Structural properties of the inner coordination sphere of indium chloride complexes in organic and aqueous solutions

Abstract

The nature of the inner coordination sphere of In³⁺ present in both the organic and aqueous solutions during the solvent extraction of In³⁺ from an aqueous HCl solution with tri-n-octyl amine (TOA) was investigated by In K-edge XAFS. This information was then used to clarify the details of the extraction properties of indium chloride anion complexes with TOA. In aqueous HCl solution (0.1–10 M), In³⁺ exists as octahedral anion complexes, [InCl_n(H₂O)_6−n]³⁻ⁿ (n ≥ 4); the [InCl₆]³⁻ complex is dominant at 10 M HCl. The extraction of In³⁺ from HCl solution with TOA was performed using two kinds of diluents: nitrobenzene (NB) or n-dodecane (DD), which contained 20 vol% of 2-ethylhexanol as an additive. The stoichiometric composition of the extracted complexes, which is estimated from the distribution ratios of In³⁺, is affected by the diluents and the HCl concentration of the aqueous phase; the apparent values of TOA/In³⁺ in the extracted complex are 3 for DD–1 M HCl (diluent–aqueous phase) and DD–5 M HCl, 2 for NB–1 M HCl and NB–5 M HCl, and 1 for NB–10 M HCl. The EXAFS analysis of these extracted complexes shows that the In³⁺ has ∼4 Cl⁻ at ∼2.336 Å and no H₂O in the inner coordination sphere; additionally, the shape of the XANES suggests that their coordination geometry is tetrahedral. Therefore, the same tetrahedral [InCl₄]⁻ complex is formed during the extraction in spite of the variation in the stoichiometric composition (TOA/In³⁺ = 1–3) of the extracted complexes.