Thermodynamics of complex formation in aqueous solution. Reactions of copper(II) with ethylenediamine, NN′-dimethylethylenediamine, and NN-dimethylethylenediamine: log K, ΔH, and ΔS values

Abstract

Equilibria in 0·5M-KNO₃ at 25° between copper(II) and the ligands (L) ethylenediamine (en), NN′-dimethylethylenediamine (dmen) and NN-dimethylethylenediamine (admn), have been investigated by e.m.f., calorimetric, and spectrophotometric measurements. In all these systems the same four complex species have been found. The equilibrium constants for Cu²⁺+nL ⇄[CuL_n]²⁺(n= 1 or 2) and nCu²⁺+nL + 2OH-⇄[Cu_n(OH)₂^–L_n](^2n-2)+(n= 1, hydrolysis; n= 2, olation) have been obtained from potentiometric data using a computer. The enthalpies of reaction were obtained by calorimetric measurements using two different kinds of calorimeters. Spectrophotometric measurements have been carried out and the spectra of individual complexes (n= 1 or 2; L = dmen, admn) have been calculated. N-methyl substitution reduces the heat of reaction and increases the positive entropy changes.