Relativistic and Solvation Effects on the Stability of Gold(III) Halides in Aqueous Solution

The redox stability of gold halide complexes in aqueous solution has been examined quantum-chemically by a systematic comparison of scalar- and nonrelativistic pseudopotential calculations, using both COSMO and D-COSMO-RS solvent models for water. After a computational benchmarking of density-functional methods against CCSD­(T) results for the gas phase decomposition AuX₄^– → AuX₂^– + X₂, B3LYP calculations have been used to establish solvent contributions. While relativity clearly enhances the stability of AuX₄^– (X = F, Cl, Br, I) complexes against X₂ elimination, solvation favors the lower oxidation state. Solvation and relativity are nonadditive, due to the relativistic reduction of bond polarity. At scalar relativistic D-COSMO-RS level, the reaction AuX₄^– ⇌ AuX₂^– + X₂ is computed to be endergonic, except for X = I, where it is slightly exergonic. Under the chosen conditions, partial hydrolysis of AuCl₄^– to AuCl₃OH^– is exergonic. The latter complex in turn is stable against Cl₂ elimination. The disproportionation 3 AuCl₂^– ⇌ AuCl₄^– + 2 Au_(s) + 2 Cl^– is clearly exergonic. All of the computed reaction energies at scalar relativistic D-COSMO-RS level agree well with the observed speciation in dilute pH-neutral solutions at ambient temperatures.