A kinetic study of the reduction of tetrachloroaurate(III) ions by the cyano complexes of iron(II), tungsten(IV) and molybdenum(IV)

Abstract

The kinetics of the reduction of tetrachloroaurate(III) by the cyano complexes of Fe(II), W(IV) and Mo(IV) have been studied in aqueous acidic medium. The reactions of Fe(CN)64− and W(CN)84− display first-order kinetics in both [AuCl4−] and [M(CN)n4−] (M = Fe and n = 6, W and n = 8) while [H+] has a retarding effect on the reaction rate for both the Fe and W reactions. An acid dissociation constant, Ka1, for the Fe(CN)64− reaction has been established kinetically to be Ka1 = (9.1 ± 0.5) × 10−3 M (pKa1 = 2.04) at µ = 1.2 M while the previously unknown Ka1 for the W(CN)84− reaction was found to be Ka1 = 0.33 ± 0.09 M (pKa1 = 0.48). Alkali metal cations such as Na+ and Cs+ and denoted as (cat+) accelerate the reaction. Some reactions were performed under second order (stoichiometric) conditions while other were performed under pseudo-first order conditions; rate constants utilizing these two techniques were mutually consistent. From the acid dependence and variation of the ionic strength of the reaction medium and the Bronsted-Bjerrum equation it has been established that AuIIICl4− and H(cat+)M(CN)n2− (M = Fe and n = 6 or M = W and n = 8) are the reactive species during the course of the reactions. The reaction between Mo(CN)84− and AuCl− is first order in [AuCl−] and second order in Mo(CN)84−. Reactions were performed under stoichiometric conditions and kinetic data was treated with a third order integrated rate law. The reaction is independent of [H+] and [cat+]. From the retarding effect of Cl− and Mo(CN)83− on the rate of the AuIIICl4− reduction, it is concluded that the molecular formula of the intermediate gold(II) species en route to AuICl2− is AuIICl3−. Activation parameters for all three reactions have been obtained from the Eyring equation.It was concluded that all three reactions follow the same mechanism but that the reverse reaction in the equilibrium between the intermediate gold(II) species, AuIICl3−, and the product MoV(CN)83− is only significant in the MoIV(CN)84− reduction of AuIIICl4−. This was also the cause for Mo(CN)84− showing a second order dependency.Graphical abstract