in your element The colours of chromium From rubies to Rolls-Royce, Anders Lennartson explores the colourful history of chromium and its coordination compounds. s a boy, I set up my own makeshift colour — by Louis Nicholas Vauquelin, laboratory in my parents’ who discovered the element in 1797. The Abasement. During my early metal was not an immediate commercial chemical investigations, I acquired some success. Fifteen years after its discovery, chromium(iii) chloride hexahydrate, Sir Humphrey Davy did not know much a green salt that gave an equally green about chromium or its compounds when solution when dissolved in water. When he wrote his famous text book Elements of I came back the next day, however, to my Chemical Philosophy, but he did remark great surprise I found that the solution was that chromic acid has a sour taste1. Tasting now a violet colour. How could that be, chemicals was obviously the order of the I wondered? OF PETRA RÖNNHOLM COURTESY IMAGE day, because in that very same year Jöns An important property of chromium(iii) Jacob Berzelius wrote in his textbook that complexes is that ligand exchange is slow. turns yellow. This colour change stems the aftertaste of the toxic chromic acid was 2 When I dissolved CrCl3∙6H2O — which is from the formation of potassium chromate, harsh and metallic . Berzelius also noted more properly represented by the formula K2CrO4, in which chromium is found in that the metal, although brittle, was very [CrCl2(H2O)4]∙Cl(H2O)2 — in water, it oxidation state vi. resistant to both acids and oxidation in air. slowly reacted with the solvent to give Other chromium(vi) compounds We now know that this property comes 3+ the violet-coloured [Cr(H2O)6] complex include the beautifully orange potassium from the fact that, when exposed to air, and free chloride ions. If I had had some dichromate, K2Cr2O7, red potassium metallic chromium forms a very thin, but chromium(iii) sulfate to hand, I would have trichromate, K2Cr3O10, and red dense, oxide layer on its surface. observed the opposite colour change; it chromium(vi) oxide, CrO3. The latter is In the 1820s it was found that addition 3+ exists as [Cr(H2O)6] in the solid hydrate, an acidic oxide, and its aqueous solutions of chromium to steel made it resistant to but when an aqueous solution of this are referred to as chromic acid — with rusting, but unfortunately the high carbon compound is heated, it turns from violet the addition of dilute sulfuric acid this content of the chromium available at the to green because of the slow dissociation becomes Jones reagent, used to convert time made these alloys brittle and useless of water ligands and coordination of alcohols to ketones or carboxylic acids. for practical applications. When methods sulfate ions. This property of Cr(iii) makes Acidic potassium dichromate is also used by developed such that carbon-free chromium it possible to isolate a wide variety of organic chemists for the same reaction, with was produced in the 1890s, the situation chromium(iii) coordination compounds, the added bonus that if a synthesis fails, a changed. Stainless steel, which typically 3+ 3+ and is why Cr , along with Co , was a solution of K2Cr2O7 in sulfuric acid can be contains 8% chromium and 18% nickel, big favourite amongst early coordination used to clean the dirty glassware, such is its soon became widely used, and it remains chemists like Alfred Werner. oxidizing power. one of the main applications of chromium In contrast to chromium(iii), It is now known that chromium(vi) today. The discovery, in the 1920s, that chromium(ii) complexes can exchange compounds are toxic and may cause a thin layer of shiny chromium could be their ligands rapidly, and addition of acetate cancer, but previously they were popular in electrolytically deposited on steel came as to blue CrCl2 solutions precipitates red pigments such as PbCrO4 and Pb2OCrO4 a delight to the automotive industry. What Cr2(OAc)4, a compound that has a Cr–Cr (chrome yellow and chrome red, would a 1930s Rolls-Royce Phantom II be quadruple bond. Chromium also exists in respectively). The colours of chromium without its chrome plating? ❐ higher oxidation states: black chromium(iv) have been highly admired since ancient oxide was used extensively in the good times — rubies are nothing but crystalline ANDERS LENNARTSON is at Chalmers old days of magnetic tapes because of aluminium oxide doped with chromium, University of Technology, Department of its ferromagnetic properties; unstable and pink hues in sapphires also originate Chemical and Biological Engineering, Polymer CrF5 (Cr(v)) is a volatile red solid. If from traces of chromium in an aluminium Technology, 41296, Gothenburg, Sweden. chromium(iii) oxide (also known as chrome oxide lattice. Emeralds, a form of beryl, e-mail: [email protected] green) is heated with potassium carbonate Be3Al2(SiO3)6, derive their green colour from and potassium nitrate, the mixture slowly small amounts of chromium. References It seems more than appropriate, 1. Davy, H. Elements of Chemical Philosophy Part 1 Vol 1, 463 (J. Johnson, 1812). therefore, that chromium was named after 2. Berzelius, J. J. Lärbok i Kemien Part 2 89 the Greek word chroma — which means (Henr. A. Nordström, 1812). Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb 942 NATURE CHEMISTRY | VOL 6 | OCTOBER 2014 | www.nature.com/naturechemistry © 2014 Macmillan Publishers Limited. All rights reserved.