used in the tips of fountain pens, instrument pivots, and electrical contacts, as they can resist wear from frequent operation. They were also used for the tips of phonograph 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 and diamond tips, so they were discontinued.[64]

electron microscopy. As a strong oxidant, it cross-links lipids mainly by reacting with unsaturated carbon–carbon bonds and thereby both fixes biological membranes in place in tissue samples and simultaneously stains them. Because osmium atoms are extremely electron-dense, osmium staining greatly enhances image contrast in transmission electron microscopy (TEM) studies of biological materials. Those carbon materials otherwise have very weak TEM contrast.[32] Another osmium compound, osmium ferricyanide (OsFeCN), exhibits similar fixing and staining action.[66]

The tetroxide and its derivative potassium osmate are important oxidants in organic synthesis. For the Sharpless asymmetric dihydroxylation, which uses osmate for the conversion of a double bond into a vicinal diol, Karl Barry Sharpless was awarded the Nobel Prize in Chemistry in 2001.[67][68] OsO4 is very expensive for this use, so KMnO4 is often used instead, even though the yields are less for this cheaper chemical reagent.

In 1898, the Austrian chemist

incandescent lamps.[46]

The light bulb manufacturer Osram (founded in 1906, when three German companies, Auer-Gesellschaft, AEG and Siemens & Halske, combined their lamp production facilities) derived its name from the elements of osmium and Wolfram (the latter is German for tungsten).[69]

Like palladium, 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.[70]

Osmium has high

UV spectrometers, which have reduced mirror sizes due to space limitations. Osmium-coated mirrors were flown in several space missions aboard the Space Shuttle, but it soon became clear that the oxygen radicals in low Earth orbit are abundant enough to significantly deteriorate the osmium layer.[72]

  • The Sharpless dihydroxylation: RL = largest substituent; RM = medium-sized substituent; RS = smallest substituent
    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.[73][74]
    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.[73][74]

Precautions

The primary hazard of metallic osmium is the potential formation of osmium tetroxide (OsO4), which is volatile and very poisonous.[75] This reaction is thermodynamically favorable at room temperature,[76] but the rate depends on temperature and the surface area of the metal.[77][78] As a result, bulk material is not considered hazardous[77][79][80][81] while powders react quickly enough that samples can sometimes smell like OsO4 if they are handled in air.[33][82]

Price

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.[83] Because osmium has few commercial applications, it is not heavily traded and prices are seldom reported.[83]

Notes

  1. ^ The thermal expansion of Os is anisotropic: 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.

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Cited sources

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