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The interaction of gold(III) complexes [Au(en)Cl2]Cl, [Au(en)2]Cl3, [Au(cis‐DACH)Cl2]Cl, and [Au(cis‐DACH)2]Cl3 (en = ethylenediamine, DACH = cis‐1,2‐diaminocyclohexane) with biologically important thiols, such as glutathione (GSH), dl‐penicillamine (PSH), mercaptoacetic acid (MAA), and N‐(2‐mercaptopropionyl)glycine (MPG), has been studied using 1H, 13C NMR, UV–vis spectroscopy and electrochemistry in aqueous solution. Kinetic data revealed that the reactivity of their substitution reaction followed the order: [Au(en)Cl2]+ > [Au(en)2]3+ > [Au(cis‐DACH)Cl2]+ > [Au(cis‐DACH)2]3+. The thiol reactivity increased with decreasing its size, viz. MAA ≫ MPG > PSH > GSH. Square wave stripping voltammetry displayed peaks for Au(III) and Au(I) at +0.875 V and +1.4 V respectively. The interaction of the complexes with thiols resulted in reduction of gold(III) to gold(I) and thiol ligands (RSH) were oxidized to disulfide (RSSR).

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The interaction of gold(III) complexes, [Au(cis‐DACH)Cl2]Cl and [Au(cis‐DACH)2]Cl3 complexes (DACH = cis‐1,2‐diaminocyclohexane), with 13C, 15N‐enriched thiourea (Tu) and 1,3‐diazinane‐2‐thione ligands was investigated. The progress of these reactions was monitored by NMR (1H, 13C, and 15N) and UV–vis spectroscopy as well as square wave stripping voltammetry. The kinetic studies of the substitution reactions between the above‐mentioned complexes with thiones in aqueous solutions containing 30 mM KCl, which is used to suppress the hydrolysis of the chloride complexes, were conducted. These reactions were followed under pseudo–first‐order conditions as functions of ligand concentration, pH, and temperature. The activation parameters (ΔH#, ΔS#) were calculated from Eyring plots, and the negative values of ΔS≠ lend support for an associative mechanism. The kinetic data also indicated a relatively higher reactivity of [Au(cis‐DACH)Cl2]Cl than that of [Au(cis‐DACH)2]Cl3 toward the thiones.