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-bissalicylidiniminato) iron(III) with sulphur(IV) in aqueous medium.

Abstract

The reaction of <i>trans</i>-[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 ≤ <i>t/ </i>^oC ≤ 35.0, 0.01 &lt; [S^IV]_T/mol dm^-3 &lt; 0.10 (<i>I= </i>0.3 mol dm^-3) in aqueous acetate buffer medium of varying <i>p</i>H (= 3.80 - 5.20). The stopped flow and rapid scan spectrophotometric measurements indicate the formation of monosulphito complex, <i>trans</i>-[Fe(Salpn<i> </i>1,2)(OH₂)(SO₃)]^-. The <i>trans-</i>diaqua complex reacts with HSO₃^-~10 times slower than its (hydroxo aqua) analogue thus displaying the labilising effect of the coordinated hydroxo group. The limiting dissociative mechanism (D) appears to be applicable for the aqua ligand substitution of the aqua-hydroxo complex, <i>trans</i>[Fe(Salpn 1 ,2)(OH₂)(OH)]. The <i>trans</i>-[Fe((Salpn 1,2)(OH₂)(SO₃)]^- undergoes redox reaction yielding Fe^II, SO₄^2- and S₂O₆^2-; <i>k</i>_obs = <i>k</i>₁ _rcd + <i>k</i>₂ _rcd [HSO₃^-] + <i>k</i>₃<i> </i>r_cd [H⁺][HSO₃^-] is obeyed with <i>k</i>₁ _rcd = (0.64 ± 0.40) ×10^-4 s^-1, <i>k</i>₂ _rcd = (1.8 ± 0.9) × 10^-4 dm^-3 mol^-1s^-1 and <i>k</i>₂ _rcd = (1.4 ± 0.1) × 10² dm⁶ mol^-2 s^-1 at 30.0^oC (<i>I= </i>0.3 mol dm^-3)· The electron transfer is innersphere type for

the uncatalysed path (<i>k</i>₁ _rcd) while the exact mechanism for the HSO₃^- and (H⁺ + HSO₃^-) dependent paths remains equivocal.