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An independent kinetic and mechanistic study of the secondary reactions in the substitution of [FeL(OH)](n−2)− (L= triethylenetetraaminehexaacetic acid) by cyanide ions

Research paper by Surendra Prasad, Prem C. Nigam, Radhey M. Naik

Indexed on: 01 Feb '90Published on: 01 Feb '90Published in: Transition Metal Chemistry



Abstract

The kinetics of reaction between [Fe(CN)5OH]3− and CN− have been investigated spectrophotometrically at pH=11.00, I=0.25 M(NaClO4) and temp.=25.0°C by disappearance of the absorption peak at 395 nm. The rate data for this reaction followed first order kinetics in both [Fe(CN)5OH3−] and [CN−]. The second order rate constant (kf) was found to be (3.44±0.08)×10−3 M−1 s−1. The pH dependence of the reaction was also investigated in the range 9–12. The activation parameters were found to be ΔH†=36.4kJ mol−1 and ΔS†=−168JK−1 mol−1.The reaction between [Fe(CN)6]3− and TTHA6− (TTHA=triethylenetetraaminehexaacetic acid) has also been followed spectrophotometrically at 420 nm, pH=11.00, I=0.1M (NaClO4) and temp.=25.0°C. This reaction also followed first order kinetics in both [Fe(CN)63−] and [TTHA6−]. The second order rate constant (kf) was found to be (3.74±0.21)×10−2 M−1 s−1. The rate of reaction was found to increase with pH in the range 9–11.5. The different reactive species of TTHA (L) are H2L4− HL5− and L6−. The rate constants for these species have been calculated and the pH profile is explained. The values of the activation parameters were found to be ΔH†= 30.9 kJmol−1 and ΔS†=−167JK−1 mol−1. Electron transfer from [Fe(CN)6]3− to the substrate followed by decomposition of the latter is proposed. The oxidation products of TTHA have been investigated by g.l.c.