ChemInform Abstract: Kinetics and Mechanism of Pentacyanohydroxoferrate(III) Formation from FeL Complexes (L: Triethylenetetraaminehexaacetic Acid and 2‐Hydroxy‐1,3‐diaminopropanetetraacetic Acid).

Abstract

Abstract The kinetics and mechanism of the displacement of two polyaminocarboxylates viz. , triethylenetetraaminehexaacetic acid (TTHA) and 2-hydroxy-1,3- diaminopropanetetraacetic acid (HPDTA) from their iron(III) complexes by the cyanide ion have been investigated spectrophotometrically. The forward reactions were carried out in the presence of excess cyanide at pH=10.5 ± 0.02 for [FeHPDTA(OH)] and 11.5 ± 0.02 for [FeTTHA(OH) 2 ] and I =0.25 M (NaClO 4 ). A variable order dependence in cyanide ranging from one to three was observed in the case of the [FeHPDTA(OH)] complex. In the case of [FeTTHA(OH) 2 ], however, a zero order dependence and only a third order dependence in cyanide was observed at low and high cyanide concentrations, respectively. The reverse reactions exhibited first order dependence on [Fe(CN) 5 OH] 3− and L n − , but an inverse first order dependence in cyanide. A five step mechanism has been proposed for the FeHPDTA(OH)CN − reaction. In the case of the FeTTHA(OH) 2 CN − reaction, however, the first step involves a slow loss of one OH − followed by five steps envisaged in the HPDTA case. The activation parameters and the effect of ionic strength have been used in support of the proposed mechanism. [Fe(CN) 5 OH] 3− reacts with excess cyanide to produce [Fe(CN) 6 ] 3− , which further reacts with the released ligand forming [Fe(CN) 6 ] 4− and the oxidation products of the ligands.