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++SO32- →(k2) ); (III) Mn(Salen)(OH2)(OH) + SO32-→(k2) in the stopped flow time scale. The fact that the mono sulphito complex does not undergo further anation with SO32-/HSO3- may be attributed to the strong trans-activating influence of the S-bonded sulphite. The values of the rate constants (10-2k/dm2mol-1s-1 at 25°C, 1 = 0.3 mol dm-3), ΔHi#/kJ mol-1 and ΔHi#/J K-1mol-1respectively 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 (la). No evidence for the formation of the O-bonded sulphito complex and the ligand isomerization in the sulphito complex, (MnIII-OSO2→ MnIII-SO3), ensures the selectivity of the MnIII centre toward the S-end of the SIV species. The monosulphito complex further undergoes slow redox reaction in the presence of excess sulphite to produce MnII, S2O62- and SO42-. The formation of dithionate is a consequence of the fast dimerization of the SO3-. generated in the rate determining step and also SO42- formation is attributed to the fast scavenging of the SO3-. by the MnIII species via a redox path. The internal reduction of the MnIII centre in the monosulphito complex is insignificant. The redox reaction of the monosulphitomanganese(III) complex operates via two major paths, one involving HSO3- and the other SO32-. 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 HSO3- and SO32- respectively) reflect a considerable degree of ordering of the reactants in the act of electron transfer