Osmium Weighs In

in your element Osmium weighs in Gregory Girolami recounts how element 76 beat a close competitor to the title of densest known metal and went on to participate in Nobel Prize-winning reactions. he road to the title of heavyweight elements to form alloys that, owing to their champion among the chemical hardness, find speciality uses as electrical Telements began with the work of contacts, wear-resistant machine parts, and English chemist Smithson Tennant (1761– tips for high-priced ink pens. An urban 1815). Tennant obtained a medical degree, legend is that osmium alloys are used in but after discovering that the sight of pain some surgical implants such as cardiac upset him too much, he abandoned the pacemakers and replacement heart valves, practice of medicine and decided to devote when in fact such alloys have never been his life to chemistry1,2. He also developed used in these devices. some unconventional laboratory habits; Although its metallic form has few for example, when a piece of linen was not applications, osmium’s chemical behaviour available for a filtration, he often used his PEN © ISTOCKPHOTO/THINKSTOCK is far from dull — it can exist in eleven handkerchief or cut off a piece of his shirt. different oxidation states, from –2 to +8; In 1800, Tennant co-founded a business the difficulty of ensuring that the samples a versatility matched only by its chemical to sell platinum metal and began to are free of internal voids. A better method cousin ruthenium and a few other transition prepare large quantities of it. He noticed, involves a calculation based on their crystal metals. Furthermore, the high +8 oxidation as had others before him, that extracting structures and atomic weights but, because state reached by osmium — along with platinum out of its crude ores with aqua the densities are so similar, refinements iron, ruthenium and xenon — is the highest regia — a mixture of concentrated nitric in these measurements have occasionally seen for any chemical element under and hydrochloric acids — left behind resulted in the champion and challenger normal circumstances. an insoluble black residue. In 1804, he reversing positions. The most useful osmium compound is announced his isolation of two new metals the tetroxide OsO4, a colourless solid used from the residue: iridium, named for “the as a stain in microscopy and in fingerprint striking variety of colours which it gives”, Osmium metal rivals diamond detection. Osmium tetroxide is also used and osmium, named after the Greek word as the least compressible of all as a catalyst in a few commercial processes, for ‘smell’, osme, for the “pungent and such as the manufacture of certain anti- peculiar smell of its oxide”. known substances. obesity and anti-diabetic drugs. These Osmium is the rarest of the stable processes are based on the Nobel Prize- elements: its average abundance in the Since its discovery, osmium has most winning work of chemist K. Barry Sharpless, Earth’s crust is about 1 gram per 200 tonnes. often held the title of densest metal, who used osmium tetroxide as a key Today, osmium is obtained commercially but for many decades in the twentieth component in a remarkable catalyst mixture as a by-product of refining nickel and some century it was iridium that occupied that that adds two hydroxyl (alcohol) groups to of the more common platinum-group position. More accurate studies carried out carbon–carbon double bonds4. metals. The back seat of an automobile could in the 1990s showed that osmium’s density So ‘heavy as lead’ turns out to be a rather easily accommodate the worldwide annual just slightly exceeds that of iridium3: the half-hearted way to describe something that production of osmium — approximately 500 current values, 22.587 (Os) and 22.562 (Ir) weighs a lot — but somehow I doubt that kilograms, which is about 5,000 times less g cm–3 at 20 °C differ by only 0.1%. ‘heavy as osmium’ will ever catch on. ❐ than its gold counterpart. Despite its rarity, osmium is currently Neighbours in the periodic table, far less expensive than gold, partly because GREGORY S. GIROLAMI is a Professor of osmium and iridium have long been known it has few commercial uses. Osmium metal Chemistry at the University of Illinois at to be the densest of all metals (both are — bluish-grey in colour, hard and brittle Urbana-Champaign and is the co-author over twice as dense as lead), but which is — lacks somewhat in visual and tactile of Synthesis and Technique in Inorganic the heavier of the two has been fiercely appeal. But element 76 does have some Chemistry, University Science Books, 2000. contested. Measuring their densities directly redeeming qualities. e-mail: [email protected] (by weighing samples of known volume) is Osmium metal strongly resists being not particularly accurate, owing in part to compressed under pressure, so much so that References it rivals diamond as the least compressible 1. McDonald, D. Notes Rec. R. Soc. Lond. 17, 77–94 (1962). 2. Griffith, W. P. Platinum Metals Rev. 48, 182-189 (2004). of all known substances. Osmium can 3. Arblaster, J. W. Platinum Metals Rev. 39, 164 (1995). be mixed with other platinum-group 4. Sharpless, K. B. Angew. Chem. Int. Ed. 41, 2024–2032 (2002). Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Ac Th Pa U Np 954 NATURE CHEMISTRY | VOL 4 | NOVEMBER 2012 | www.nature.com/naturechemistry © 2012 Macmillan Publishers Limited. All rights reserved.

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