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

Abstract

The equivalent conductivities of tris-(ethylenediamine)chromium complexes, [Cr(en)(3)]X-3 (where X-= Cl-, Br-, I-; en = ethylenediamine) were measured as functions of temperature (278.15 to 328.15 K) and concentration [(1.948 x10-4 to 10.728 x10-4 mol.dm(-3)) and (2.282 x10-4 to 11.246 x10-4 mol.dm(-3))] in N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMAC), respectively. Equivalent conductivity values for [Cr(en)(3)]X-3 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)]X-3, the limiting equivalent ionic conductivities of [Cr(en)(3)](3+) and the ion-association constants (K-A) of the ion-pair between [Cr(en)(3)](3+) and the monovalent halide anions were determined in DMF and DMAC. The values of K-A 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 K-A at 298.15 K decreased in the order Cl- > Br- > I- in DMF and Cl- > I- > Br- in DMAC. The K (A) values for [Cr(en)(3)]Cl-3 increased with increasing temperature in both DMF and DMAC. For [Cr(en)(3)]X-3(X- = Br-, I-) in both solvents, this indicates increasing disorder occurs with increasing temperature.