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in your element
The colours of chromium
From rubies to Rolls-Royce, Anders Lennartson explores the colourful history of chromium and its coordination compounds.
As a boy, I set up my own makeshi laboratory in my parents basement. During my early chemical investigations, I acquired some chromium(iii) chloride hexahydrate,a green salt that gave an equally green solution when dissolved in water. WhenI came back the next day, however, to my great surprise I found that the solution was now a violet colour. How could that be,I wondered?
An important property of chromium(iii) complexes is that ligand exchange is slow. When I dissolved CrCl36H2O which is more properly represented by the formula [CrCl2(H2O)4]Cl(H2O)2 in water, it slowly reacted with the solvent to givethe violet-coloured [Cr(H2O)6]3+ complex and free chloride ions. If I had had some chromium(iii) sulfate to hand, I would have observed the opposite colour change; it exists as [Cr(H2O)6]3+ in the solid hydrate, but when an aqueous solution of this compound is heated, it turns from violetto green because of the slow dissociationof water ligands and coordination of sulfate ions. This property of Cr(iii) makes it possible to isolate a wide variety of chromium(iii) coordination compounds, and is why Cr3+, along with Co3+, was abig favourite amongst early coordination chemists like Alfred Werner.
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