Reduction of dicyanoaurate(I) in aqueous solution: formation of nonmetallic clusters and colloidal gold

Au(CN){sub 2}{sup {minus}} is reduced by hydrated electrons and hydrogen atoms. Three primary reduction products, which are in protolytic equilibrium, are formed depending on pH: Au(CN){sub 2}{sup 2{minus}} + 2H{sup +} {l reversible} Au(CN){sub 2}H{sup {minus}} + H{sup +} {l reversible} Au(CN){sub 2}H{sub 2}. The absorption spectra of the three species, the pK values of their protolytic equilibria, and their rates of decay into Au{sup 0} + 2CN{sup {minus}} (or HCN) were measured by the method of pulse radiolysis. The absorption spectrum of the gold atom is also reported. The agglomeration of gold atoms yields intermediate clusters of nonmetallic gold and finally colloidal metal. Hydroxymethyl and 2,2-methylhydroxypropyl radicals can also reduce Au(CN){sub 2}{sup {minus}}, although the specific rates are low. Gold atoms have strong reducing properties, the standard potential of the system Au{sup +}/Au{sup 0} being -1.5 V. Various organic radicals are reduced by Au{sup 0}. It is proposed that these reductions do not occur as simple electron-transfer reactions but through hydrolysis of intermediate organo-gold compounds.