Kinetic studies on the oxidation of ethylenediaminebis(pyridine)- and bis(ethylenediamine)-platinum(II) complexes by hexachloroiridate(IV)
Abstract
The oxidation reactions of [Pt(en)L2]2+[L =½ ethylenediamine (en) or pyridine (py)] by IrCl62– in the presence of chloride ions have been kinetically investigated in aqueous solution. In the absence of added IrCl63– the reactions obey the rate law: –d[IrCl62–]/dt= 2 (ka+kb[Cl–])[Pt(en)L22+]-[lrCl62–]; a further rate term, second order in [Cl-], operates at high chloride concentrations. Hexachloroiridate(III) lowers the rates and its kinetic effect increases at lower IrCl62– and/or Cl– concentrations. A consistent reaction mechanism is proposed. The complex [Pt(en)2]2+ reacts much faster than [Pt(en)(py)2]2+, e.g. at 20 °C Kb/dm6 mol–2 s–1= 2.81 × 104(L =½ en) or 5.7 × 101(L = py), owing to a very favourable activation enthalpy [ΔH‡/kJ mol–1= 2.0 (L =½ en) or 26.2 (L = py)]. The kinetic and thermodynamic tendencies towards the oxidation of such complexes appear to be correlated.