COMPLEX-FORMATION BETWEEN THE AQUOPENTACYANOFERRATE(II) ION AND THIOUREA AND ITS N-SUBSTITUTED DERIVATIVES

The results of a kinetic investigation of the substitution reactions of pentacyanoaquoferrate(II) with thiourea and N-substituted thioureas are presented. The substitution rate constants, measured in the pH range 2.8-9.0, show a hydrogen ion dependence [formula ommited] which is consistent with the presence of three reacting species: HFe(CN)5OH22-, Fe(CN)5OH23-, and Fe(CN)5OH4- The acid dissociation constant for the aquo species has been determined (KAH = (1.37 ± 0.20) × 10-8 M). The aquo species is found to be more reactive toward thiourea (KAH(25 °C) = 202 M-1 s-1) than the hydroxy species (&A(25 °C) = 100 M-1 s-1) while the protonated aquo species (kAH2(25 °C) = 2 M-1 s-1) is much less reactive. The dissociation rates of the Fe(CN)5TU3- (TU = thiourea) species are larger than those of complexes with N-donor ligands, indicating labile Fe-S bonds. An isokinetic plot of the activation parameters suggests different mechanisms operating in the formation (Id mechanism) and dissociation (D mechanism) reactions of Fe(CN)5TU3- species. © 1979, American Chemical Society. All rights reserved.