The method of continuous variation, in conjunction with UV-visible, Raman and infrared spectroscopy, has been applied to the structural characterization of complexes resulting from tributyl phosphate (TBP), Li+ and FeCl3 in an organic extraction solvent. Data derived from various spectroscopic analyses confirmed the presence of FeCl4- in the solvent, and also demonstrated that the combination of Li+ and FeCl3 produces LiFeCl4. From the method of continuous variation, it was determined that more than one complex results during the extraction of Li+ from aqueous solution, and that these complexes have the general structural formula LiFeCl4 center dot nTBP, where n is greater than or equal to 1. The coordination mode of the complex was investigated by applying a curve-fitting algorithm to the IR spectra of the organic solvent following the extraction of various cations. The results showed that high molecular weight organic cations are produced by the coordination of the TBP extractant and the cation, and that the selective recovery of Li+ occurs via an ion association reaction involving the organic cation and the FeCl4- anion. Finally, the structural formulae of the complexes formed during the extraction were confirmed to be [Li(TBP)(H2O)(3)](+) center dot FeCl4- and [Li(TBP)(2)(H2O)(2)](+) center dot FeCl4-, and the planar and stereo structures of both are given. (C) 2013 Elsevier Ltd. All rights reserved.