A Raman Study on the Speciation of Different Metal Ions in an AlCl 3 –Based Ionic Liquid

The speciation of Cr, Zn and Sn in AlCl 3 /1-ethyl-3-methylimidazolium chloride containing CrCl 2 , ZnCl 2 and SnCl 2 , respectively, has been studied by cyclic voltammetry (CV), Raman spectroscopy and density functional theory (DFT) calculations. Addition of the respective metal salt causes the current waves in the CV to decrease, indicating a reaction of the metal salts with Al 2 Cl 7 − . Compared to the neat electrolyte, the Raman peaks of Al 2 Cl 7 − decrease while the AlCl 4 − peak increases in intensity, broadens and shifts towards lower wavenumbers. Calculated wavenumbers of metal complexes [Me(AlCl 4 ) 3 ] − reflect these observations. DFT calculations of the Gibbs free energies of formation, solvation and reaction support the formation of the proposed complexes. The central ions are coordinated by three bidentate AlCl 4 − ligands that are arranged planar–trigonally. Due to the occupied Sn–5s orbital, repulsive forces cause a trigonal–pyramidal geometry in case of the Sn complex. Based on the similarities in the experimental observations and the orbital configuration of Zn 2+ compared to Cr 2+ , the spontaneous formation of the species [Cr(AlCl 4 ) 3 ] − can be assumed.