Kinetics and mechanism of the reaction between silver(III) and thiourea in aqueous alkaline media

The tetrahydroxoargentate(III) ion, Ag(OH)₄^?, is rapidly reduced by thiourea (tu) in accordance with the three term rate law RATE = {k₁+(k₂+k₃OH^?])tu]}Ag^III] where k₁ = 1.08 s^?1, k₂ = 1.46 x 10³ M^?1 s^?1, and k₃ = 2.02 x 10³ M^?2 s^?1. The k₁ path occurs via the rate-determining aquation of Ag(OH)₄^? while the other two paths involve axial attack of thiourea on silver. The higher values of k₂ and k₃ compared to the ethylenediamine reaction, which obeys the same rate law, is a reflection of the greater nucleophilicity of tu.

Following the redox reaction, solutions become brown in a reaction that obeys pseudo-first-order kinetics. Similar behaviour is observed when tu is replaced by Na₂S or thio-acetamide and when Ag^I reacts with any of these sulphur containing compounds. We attribute this process to the Ag^I promoted formation of sulphide species which eventually precipitate as Ag₂S.