Cyanide and chloride exchange on homoleptic gold(III) square-planar complexes: variable pressure kinetic investigation by heteronuclear NMR

Kinetic studies of X− exchange on [AuX4]− square-planar complexes (where X=Cl− and CN−) were performed at acidic pH in the case of chloride system and as a function of pH for the cyanide one. Chloride NMR study (330–365 K) gives a second-order rate law on [AuCl4]− with the kinetic parameters: (k2Au,Cl)298=0.56±0.03 s−1 mol−1 kg; ΔH2‡ Au,Cl=65.1±1 kJ mol−1; ΔS2‡ Au,Cl=−31.3±3 J mol−1 K−1 and ΔV2‡ Au,Cl=−14±2 cm3 mol−1. The variable pressure data clearly indicate the operation of an Ia or A mechanism for this exchange pathway. The proton exchange on HCN was determined by 13C NMR as a function of pH and the rate constant of the three reaction pathways involving H2O, OH− and CN− were determined: k0HCN,H=113±17 s−1, k1HCN,H=(2.9±0.7)×109 s−1 mol−1 kg and k2HCN,H=(0.6±0.2)×106 s−1 mol−1 kg at 298.1 K. The rate law of the cyanide exchange on [Au(CN)4]− was found to be second order with the following kinetic parameters: (k2Au,CN)298=6240±85 s−1 mol−1 kg, ΔH2‡ Au,CN=40.0±0.8 kJ mol−1, ΔS2‡ Au,CN=−37.8±3 J mol−1 K−1 and ΔV2‡ Au,CN=+2±1 cm3 mol−1. The rate constant observed varies about nine orders of magnitude depending on the pH and HCN does not act as a nucleophile. The observed rate constant of X− exchange on [AuX4]− are two or three orders of magnitude faster than the Pt(II) analogue.