The Effect of Temperature on the Equivalent Conductivities and Ion-Association Constants of Some Tris-(ethylenediamine)chromium(III) Complexes in N,N-dimethylformamide and N,N-dimethylacetamide

The equivalent conductivities of tris-(ethylenediamine)chromium complexes, [Cr(en)3]X3 (where X−= Cl−, Br−, I−; en = ethylenediamine) were measured as functions of temperature (278.15 to 328.15 K) and concentration [(1.948 ×10−4 to 10.728 ×10−4 mol⋅dm−3) and (2.282 ×10−4 to 11.246 ×10−4 mol⋅dm−3)] in N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMAC), respectively. Equivalent conductivity values for [Cr(en)3]X3 in DMF were found to be higher than those in DMAC. The conductivity data were analyzed with the Robinson-Stokes equations. For [Cr(en)3]X3, the limiting equivalent ionic conductivities of [Cr(en)3]3+ and the ion-association constants (KA) of the ion-pair between [Cr(en)3]3+ and the monovalent halide anions were determined in DMF and DMAC. The values of KA for three complex salts in DMF were higher than those in DMAC. This can be ascribed to an increase of the ion-association constants with a decrease of the relative permittivity of the solvents. The values of KA at 298.15 K decreased in the order Cl−> Br−> I− in DMF and Cl−> I−> Br− in DMAC. The KA values for [Cr(en)3]Cl3 increased with increasing temperature in both DMF and DMAC. For [Cr(en)3]X3(X−= Br−, I−) in both solvents, this indicates increasing disorder occurs with increasing temperature.