Kinetics and mechanism of the reaction between tetrachloro- and tetrabromo-aurate(III) and thiocyanate

Abstract

The kinetics and mechanism for the overall reaction (i)(X = Cl or Br) have been studied at 25.0 °C using stopped-flow spectrophotometry. 3[AuX₄]^–+ 7SCN^–+ 4H₂O → 3[Au(SCN)₂]^–+ HSO₄^–+ HCN + 12X^–+ 6H⁺(i), The reaction takes place in two kinetically well separated steps. The initial, rapid process can be identified as stepwise ligand substitutions (ii)(n= 0–3) which take place via direct ligand displacements, the solvent path being negligible. [AuX_4–n(SCN)_n]^–+ SCN^–⇌[AuX_3–n(SCN)_n+1]^–+ X^–(ii) The substitution kinetics give no evidence for formation of persistent five-co-ordinate intermediates. The subsequent slower reaction is due to reduction of gold(III) to gold (I) thiocyanato-species. The rate of this step varies by four orders of magnitude within the accessible concentration interval of gold (10^–6–10^–2 mol dm^–3). At high gold concentrations the reduction is slow and follows no simple-order kinetics due to inhibition by the cyanide formed as a product. This inhibition is eliminated for gold concentrations less than 5 × 10^–6 mol dm^–3, where the redox reaction is rapid and strictly first order with respect to the concentrations of thiocyanate and gold complex. The mechanism for the reductive elimination is intermolecular involving a reaction between the gold(III) complex and an outer-sphere thiocyanate. Rate constants for reduction of [AuBr₄]^– and [Au(SCN)₄]^– by thiocyanate at 25 °C are (5 ± 2)× 10⁴ and (2.4 ± 0.2)× 10³ dm³ mol^–1 s^–1 respectively, for a 1.00 mol dm^–3 perchlorate medium.