Inner-Sphere Structure of Rhodium Complexes with Tin(II) Chloride in Concentrated Hydrochloric Acid Solution

Abstract

Abstract The coordination properties of highly concentrated rhodium complexes (Rh: ca. 10 g L−1) in concentrated hydrochloric acid solution upon addition of tin(II) chloride were investigated using Rh and Sn K-edge XAFS spectra. For Rh solutions in the absence of Sn, the Rh ion shows no inner-sphere H2O complexation for [HCl] ≥ 6 M. The coordination structure of Rh in 3 M HCl solution (ca. 1 O(H2O) at 2.06(3) Å and ca. 5 Cl− at 2.321(8) Å) drastically changes with increasing amount of tin(II) chloride added; the Sn directly coordinates to Rh by replacing the H2O/Cl−. At [Sn]/[Rh] = 12.4, the inner coordination sphere of Rh is occupied only by Sn (ca. 5 Sn at 2.510(7) Å). The curve fits of the Sn K-edge EXAFS spectra for the Rh–Sn system show that the Sn has ca. 3 Cl− and ca. 1 Rh in the inner coordination sphere. XANES analysis reveals that the Rh ion is reduced from trivalent to monovalent with concomitant oxidation of the Sn ion, divalent to tetravalent. These results demonstrate that the Rh complexes in 3 M HCl solution transform from [RhIIICl5(H2O)]2− to [RhI(SnIICl3)5]4− through the [RhIIICln(H2O)m(SnIICl3)6−n−m]n−3 complex (1 ≤ n ≤ 5, 0 ≤ m ≤ 1) with an increase in the Sn concentration.