The thermodynamic and electronic properties of some copper(II) five-co-ordinate cations in non-aqueous solution

Abstract

The standard thermodynamic functions for the equilibrium [CuL₂]²⁺+ X^–⇌[CuL₂X]⁺ with L = en, meen, dmen, amen (ethylenediamine, N-methylethylenediamine, NN′-dimethylethylenediamine, and NN-dimethylethylenediamine respectively) and X^–= Cl^–, Br^–, I^–, or SCN^– have been obtained in methanol at 25 °C. The equilibrium constants were measured spectrophotometrically. The heats of reaction were determined by calorimetric titration. The spectroscopic parameters and thermodynamic functions of the five-co-ordinate cations except for those of [Cu(amen)₂I]⁺ show a regular trend in each series of anions considered. These regularities indicate that the five-co-ordinate cations with a given chelate have the same stereochemistry. The anomalous values for the five-co-ordinate [Cu(amen)₂I]⁺ are probably due to a large distortion of the cation towards a trigonalbipyraidal configuration. The thermodynamic functions clearly show that the driving force of these reactions is the desolvation process, as all are entropy-stabilized. There is a linear correlation between the enthalpies of reaction and the enthalpies of anion desolvation. The different slope found for each [CuL₂]²⁺ complex is explained in terms of the steric hindrance of the methyl groups to the introduction of the donor anion into the co-ordination sphere of copper(II).