Kinetics and mechanism of anation of (α, β)S-(hydroxo) (tetraethylenepentamine)cobalt (III) by sulphite in basic medium, the role of anionic micelles

Abstract

The anation of (α, β)S-(tetren)CoOH²⁺ by SO ²⁻₃ in alkaline medium (pH>12) produced the S-bonded sulphito complex exclusively. The reaction is reversible andk _obs=(k ₁+k ₂[OH⁻])[SO ²⁻₃ ] _T +k ₋₁[OH⁻] is obeyed at [SO ²⁻₃ ]=0.005–0.006, [OH⁻] _T =0.02−0.30 mol dm⁻³ (30–45°C,I=1.0 mol dm⁻³). At 35°C,k ₁=(3.6±0.6)×10⁻³ dm³ mol⁻¹ s⁻¹,ΔH ^≠=55±10 kJ mol⁻¹,ΔS ^≠=−112±34 JK⁻¹ mol⁻¹,k ₂=(4.3±0.6)×10⁻²dm⁶ mol⁻² s⁻¹,ΔH ^≠=44±11 kJ mol⁻¹,ΔS ^≠=−130±36 JK⁻¹ mol⁻¹. The unusually high rate of sulphite substitution particularly in the OH⁻ independent path is attributed to the involvement of a reactive internal conjugate base due to the coordinated OH⁻ assisted NH-deprotonation of the tetren moiety (Co(tetren)OH²⁺⇋Co(tetren-H)OH ²⁺₂ ). Substantially low values of the activation parameters (ΔH ^≠ andΔS ^≠) particularly the activation entropy is consistent with associative interchange mechanism (I _a) which is further supported by the rate retardation by anionic micelles of SDS. For the pseudo-phase ion-exchange equilibrium (tetren) in the reaction CoOH ²⁺ _W +2Na ⁺ _M ⇋(tetren)CoOH ²⁺ _M +2Na ⁺ _W , the calculated value ofK′_ex is 219±60 at 35°C.