Complex formation equilibria between 2-amino-N-hydroxyacetamide and 2–amino-N-hydroxypentanamide and cobalt (II), nickel(II), copper(II), and hydrogen ions in aqueous solutions

Abstract

The formation constants of the complexes formed between H⁺, Co²⁺, Ni²⁺, and Cu²⁺ and 2-amino-N-hydroxyacetamide (aha) and 2-amino-N-hydroxypentanamide (ahp) have been measured at 25 °C and I= 0.5 mol dm^–3(KCl) using potentiometric techniques. Their acid–base and metal–ligand reaction stoicheiometries have been determined, and the equilibrium constants of the corresponding proton and metal ion association constants have been calculated from potentiometric data with the aid of the program SUPERQUAD. The equilibrium chemical models are selected on the basis of a critical evaluation of the least-squares results and of a statistical analysis of the weighted residuals. Interpretation of the results was facilitated by visible spectrophotometry of solutions containing metal ions (M), and aha or ahp (HL) at various pH. All metals form the mononuclear ([ML]⁺ and [ML₂]) and hydroxo complexes with both ligands, whereas with copper(II) polynuclear species are also present. The ligands are bound to the metals through co-ordination of the aminohydroxamate moiety, via the N atom of the amino group, and by the deprotonated NHO^– group; the square-planar complexes are probably held together by quite strong intermolecular hydrogen bonds involving the unco-ordinated ketonic oxygen atom.