Revisiting the Assignment of Innocent and Non-Innocent Counter ions in Lanthanide(III) Solution Chemistry

Lanthanides are found in critical applications from display technology to renewable energy. Often these rare earth elements are used as alloys or functional materials, yet the access to them are trough solution processes. In aqueous solution the rare earths are found predominantly as trivalent ions and charge balance dictates that counter ions are present. The fast ligand exchange and lack of directional bonding in lanthanides complexes has led to questions regarding the speciation of Ln3+ solvates in the presence of various counter ions, and to the distinction between innocent = non-coordinating, and non-innocent = coordinating counter ions. There is limited agreement as to which counter ions that belong to each group, which lead to this report. By using Eu3+ luminescence, it was possible to clearly distinguish between coordinating and non-coordinating ions. To interpret the results it was required to bridge the descriptions of ion pairing and coordination. The da-ta—in form of Eu3+ luminescence spectra and luminescence lifetimes from solutions with varying concentrations of acetate, chloride, nitrate, fluoride, sulfate, perchlorate and triflate—were contrasted to those obtained with ethylenediaminetet-raaceticacid (EDTA), which allowed for the distinction between three Ln3+-anion interaction types. It was possible to con-clude which counter ions are truly innocent (e.g. ClO4- and OTf-), and which clearly coordinate (e.g. NO3- and AcO-). Finally, the considerate amount of data from systems studied under similar conditions allowed the minimum perturbation arising from inner sphere or outer sphere coordination in Eu3+ complexes to be identified.