Thio derivatives of β-diketones and their metal chelates. XI. Stability constants of nicke(II), copper(II). zinc(II), cadniium(II), and lead(II) chelates of two monothio-β-diketones

Abstract

The acid dissociation constant (pK_D) of the monothio derivative of benzoyl-acetone, viz. 3-mercapto-1-phenylbut-2-en-1-one (I), was found to be 10.43 in 74.5 vol. % dioxan-water solution. Stoicheiometric stability constants (log k_n) of five metal chelates of (I) and of the monothio derivative of dibenzoylmethane, viz. 3-mercapto-1,3- diphenylprop-2-en-1-one (II), have been measured in 74.5 vol. % dioxan- water solution with both chloride and perchlorate as supporting electrolyte.     With chloride as supporting electrolyte the values obtained for log k_av are as follows: with ligand (I), copper(II) 10.11, nickel(II) 9.25, zinc(II) 6.68, lead(II) 6.06, cadmium(II) 6.12; with ligand (II), copper(II) 10.66, nickel(II) 10.20, zinc(II) 7.44, lead(II) 7.11, cadmium(II) 6.90. With perchlorate as supporting electrolyte zinc, lead, and cadmium give values of log k_av which are 1.5-1.9 units greater. The thermodynamic stability constants (logK_n) were calculated from the stoicheiometric constants by use of the equations            Log K₁ = logk₁ + 2.30        logK₂ = log k₂ + 1.15 For the copper and nickel complexes of both (1) and (II) log K_av is greater than pK_D; such a situation has been reported in only a few instances.     From the data obtained with chloride as a supporting medium, approximate stoicheiometric stability constants were calculated for the chloro complexes MCl⁺ (M = Zn, Cd, Pb) in 74.5 vol. % dioxan. The values obtained were: ZnCl⁺, 5.0; CdCl⁺, 5.7; PbCl⁺, 5.0.