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 SO₃ ^2- in alkaline medium (pH > 12) produced the S-bonded sulphito complex exclusively. The reaction is reversible and k_obs=(k₁+k₂[OH^-])[SO ₃^2-]_T+k^-1[OH^-] is obeyed at [SO₃^2- ]=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, k₁=(3·6±0·6)×10^-3 dm³ mol^-1 s^-1,ΔH ^≠=55± 10 kJ mol^-1, ΔS ^≠ = -112±34 JK^-1 mol^-1,k₂=(4·3±0·6)×10^-2dm⁶ mol^-2s^-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)OH₂²⁺). Substantially low values of the activation parameters (ΔH ^≠ and ΔS^≠) 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_W²⁺ + 2Na_M⁺ □(tetren)CoOH_M²⁺ + 2Na_W⁺ , the calculated value of K'_ex is 219±60 at 35°C.