Determination of structures of cupric-chloro complexes in hydrochloric acid solutions by UV-Vis and X-ray absorption spectroscopy

Ultrahigh-purity metals are indispensable to understanding the nature of materials, but the purity of contemporary metals is insufficient for determining their intrinsic properties. The fundamentals of the anion-exchange reaction must be clear in order to increase the refining efficiency of anion-exchange separation for purification of metals. The thermodynamic analysis of the anion-exchange reaction needs to take account of the distributions of metal-chloro complexes in the solution phase in addition to those between the resin and the solution phase. The structures of metal-chloro complexes in the solution phase must be determined as the first step in determining what species is adsorbed on the resin. Copper is one of the base metals most commonly used in modern society, and the anion-exchange behavior of cupric species is representative. The distribution and the molar attenuation coefficients of cupric-chloro complexes in hydrochloric acid solutions were obtained employing factor analysis followed by fitting a thermodynamic model to ultraviolet-visible absorption spectra. The cumulative formation constants were determined as follows: log ₁₀β₁= 0.599 , log ₁₀β₂= 0.343 , log ₁₀β₃= − 1.88 , and log ₁₀β₄= − 5.25 , and the Setchénow coefficient for the neutral species of [Cu^IICl₂]⁰ is 0.188. Using the distribution assessed by the above thermodynamic parameters, the X-ray absorption spectra (XAS), consisting of X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) of individual species, were assessed by factor analysis. EXAFS theoretical models were fitted to experimental spectra of individual species to determine their structures and coordination geometries for the first time, although it is generally very difficult to obtain the spectrum of the individual species in a matrix containing other species simultaneously. The structures of five cupric-chloro complexes were determined to be distorted octahedrons of [CuII(H2O)4eq(H2O)2ax]2+,[CuII(H2O)4eq(H2O)axClax]+, and [CuIICl2ax(H2O)4eq]0, a planar triangle of [CuIICl3]−, and a tetrahedron of [Cu^IICl₄]²⁻. The obtained spectra can be used as standards. The XANES spectra of cupric-chloro complexes were interpreted qualitatively. The Cl⁻ ligand in [CuII(H2O)4eq(H2O)axClax]+ is attracted to the center atom of Cu^II by electrostatic force, as the H₂O ligands are. Contrastingly, the bonding system of the Cl⁻ ligands in the latter three species involves covalency.