Kinetics and mechanism of ligand exchange of Au (III), Zn(II), and Cd(II) chlorides in aqueous solution: An NMR study from 28–98°C

Aqueous chloride complexes of Au(III), Zn(II), and Cd(II) are labile. There is a constant, dynamic exchange of Cl ligands coordinated to the metal with excess Cl− in the surrounding solution. Nuclear magnetic resonance (NMR) spectroscopy of Cl was used as a probe of this exchange to reveal the nature of bonding between Cl and each metal, the rate of Cl exchange out of the metal binding site (gkex), and the mechanism of exchange. All solutions were at low pH, and contained 1 M Cl and various concentrations of metals. NMR relaxation rate measurements were made on both the 35Cl and 37Cl nuclei over the temperature range 28–98°C. These measurements indicate that Zn forms inner sphere (directly bonded) complexes with Cl over the concentration range 10−3 to 0.25 M Zn. This is similar to the bonding behavior of Au and Cd with Cl, but contrasts with that of metals such as Mg and Na, which coordinate with only H2O in low pH, chloride solutions. From the variation of gkex with temperature, the enthalpy (ΔH∗) and entropy (ΔS∗) of Cl ligand exchange were estimated (Appendix) for Au and Zn. The signs and magnitudes of these values (Table 1) indicate that ligand exchange is via a dominantly associative mechanism for both metals. This suggests that neither of the dominant Au or Zn species is sterically hindered to coordination by another ligand in solution, or to an atom or functional group at a mineral surface, that is of equal or lesser size than Cl−. Ligand exchange provides a first order model of the mechanism of deposition of Au from solution onto mineral surfaces. Previously it has been observed that in aqueous chloride solutions, Zn undergoes a concentration dependent change in speciation and coordination number in the composition range 0.01–1.0 M Zn. This change is promoted by the formation of polymeric networks of tetrahedral Zn-Cl units with increasing Zn concentration. Concentration-dependent changes in gkex, ΔH∗, and ΔS∗ in these solutions are consistent with the onset of speciation changes and/or polymer formation at <0.1 M Zn.