Study of mechanistic features of tetrahedral-octahedral interconversion of tetracyanozincate (II) tobis(4-(2hyphen;Pyridylazo)hyphen;resorcinol)zincate(II) by stopped-flow technique

Abstract

The present study examines the kinetics and mechanism of the formation of [Zn(CN)4]2− from ZnR22− (R represents 4-(2-Pyridylazo)resorcinol, short named as Par) and vice versa. The reactions have been followed at 494 nm (λmax of [Zn(Par)2]2−, e = 7.8 × 104 M−1 cm−1) using stopped-flow spectrophotometer in presence of excess of incoming ligand. The data show that the formation of [Zn(CN)4]2− complex is first order in [ZnR22−] and the order with respect to [CN−] varies from one at higher cyanide concentration to zero at low [CN−]. These observations suggest a slow dissociation of [ZnR2]2− to ZnR and R2− and a cyanide assisted rapid dissociation of [ZnR2]2− to [ZnR(CN)x]x− followed by their rapid conversion to [Zn(CN)4]2−. The reaction of Zn(Par) with cyanide ions also follow first order kinetics at higher as well as lower cyanide concentrations. The reverse reaction exhibits first order dependence each in [Zn(CN)42−] and [Par2−], but an inverse first order dependence in [CN−] at low Par concentrations. However, at higher Par concentration the reaction rate tends to be independent of [Par2−]. On the basis of forward and reverse rate studies, a five step mechanism consistent with these results has been proposed. The activation parameters and the effect of ionic strength have been used in further support to the proposed mechanism. The effect of pH on the rates of forward and reverse reaction has also been investigated.