Kinetics and mechanism of the reaction of sulphur(IV) with N,N′‐ethylene‐bis(sali‐cylidiniminato)manganese‐(III) in aqueous solution

The reaction of (diaqua)(N,N′‐ethylene‐bis(salicylidiniminato)manganese(III) with aqueous sulphite buffer results in the formation of the corresponding mono sulphito complex, Mn(Salen)(SO₃)]^? (S‐bonded isomer) via three distinct paths: (i) Mn(Salen)(OH₂)₂⁺ + HSO₃^? → (k₁); (ii) Mn(Salen)(OH₂)₂⁺ + SO₃^2? → (k₂); (III) Mn(Salen)(OH₂)(OH) + SO₃^2? → (k₃) in the stopped flow time scale. The fact that the mono sulphito complex does not undergo further anation with SO₃^2?/HSO₃^? may be attributed to the strong trans‐activating influence of the S‐bonded sulphite. The values of the rate constants (10^?2k_i/dm² mol^?1 s^?1 at 25°C, I = 0.3 mol dm^?3), ΔH_i^#/kJ mol^?1 and ΔS_i^#/J K^?1 mol^?1 respectively are: 2.97 ± 0.27, 42.4 ± 0.2, ?55.3 ± 0.6 (i = 1); 11.0 ± 0.8, 33 ± 3, ?75 ± 10 (i = 2); 20.6 ± 1.9, 32.4 ± 0.2, ?72.9 ± 0.6 (i = 3). The trend in reactivity (k₂ > k₁), a small labilizing effect of the coordinated hydroxo group (k₃/k₂ < 2), and substantially low values of ΔS^# suggest that the mechanism of aqua ligand substitution of the diaqua, and aqua‐hydroxo complexes is most likely associative interchange (I_a). No evidence for the formation of the O‐bonded sulphito complex and the ligand isomerization in the sulphito complex, (Mn^III‐OSO₂ → Mn^III‐SO₃), ensures the selectivity of the Mn^III centre toward the S‐end of the S^IV species. The monosulphito complex further undergoes slow redox reaction in the presence of excess sulphite to produce Mn^II, S₂O₆^2? and SO₄^2?. The formation of dithionate is a consequence of the fast dimerization of the SO_3?. generated in the rate determining step and also SO₄^2? formation is attributed to the fast scavenging of the SO_3?. by the Mn^III species via a redox path. The internal reduction of the Mn^III centre in the monosulphito complex is insignificant. The redox reaction of the monosulphitomanganese(III) complex operates via two major paths, one involving HSO_3? and the other SO₃^2?. The electron transfer is believed to be outersphere type. The substantially negative values of activation entropies (ΔS^# = ?(1.3 ± 0.2) × 10² and ?(1.6 ± 0.2) × 10² J K^?1 mol^?1 for the paths involving HSO_3? and SO₃^2? respectively) reflect a considerable degree of ordering of the reactants in the act of electron transfer. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 627–635, 1999