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Title: Kinetics and mechanism of pentacyanohydroxoferrate(III) formation from the reaction of [FeL(OH)]2-n complexes with cyanide ions [Ln- =trans-1 ,2-diaminocyclohexanetetraacetic acid (CYDTA) and nitrilotriacetic acid (NTA)]
Authors: Naik, Radhey M
Chaturvedi, Desh D
Srivastava, Neena
Verma, Amit K
Tewari, Anjani K
Agarwal, Abhinav
Issue Date: Nov-2004
Publisher: NISCAIR-CSIR, India
Abstract: The kinetics and mechanism of exchange of CYDTA4- in [FeCYDTA (OH)]2- and NTA3- in [FeNTA(OH)]l- with cyanide ions (CYDTA 4- = trans-1,2-diaminocyclohexanetetraacetic acid and NTA = nitrilotriacetic acid) have been investigated spectrophotometrically at 395 nm (λmax of [Fe(CN)5OH]3-) under the conditions, temp. = 45 ± 0.1 °C, pH = 11±0.02 and I= 0.25 M(NaClO4) for CYDTA and temp. 25±0.1oC, pH =9.0 ± 0.02 and I=0.4 M (NaClO4) for NTA. Both the reactions exhibit three observable stages leading to the formation of [Fe(CN)5OH]3-, [Fe(CN)6]3- and [Fe(CN)6]4- respectively. The [FeCYDT(OH)]2--CN- as well as [FeNTA(OH)]1--CN- systems show variable order dependences in [CN-] in their first stage, ranging from one to two and zero to one at low and high cyanide concentrations. The second stage of reaction is common for both the systems and follow a first order dependence each in [Fe(CN)5OH3-] and [CN-].The third stage of reaction follows a overall second order kinetics, first order each in [Fe(CN)5OH3-] and [Ln-] (Ln- = CYDTA 4- and NTA3-). The thermodynamically unfavourable reverse reaction of [Fe(CN)5OH3-] with [CYDTA]4- and [NTA3-] have also been studied under forcing conditions by taking large excess of [CYDTA]4- and [NTA]3-.These reactions exhibit first order dependence each in [Fe(CN)5OH3-]  and [CYDTA4-] or [NTA3-] and an inverse first order dependence in [CN-] which makes it possible to identify the fourth step as rate determining one. The dependence of forward rate on ionic strength also confirms that the fourth step is rate determining in the porposed reaction mechanism for the first stage of reaction. A five step mechanistic scheme is proposed for both the systems in their first stage of reaction involving the presence of four cyanide ions around the central iron atom in the rate determining step. The activation parameters for both forward and backward reactions of the first stage of reaction are evaluated and they support the proposed mechanism.
Page(s): 2307-2314
ISSN: 0975-0975(Online); 0376-4710(Print)
Appears in Collections:IJC-A Vol.43A(11) [November 2004]

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