Ligand control on the reactions of transition metal complexes with sulphur(IV): Kinetics and mechanism of the reaction of trans-(diaqua) (N,N'-propylene 1,2-bis-salicylidiniminato)iron(III) with sulphur(IV) in aqueous medium.

The reaction of trans-[Fe(Salpn 1,2)(OH2)(2)]+ (Salpn 1,2 = N,N'-propylene 1,2 -bis-salicylidiniminate) with S-IV has been studied at 20.0 less than o r equal to t/ degreesC less than or equal to 35.0, 0.01 < [S-IV](T)/mol dm( -3) < 0.10 (I = 0.3 mol dm(-3)) in aqueous acetate buffer medium of varying pH ( = 3.80 - 5.20). The stopped flow and rapid scan spectrophotometric me asurements indicate the formation of monosulphito complex, trans-[Fe(Salpn 1,2)( OH2)(SO3)](-). The trans- diaqua complex reacts with HSO3- similar to 10 times slower than its (hydroxo aqua) analogue thus displaying the labil ising effect of the coordinated hydroxo group. The limiting dissociative me chanism (D) appears to be applicable for the aqua ligand substitution of th e aqua-hydroxo complex, trans-[Fe(Salpn 1,2)( OH2)(OH)]. The trans-[Fe((Sal pn 1,2)( OH2)(SO3)](-) undergoes redox reaction yielding Fe-II, SO42-. and S2O62-; k(obs) = k(1red) + k(2) (red) [HS0(3)(-)] + k(3) (red) [H+][HSO3-] is obeyed with k(1red) = (0.64 +/- 0.40) x 10(-4) s(-1), k(2) (red) = (1.8 +/- 0.9) x 10(-4) dm(3) mol(-1)s(-1) and k(2 red) = (1.4 +/- 0.1) x 10(2) d m(6) mol(-2) s(-1) at 30.0 degreesC (I = 0.3 mol dm(-3)). The electron tran sfer is innersphere type for the uncatalysed path (k(1 red) ) while the exa ct mechanism for the HSO(3)(-)and (H+ + HSO3-) dependent paths remains equi vocal.