Reactions of sulphur(IV) with (tetraethylenepentamine)iron(III)

Abstract

The reactions of S(IV) with Fe(III) in the presence of tetraethylenepentamine(T) have been investigated at pH = + 3.6-4.2 Fe(III) is bound by the amine in a tridentate fashion (Fe(TH2)(5+)) with the two terminal primary amino groups remaining in the protonated form. The species Fe(TH2)(5+) undergoes sulphite substitution to yield Fe(TH2)(SO3)(3+). The rate and activation parameters for the formation and dissociation of the sulphito complex are reported. Reduction of Fe(III) by the coordinated sulphite is slow. The redox reaction reveals the existence of two isomers of Fe(TH2)(SO3)(3+) in which SO32- is bonded trans-or cis to the coordinated H2O (axial or equatorial). From the reactivity trend it is presumed that the coordinated sulphite in the trans isomer reduces Fe(III) centre faster (by a factor of similar to 10) than the same in the cis isomer. The product distribution experiments show that SO3- dimerises to S2O62- besides being oxidised to SO42- by Fe(III).