Iridium's Impact

in your element Iridium’s impact David Payne relates iridium’s role in two very different ages.

he story of iridium is both modern and have been predicted to display exotic prehistoric. It was discovered along magnetic and electronic physics4. with osmium in 1803 by the British Recently element 77 had also made the T 5 chemist Smithson Tennant, born in 1761 in news in chemistry through the isolated + Selby, Yorkshire. Tennant studied medicine compound [IrO4] , which features iridium but after graduating found it unsuitable for in the highest oxidation state observed so his temperament, and focused on chemistry far, +9. This makes iridium the element that instead. In 1785, despite not having yet displays the widest range of oxidation states published any scientific papers, he was (from –3 to +9). elected a Fellow of the Royal Society at the Its modern developments are young age of 23. Although he published few counterbalanced by the (literal) impact of papers during his career, they were of great iridium in the much older history of our significance — he demonstrated for example planet: one of the great global catastrophic that diamond consisted solely of carbon1. events. Sixty five million years ago, the Tennant discovered elements 76 and Cretaceous–Paleogene (K–Pg) extinction 77, osmium and iridium respectively, by event occurred, and marked the loss of © 2007 LYNN A. MACINTYRE LYNN © 2007 collecting insoluble black impurities derived 70% of the plant and animal species on from the treatment of platinum ore with Earth including all non-avian dinosaurs. aqua regia. He treated them with caustic than gold. It is thought to have sunk to The geological record is marked by the soda before carrying out an extraction step the Earth’s core during the formation of K–Pg boundary, a thin layer of sediment, with hydrochloric acid, which was repeated the planet, owing to its siderophilic (iron- which was found to contain very high a number of times. From this he obtained loving) nature. Only three tonnes are levels of iridium — much higher than the

red crystals, most probably Na2[IrCl6]·nH2O, produced annually, mainly as a by-product natural abundance in the Earth’s crust. that when heated gave a white powder, of the electrorefining process of copper Asteroids are notably rich in iridium, and which Tennant stated was “not capable of and nickel, therefore making iridium an Luis Alvarez and colleagues6 postulated being melted, by any degree of heat I could expensive commodity. that the K–Pg extinction was caused by an apply”2. He named the element iridium Despite its rarity, iridium has found asteroid impact; subsequently a suitably after Iris, messenger of the gods in Greek uses in technology — albeit limited sized crater was discovered in the Yucatán mythology and the personification of the ones. Its high resistance to corrosion, Peninsula, Mexico. The theory that the rainbow, owing to, in his own words, the including under high temperatures, extinction of the dinosaurs was caused by “striking variety of colours it gives” when the makes it a desirable constituent in spark an iridium-rich asteroid strike is further metal is dissolved in hydrochloric acid2,3. plugs and engine parts, for example, backed up by the fact that no dinosaur in aviation applications. Iridium is fossils are ever found above the K–Pg also commonly found in homogenous boundary. A significant amount of iridium Insulating states of iridium catalysis, with organoiridium compounds present in the Earth’s crust today came oxides are predicted to display finding use in the carbonylation of from this asteroid strike. exotic physics. methanol to acetic acid. Vaska’s complex, Element 77, which participated in the end IrCl(CO)[P(C6H5)3]2, used in oxidative of one age, finds applications and still holds addition reactions, has the unusual surprises in the modern age. ❐ The physical properties of iridium were property of being able to reversibly bind apparent from the moment of its discovery diatomic oxygen. DAVID PAYNE is in the Department with a very high melting point (2,447 °C), The oxides of iridium have also attracted of Materials, Imperial College London, hardness (1,760 MPa) and density great interest in the last 10–15 years. As Exhibition Road, London SW7 2AZ, UK. (22.56 g cm–3) — over twice that of lead a heavy transition metal with partially e-mail: [email protected] and only just beaten by osmium. It is one of filled d5 orbitals, the oxides would be References the least abundant elements in the Earth’s expected to display metallic conductivity, 1. Tennant, S. Phil. Trans. 87, 123–127 (1797). crust — about forty times less abundant yet relativistic effects (spin–orbit coupling 2. Tennant, S. Phil. Trans. 94, 411–418 (1804). to be precise) on the electronic structure, 3. Griffiths, W. P. Platinum Metal Rev. 48, 182–189 (2004). 4. Zhao, L. et al. Nature Phys. 12, 32–36 (2016). drive a number of the oxides into an 5. Wang, G. et al. Nature 514, 475–477 (2014). insulating state. It is these insulators that 6. Alvarez, L. W. et al. Science 208, 1095–1108 (1980). W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Ac Th Pa U Np Pu 392 NATURE CHEMISTRY | VOL 8 | APRIL 2016 | www.nature.com/naturechemistry