Gold-chloride complexes in very acidic aqueous solutions and at temperatures 25-300 °C: A laser Raman spectroscopic study
Raman spectra of gold-chloride complexes in very acidic solutions ( mhcl = 2-5) have been recorded at temperatures 25-300°C and at pressures on the liquid vapor curve for the system. At temperatures ⪯ 100°C, the square planar Au(III)-chloride complex, AuCl -4, is predominant in solution, showing bands at approximately 171, 324, and 347 cm -1. However, in these acidic solutions, when the temperature is >100°C, the square planar Au(III)-chloride complex is partially transformed into the linear Au(I)-chloride complex, AuCl -12, with a single band near 332 cm -1. The transformation of the Au(III) square planar tetrachloro complex into the Au(I) linear dichloro complex is also favoured by a reduction of the oxygen fugacity and an increase of pH. The experimental observations are in full agreement with thermodynamic calculations and provide the first direct evidence for the existence of the AuCl -2 complex in hydrothermal solutions.