Complex Formation and Solvation of [CuCln](2–n)+ in Acetonitrile and in N,N-Dimethylformamide

Abstract

Abstract Complex formation equilibria between copper(II) and chloride ions have been studied by calorimetry and spectrophotometry in acetonitrile(AN) containing 0.1 mol dm−3 (C2H5)4NClO4 as a constant ionic medium at 25 °C. Both calorimetric and spectrophotometric titration curves could be well explained in terms of formation of a series of four mononuclear complexes, [CuCln](2−n)+ (n=1–4), and formation constants, enthalpies and entropies of formation of the complexes were determined. Stepwise enthalpies ΔHn° (n=1–4) were all negative, the ΔH4° value being especially largely negative. On the other hand, the stepwise entropy ΔSn° was largely positive for n=1–3, but largely negative for n=4. The result suggested that solvation of the [CuCl3]− complex was extremely weak in AN and that release of solvent molecules from [CuCl3]− was not so significant that the enthalpy and entropy of formation of [CuCl4]2− were mainly attributable to those of desolvation of Cl− followed by a bond formation between the [CuCl3]− complex and the chloride ion. The [CuCl4]2− complex might have no solvent molecules in the primary coordination sphere of the metal ion within the complex in AN. Enthalpies of solution of anhydrous CuCl2 crystals were measured in AN and N,N-dimethylformamide(DMF) in order to evaluate enthalpies of transfer ΔHt° of Cu2+ from water to the aprotic solvents. The ΔHt° values well correlated with Gutmann’s donor number of the solvents. The enthalpies of transfer ΔHt° of [CuCln](2−n)+ (n=0–4) from AN to DMF were evaluated, and the enthalpy values monotonously increased with n. The ΔHt°(CuCl42−) value was only slightly positive as well as ΔHt°(Cl−), and the result also suggested that the metal ion within the complex was not primarily solvated with solvent molecules in both AN and DMF. Electronic spectra of individual copper(II) chloride complexes were determined in AN.