Potentiometric studies in transition metal ion ( d5− d10) chelates of substituted salicylhydroxamic acids

Abstract

The interactions of transition metal ions (Mn 2+Zn 2+) with salicylhydroxamic acid (SHA) and 5-methyl, 5-chloro, 5-bromo, 5-nitro, 4-chloro, 4-bromo and 3-chloro salicylhydroxamic acids were investigated potentiometrically by the Calvin-Bjerrum titration technique at 30±0.1°C and ionic strength μ = 0.1 M(NaClO 4) in 50% v v dioxane-water mixtures. It was observed that all the metal ions form only 1:1 chelates with these ligands. The dissociations of the -NH and the -OH groups are observed potentiometrically. The NOH group does not dissociate even at very low concentrations of hydrogen ions. In SHA and its derivatives, p K 1 is ascribed to the dissociation of the -NH group and p K 2 to the dissociation of the phenolic -OH group. Arguments are put forward for these assignments. The effects of various substituting groups on the p K 1 and p K 2 values are discussed. Irving-Williams order is not completely obeyed, though the latter part of the order, i.e. Ni 2+ < Cu 2+ > Zn 2+ is followed by the chelates. Explanations are suggested. The validity of the log K = a p K + b relation is examined for these chelates and the significance of dπ − pπ interaction in the case of Cu 2+-5-nitro-SHA chelate is discussed.