Kinetics of the Fe(II) reduction of trans-halogenopyridinebis(dimethylglyoximato)Co(III) complexes

The kinetics of the Fe(II) reduction of trans-chloro, bromo and iodopyridinebis(dimethylglyoximato)Co(III) have been studied at 30.0±0.1°C and I = 1.0 mol. dm⁻³(NaClO₄) in the [H⁺] range 0.0043–0.115 mol. dm⁻³. The reaction showed an inverse dependence on [H⁺]. The second order rate constant could be expressed in the form k_II = k₁ + k₂(1 + K_B[H⁺])^{−1. The kinetic data were found to be:} Co(DH)₂(py)Cl−k₁ = 0.051 ±0.003 dm³ mol⁻¹s⁻¹, 0.051±0.003 dm³ mol⁻¹ s⁻¹,k₂ = 0.76±0.04 dm³ mol⁻¹ s⁻¹ K_B = 325±8 dm³ mol⁻¹;Co(DH)₂(py)Br-k₁ = 0.071±0.004 dm³mol⁻¹ s⁻¹,k₂ = 1.21±0.04 dm³ mol⁻¹ s⁻¹ K_B = 460±15 dm³ mol⁻¹; Co(DH)₂(py)I-k₁ = 0.075±0.006 dm³ mol⁻¹ s⁻¹,k₂ = 1.91±0.09 dm³ mol⁻¹ s⁻¹ K_B = 625±30 dm³ mol⁻¹. The inverse dependence on [H⁺] suggests an inner-sphere mechanism involving protonated and unprotonated species of the complex. The order of rates for the three complexes was found to be Co(DH)₂(py)I > Co(DH)₂(py)Br > Co(DH)₂(py)Cl.