Investigation on Gold-Ligand Interaction for Complexes from Gold Leaching: A DFT Study

Gold leaching is an important process to extract gold from ore. Conventional alkaline cyanide process and alternative nontoxic lixiviants including thiosulfate, thiourea, thiocyanate, and halogen have been widely investigated. However, density functional theory (DFT) study on the gold complexes Au(CN)(2)(-), Au(S2O3)(2)(3-), Au[SC(NH2)(2)](2)(+), Au(SCN)(2)(-), and AuCl2- required for discovering and designing new highly efficient and environmentally friendly gold leaching reagents is lacking, which is expected to support constructive information for the discovery and designation of new high-efficiency and environmentally friendly gold leaching reagents. In this study, the structure information, electron-transferring properties, orbital interaction, and chemical bond composition for complexes Au(CN)(2)(-), Au(S2O3)(2)(3-), Au[SC(NH2)(2)](2)(+), Au(SCN)(2)(-), and AuCl2- depending on charge decomposition analysis (CDA), natural bond orbital (NBO), natural resonance theory (NRT), electron localization function (ELF), and energy decomposition analysis (EDA) were performed based on DFT calculation. The results indicate that there is not only sigma-donation from ligand to Au+, but also electron backdonation from Au+ to ligands, which strengthens the coordinate bond between them. Compared with Cl-, ligands CN-, S2O32-, SC(NH2)(2), and SCN- have very large covalent contribution to the coordinate bond with Au+, which explains the special stability of Au-CN and Au-S bonds. The degree of covalency and bond energy in Au-ligand bonding decreases from Au(CN)(2)(-), Au(S2O3)(2)(3-), Au[SC(NH2)(2)](2)(+), Au(SCN)(2)(-), to AuCl2-, which interprets the stability of the five complexes: Au(CN)(2)(-) > Au(S2O3)(2)(3-) > Au[SC(NH2)(2)](2)(+) > Au(SCN)(2)(-) > AuCl2-.