KINETICS AND MECHANISM OF THE REACTION BETWEEN MONO ((ETHYLENEDIOXY) DIETHYLENEDINITRILO)-TETRAACETATODINICKELATE(II) AND CYANIDE ION

Abstract

The kinetics of ligand substitution at the binuclear nickel(II) chelate Ni₂L, where L is EGTA ((Ethylenedioxy)diethylenedinitrilo)tetraacetic acid with cyanide ion has been investigated spectro-photometrically. The reaction conditions were pH = 11.0 ± 0.1, μ = 0.1 M (NaClO₄) and T = 25 ± 0.1°. The formation of Ni(CN)₄ ^2- is first order in Ni₂L and first-order in cyanide over a large range of cyanide concentration, but below 10⁻³ M the reaction becomes zero order in cyanide. These results indicate the presence of a slow step in which Ni₂ L dissociates to give NiL^2-n and Ni_(aq) ²⁺ at low cyanide concentrations and a cyanide assisted rapid dissociation of Ni₂L to produce NiL(CN)_x ^2-n-x and Ni_(aq) ²⁺ at higher cyanide concentrations. These species react with excess cyanide finally producing Ni(CN)₄ ^2-. On the other hand, reaction of Ni(CN)₄ ^2- with EGTA is first-order in both reactants and inverse-order in cyanide. These results lead to a five step mechanism in which the fourth step has been identified as rate determining. The rate constants have been calculated and results compared with earlier work on binuculear complexes.