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(I1) chelate Ni,L, where L is EGTA ((Ethy1enedioxy)diethylenedinitrilo)tetraacetic acid with cyanide ion has been investigated spectrophotometrically. The reaction conditions were pH = 11.0 i 0.1, g = 0.1 M (NaCIO,) and T = 25 t 0.1". The formation of Ni(CN):-is fjlst order in Ni,L and first-order in cyanide over a large range of cyanide concentration, but below M the reaction becomes zero order in cyanide. These results indicate the presence of a slow step in which Ni,L dissociates to give NilZ-" and Nifi at low cyanide concentrations and a cyanide assisted rapid dissociation of Ni,L to produce NiL(CN$-n-x and Niy8) at higher cyanide concentrations. These species react with excess cyanide finally producing NI(CN , On the other hand, reaction of Ni(CN):-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.