Chlorine Dioxide Reduction by Aqueous Iron(II) through Outer-Sphere and Inner-Sphere Electron-Transfer Pathways

The reduction of ClO2 to ClO2- by aqueous iron(II) in 0.5 M HClO4 proceeds by both outer-sphere (86%) and inner-sphere (14%) electron-transfer pathways. The second-order rate constant for the outer-sphere reaction is 1.3 × 106 M-1 s-1. The inner-sphere electron-transfer reaction takes place via the formation of FeClO22+ that is observed as an intermediate. The rate constant for the inner-sphere path (2.0 × 105 M-1 s-1) is controlled by ClO2 substitution of a coordinated water to give an inner-sphere complex between ClO2 and Fe(II) that very rapidly transfers an electron to give (FeIII(ClO2-)(H2O)52+)IS. The composite activation parameters for the ClO2/Fe(aq)2+ reaction (inner-sphere + outer-sphere) are the following:  ΔHr⧧ = 40 kJ mol-1; ΔSr⧧ = 1.7 J mol-1 K-1. The FeIIIClO22+ inner-sphere complex dissociates to give Fe(aq)3+ and ClO2- (39.3 s-1). The activation parameters for the dissociation of this complex are the following:  ΔHd⧧ = 76 kJ mol-1; ΔSd⧧ = 32 J K-1 mol-1. The reaction of Fe(aq)2+ with ClO2...