Extraction and Structural Properties of Rhodium–Tin Complexes in Solution

Abstract

The extraction and structural properties of rhodium complexes with stannous chloride using N,N‐dioctyl hexanamide (DOHA) were investigated by solvent extraction and Rh K‐edge X‐ray absorption fine structure (XAFS) spectroscopy. In the absence of Sn(II), Rh was hardly extracted from a 1 M HCl solution. However, the Rh extraction was enhanced with an increase in the Sn(II) concentration in the initial aqueous phase. The high extractability of Rh is achieved using the 1 M solution at [Sn(II)]/[Rh] ≥ 6 as the aqueous phase. At the [Sn(II)]/[Rh] = 12, the extraction of Rh was almost 100% over the HCl concentration range from 1 to 10.5 M. The X‐ray absorption near edge structure (XANES) spectra changed with an increase in the [Sn(II)]/[Rh] value in the 1 M HCl solution, suggesting that the Rh ion was reduced from trivalent to monovalent. In the DOHA complex prepared by solvent extraction from the [Sn(II)]/[Rh] = 12–1 M HCl solution, the XANES spectrum indicated that Rh(I) is predominant. The extended X‐ray absorption fine structure (EXAFS) spectra and the obtained structural parameters showed nearly the same result for the 1 M HCl solution at [Sn(II)]/[Rh] = 12 (Rh–Sn 5.3(6) at 2.54(1) Å) and the DOHA complex (Rh–Sn 4.7(8) at 2.56(1) Å). These results suggest that the [RhI(SnCl3)5]4− complex is the predominant species in both the organic and aqueous phases when the 1 M HCl solution at [Sn(II)]/[Rh] = 12 is used as the aqueous phase and that the [RhI(SnCl3)5]4− complex is highly extractable.