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

Abstract

Kinetic studies of X − exchange on [AuX 4] − 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 [AuCl 4] − with the kinetic parameters: ( k 2 Au,Cl) 298=0.56±0.03 s −1 mol −1 kg; Δ H 2 ‡ Au,Cl =65.1±1 kJ mol −1; Δ S 2 ‡ Au,Cl =−31.3±3 J mol −1 K −1 and Δ V 2 ‡ Au,Cl=−14±2 cm 3 mol −1. The variable pressure data clearly indicate the operation of an I a 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 H 2O, OH − and CN − were determined: k 0 HCN,H=113±17 s −1, k 1 HCN,H=(2.9±0.7)×10 9 s −1 mol −1 kg and k 2 HCN,H=(0.6±0.2)×10 6 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: ( k 2 Au,CN) 298=6240±85 s −1 mol −1 kg, Δ H 2 ‡ Au,CN=40.0±0.8 kJ mol −1, Δ S 2 ‡ Au,CN=−37.8±3 J mol −1 K −1 and Δ V 2 ‡ Au,CN=+2±1 cm 3 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 [AuX 4] − are two or three orders of magnitude faster than the Pt(II) analogue.