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 IrCl₆^2– in the presence of chloride ions have been kinetically investigated in aqueous solution. In the absence of added IrCl₆^3– the reactions obey the rate law: –d[IrCl₆^2–]/dt= 2 (k_a+k_b[Cl^–])[Pt(en)L₂²⁺]-[lrCl₆^2–]; 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 IrCl₆^2– 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 K_b/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.