The hydrolysis of gold(I) in aqueous acetonitrile solutions

Abstract

The hydrolysis and subsequent decomposition of gold(I) in aqueous solutions with acetonitrile were found not to yield the expected disproportionation products gold(0) and gold(III) in a ratio of 2∶1, but more than 98% gold(0), and also oxygen when acetonitrile from the gold(I) stock solution was removed first. The pH dependence of the reaction rate is attributed to a hydrolysis equilibrium established before the redox decomposition. The species [Au(CH ₃ CN)(OH)] and Au(OH) were identified by alkalimetric titrations and numerical analysis, with the logarithms of their formation constants being 10.7 and 10.2 respectively. These species seem to be sources of the OH˙ radical, which explains the oxygen formation in the absence of acetonitrile, and an alteration of its UV spectrum when it is left in the reaction mixture. The high complex stability of gold(I) with hydroxide ions means that it is one of the most acidic univalent cations, similar to HgCH ₃ ⁺ . The known relativistic contraction of the 6s orbital in gold(I) may account for this unusual property.