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: (k₂^Au,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 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 H₂O, OH⁻ and CN⁻ were determined: k₀^HCN,H=113±17 s⁻¹, k₁^HCN,H=(2.9±0.7)×10⁹ s⁻¹ mol⁻¹ kg and k₂^HCN,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: (k₂^Au,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.