in your element Thoroughly enthralling thulium Named after a mysterious place, thulium — one of the rarest rare earths — has some exotic chemistry in store for us, says Polly Arnold.

hulium — isolated by a Swedish chemist ligand choice for example can shift the formal like so many of the other rare earths — is potential of a particular metal redox couple Tnamed after a mysterious place in the by as much as 1.5 eV. The first organometallic far northern corners of Europe. The Thule thulium(ii) complex reported was isolated

of medieval geography (and its variations by replacement of the iodides of TmI2(THF)3 such as Tile, as seen in the 1539 map by by a silyl-functionalized cyclopentadienyl , OLAUS MAGNUS, 1539, 1539, MAGNUS, , OLAUS cartographer Olaus Magnus, pictured) has anion, which is well-known for its capacity to since been attributed to a variety of places stabilize formally low-oxidation-state metal including Iceland, islands north of Scotland, cations through electron-density acceptance

and Scandinavia. It is the latter that Per Teodor from the metal (back-donation). At low MARINA CARTA Cleve referred to when he proposed a name for temperatures, dark purple crystals of the THF element 69, which he identified in 1879 while solvate were grown and characterized. These studying the mineral ‘erbia’ from the Swedish colours are typical of rare-earth(ii) systems; town of Ytterby, working on isolating different at room temperature these slowly react to

rare-earth trications from each other — a and thulium(ii). Although they are extremely deoxygenate the solvents, returning to the THE FROM DETAIL LAFRERI, 1579 ANTONIO BY COLOURED notoriously challenging endeavour, given their difficult to reduce, and can only be isolated pale-yellow colours of 5d0–RE(iii) ions. very similar behaviours. with the right ligands, they can be accessed by The high reactivity of these complexes

Thulium is relatively expensive and has comproportionation (2REX3 + RE = 3 REX2), means that no suitable method to record only found commercial application in low- or through REX3 reduction by a very strongly the potential of the metal’s redox couple in volume uses in the medical field as a dopant to reducing metal such as potassium. solution has yet been developed. However,

the yttrium aluminium garnet (YAG; Y3Al5O12) In 1997, with the report that TmI2 could pleasingly, there has been a resurgence in used in surgical lasers, and its radioactive be dissolved and solvated in the ether solvents the development of new ligands that can isotope 170Tm serves as a source of X-rays for dimethoxyethane (DME) and tetrahydrofuran shift these redox couples by manipulating portable devices. Its sharp inter-4f-orbital (THF)2, Bochkarev made the breakthrough geometries and orbital overlap5–8, and in electronic transitions are also exploited in that launched the reactivity of compounds the search for other metals in the f-block — anti-counterfeiting inks in Euro currency of all of the rare earths(ii) in solution. Until including even the highly radioactive banknotes, whose blue fluorescence under then, formally zero-oxidation-state complexes transuranic elements9 — that can exhibit new, UV light arises from Tm3+ ions. (with strong colours arising from metal–ligand low formal oxidation states. New data suggest Rare-earth chemistry in solution mostly charge transfers3) made using metal vapours differentd /f electron configurations are now occurs in the +3 oxidation state. Nevertheless, had been the only known low-oxidation-state achievable10; future undergraduates may be

divalent rare-earth halides (REX2, with RE complexes of these ‘non-reducible’ rare earths. disappointed to have to learn ligand-field a rare-earth element, X a halide F–I) are As one of the rarest rare earths, thulium theory for the f-block as well as the d-block. ❐ accessible for europium, ytterbium and will not be replacing samarium as organic samarium, which approach or achieve a chemists’ reductant of choice any time soon POLLY ARNOLD is at the EaStCHEM School full- or half-full f-shell in this oxidation state. due to its high cost4. Yet its potential for of Chemistry, University of Edinburgh, The Indeed, samarium diiodide and its solvates exotic new chemistry is enormous. Using King’s Buildings, Edinburgh EH9 3FJ, UK. (which help tune its solubility and reducing organometallic ligands and techniques, we e-mail: [email protected] power) have been used for around fifty years can make compounds that do not follow Twitter: @ProfArno now by organic chemists for controlled one- the well-established rules of the d-block electron reduction reactions of a range of metals. Studying their fundamental electronic References 1. Strekopytov, S. Nat. Chem. 8, 816–816 (2016). functional groups, including carbonyl, alkyl structures and bonding subtleties can then 2. Bochkarev, M. N. et al. Angew. Chem. Int. Ed. 36, 133–135 (1997). 1 halide and nitro groups . help us gain a better understanding of — and 3. Cloke, F. G. N. Chem. Soc. Rev. 22, 17–24 (1993). Although textbooks stipulate the absence of in turn a better ability to manipulate — the 4. Kagan, H. B. Tetrahedron 59, 10351–10372 (2003). 5. La Pierre, H. S., Scheurer, A., Heinemann, F. W., Hieringer, W. & the +2 oxidation state for the other rare earths heavy, relativistic and often radioactive metals Meyer, K. Angew. Chem. Int. Ed. 53, 7158–7162 (2014). in solution, research chemists know of three of the f-block metals, whose reactivities 6. Dutkiewicz, M. S. et al. Nat. Chem. 8, 797–802 (2016). others — neodymium(ii), dysprosium(ii) are critical to renewable energy, magnet 7. Anderson, N. H. et al. Nat. Chem. 6, 919–926 (2014). technologies and nuclear waste management. 8. Goodwin, C. A. P. et al. Inorg. Chem. 55, 10057–10067 (2016). 9. Arnold, P. L., Dutkiewicz, M. S. & Walter, O. Chem. Rev. The capacity for ligands to tune 117, 11460–11475 (2017). organometallic species is phenomenal — 10. Fieser, M. E. et al. Chem. Sci. 8, 6076–6091 (2017). Dy Ho Er Tm Yb Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn 1288 NATURE CHEMISTRY | VOL 9 | DECEMBER 2017 | www.nature.com/naturechemistry ©2017 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts reserved.