Ligand control on the reactions of transition metal complexes with sulphur(IV): Kinetics and mechanism of ligand substitution and electron transfer reactions of trans-(diaqua)-(N,N '-ethylene-bis-salicylidiniminato)iron(III)with sulphur(IV) in aqueous medium

The reaction of trans-[Fe(Salen)(OH2)(2)](+) with S-IV has been studied at 20.0 less than or equal to t (degreesC) < 35.0, I = 0.3 mol dm(-3) in aqueo us acetate buffer medium of pH = 3.9-5.3. Stopped flow and rapid scan spect rophotometry indicate the formation of a monosulphito complex, trans-[Fe(sa len)(OH2)(SO3)](-). The rate constant (k (dm(3) mol(-1) s(-1))), and activa tion parameters (<Delta>H-not equal (kJ mol(-1)), DeltaS(not equal) (J K-1 mol(-1))) for the formation of the sulphito complex by the reaction of tran s[Fe(Salen)(OH2)OH] with HSO3- and SO32- at 25 degreesC are: (4.5 +/- 0.6) x 10(2), 39 +/- 8, -65 +/- 28 (HSO3-) and (1.8+/- 0.4) x 10(4) 33 +/- 8, -5 4 +/- 28 (SO32-) respectively. The diaqua complex, [Fe(Salen)(OH2)(2)](+), reacts with HSO3- (k(1) = 6.3 +/- 3.4 dm(3) mol(-1) s(-1) at 25 degreesC) s imilar to 10(2) times slower than its aqua hydroxo analogue, thus displayin g the strong labilising action of the coordinated hydroxo group. The limiti ng dissociative mechanism (D) appears to be applicable for the aqua ligand substitution of trans-[Fe(salen)(OH2)OH]. A value of 2.5 x 10(4) s(-1) (25 degreesC) is calculated for the rate constant of water dissociation from tr ans-[Fe(salen) (OH2)OH] when HSO3- is the entering nucleophile. The observe d reactivity difference for HSO3- and SO32- is indicative of the discrimina tion of the five coordinate intermediate, Fe(Salen)OH, for the entering nuc leophiles. The trans-[Fe(Salen)(OH2)(SO3)](-) undergoes a redox reaction yi elding Fe-II, SO24- and S2O62-, k(obs) = k(1red) + k(2red)[HSO3-] + k(3red) [H+][HSO3-] is obeyed with k(1re)d = (0.77 +/- 0.31) x 10(-4) dm(3) mol(-1 ) s(-1), k(2red) = 21.6 +/- 8.0 dm(3) mol(-1) s(-1) and k(3red) = 99 +/- 9 dm(6) mol(-2) s(-1) at 30 degreesC (I = 0.3 mol dm(-3)). The last reaction is indistinguishable from the reduction of the sulphito complex by SO2.