in your element The riches of uranium Uranium is best known, and feared, for its involvement in nuclear energy. Marisa J. Monreal and Paula L. Diaconescu take a look at how its unique combination of properties is now increasingly attracting the attention of chemists. t is nearly impossible to find an uplifting, and can be arrested by the skin, making found about uranium’s superior catalytic funny, or otherwise endearing quote on depleted uranium (composed mainly of 238U) activity may not be an isolated event. The Iuranium — the following dark wisecrack1 safe to work with as long as it is not inhaled organometallic chemistry of uranium was reflects people’s sinister feelings about this or ingested. born during the ‘Manhattan project’ — code element: “For years uranium cost only a few Studying the fundamental chemistry of name of the development of the first nuclear dollars a ton until scientists discovered you uranium is an exotic endeavour, but those who weapon during the Second World War. This could kill people with it”. But, in the spirit of embrace it will reap its benefits. Haber and field truly began to attract interest in 1956 rebranding, it is interesting to note that the Bosch found that uranium was a better catalyst when Reynolds and Wilkinson reported the main source of Earth’s internal heat comes than iron for making ammonia2. The preparation of the first cyclopentadienyl from the radioactive decay of uranium, isolation of an η1-OCO complex derivatives6. The discovery of thorium and potassium-40 that keeps the of uranium3 also showed uranocene electrified the field outer core liquid, induces mantle convection that, even though it is as much as that of ferrocene and, subsequently, drives plate tectonics. not involved in natural had pushed forward the Uranium is the naturally occurring processes, it can help organometallic chemistry element with the highest atomic number. Its shed light on their of transition metals. Today, discovery in 1789 in the mineral pitchblende mechanism: similar examples of processes (or uraninite) is credited to the German complexes are proposed catalysed by uranium chemist Martin Heinrich Klaproth who to be instrumental in complexes include the named it after the planet Uranus, itself only the photosynthetic hydrogenation of alkenes discovered eight years earlier. Although at the fixation of CO2, but and the oligomerization, time Klaproth thought that he had discovered transition-metal models dimerization, hydrosilation uranium metal, its actual isolation was not have remained elusive. and hydroamination of achieved until 1841, by the French chemist Synthetic chemists are excited terminal alkynes. Eugène-Melchior Péligot who reduced the about uranium’s properties. Uranium Reports of researchers taking advantage anhydrous uranium tetrachloride present brings the best of many worlds to the table: of the unique properties of uranium are in the ore with potassium. Through the like the lanthanides, it generates electrophilic appearing with increasing frequency, and it is end of the nineteenth century and until the complexes that show high reactivity towards only a matter of time until these processes are mid-twentieth century, uranium was widely normally inert substrates. Like the transition transformed from stoichiometric to catalytic. used, for example, in the colouring of glass, metals, it benefits from multiple oxidation Uranium may still cost more than a few giving it a greenish–yellow hue (from UO2; states (iii to vi), a property that has been dollars a ton, but scientists are using it for a lot ‘Vaseline glass’). greatly exploited in redox reactions. However, more than just weapons. ❐ The discovery of radioactivity in 1896 by unlike the lanthanides, which generate Henri Becquerel from a uranium sample, mostly ionic compounds, and unlike the MARISA J. MONREAL AND and subsequent research by Enrico Fermi transition metals, which use their d orbitals PAULA L. DIACONESCU are at the and others from 1934 on, eventually showed for coordination bonds, uranium can use its f Department of Chemistry & Biochemistry, that 235U can undergo a chain reaction of orbitals (pictured) to form covalent bonds4. University of California, Los Angeles, nuclear fission. This made it attractive for Furthermore, uranium’s large radius makes California 90095, USA. the nuclear power industry but also for unique coordination environments possible. e-mail: [email protected] Little Boy, the first nuclear weapon used in One class of compounds in particular a war. Uranium’s use in nuclear weapons is, encompasses the many unusual properties of References understandably, the source of the widespread uranium: the inverted sandwiches of arene- 1. http://www.todayinsci.com phobia felt by most for uranium. However, it bridged diuranium complexes, which showed 2. http://go.nature.com/etvnrj is only weakly radioactive — the half-life of that δ back-bonding with f orbitals is the 3. Castro-Rodriguez, I., Nakai, H., Rheingold, A. & Meyer, K. 238U is 4.468 × 109 years — and emits alpha equivalent of π back-bonding in transition Science 305, 1757–1759 (2004). 4. Kozimor, S. A. et al. J. Am. Chem. Soc. 131, 12125–12136 (2009). 5 particles. These have a low penetration depth metal organometallic chemistry . 5. Diaconescu, P. L., Arnold, P. L., Baker, T. A., Mindiola, D. J. & Although the use of uranium catalysts Cummins, C. C. J. Am. Chem. Soc. 122, 6108–6109 (2000). for drug synthesis seems unlikely, it is worth 6. Reynolds, L. T. & Wilkinson, G. J. Inorg. Nucl. Chem. considering that what Haber and Bosch 2, 246–253 (1956). Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Rf Db Sg Bh Hs Mt 424 NATURE CHEMISTRY | VOL 2 | MAY 2010 | www.nature.com/naturechemistry © 2010 Macmillan Publishers Limited. All rights reserved.