Visible spectrophotometric determination of metal ions: the influence of structure on molar absorptivity value of copper(II) complexes in aqueous solution

Abstract

In this study, the structure–absorptivity relationship problem is dealt with, on the basis of a considerable mass of data (117 complexes) concerning equatorial complexes of copper(II) in aqueous solution. Some spectrophotometric measurements have been carried out and as well as some molar absorptivity calculations on copper(II)–ammonia ( ε 640=77 M −1 cm −1 for the Cu(NH 3) 4 2+ species) and copper(II)–biuret ( ε 505=46 M −1 cm −1 for the Cu(C 2H 3N 3O 2) 2 2− species) systems have been made, in order to complete the quantitative information necessary for the discussion. It was found that the qualitative relationship between ε max and λ max, for the complexes in question is traceable back to a hyperbolic type of path; the dispersion of the data in the cartesian plane ( ε max, λ max) is analyzed and discussed. It has been found that the dissociated peptide nitrogen is, amongst those under study (namely: amino nitrogen; peptide nitrogen; imidazole nitrogen; carboxylate oxygen; alcoholate oxygen; hydroxide oxygen; water oxygen), that which corresponds to the major increase of ε max for copper(II). On the contrary, the one which corresponds to the minor increase is the imidazole donor (that is pyridine nitrogen of imidazole residue), which manifests the ability to lower the ε max values of the equatorial complexes of copper(II). Moreover, contrary to what has been previously demonstrated for λ max, for ε max it does not appear to be correct to schematize the ligand with its donor groups in order to assess spectrum–structure correlations. Consequently, this study contains a type of methodological proposal, even beyond the metal ion specifically in question, for the analyses of the selection criteria of the molecular characteristics that a ligand must possess in order to form a co-ordination compound of analytical interest with a determined metal ion.