Conductometric Study of Ion Association between Tris(ethylenediamine)cobalt(III) Ion and Monovalent Anions at Temperatures from 0 to 50 °C

Abstract

Abstract The conductivities for aqueous solutions of the chloride, bromide, iodide, nitrate, and perchlorate of the tris(ethylendiamine)cobalt(III) complex, [Co(en3)]3+, were measured with varying temperatures from 0 to 50 °C. The ion-association constants (KA) of the complex ion with the anions had minimum values at temperatures (tmin) characteristic of the salts. The values of tmin increased in the order Cl− < Br− < I− < NO3− < ClO4−. The order in magnitude of KA at 25 °C was ClO4− < NO3− < I− < Cl− < Br−, but was somewhat dependent on the temperature. Both the KA and tmin values for the halides of [Co(en)3]3+ were larger than those of the hexaamminecobalt(III) complex, [Co(NH3)6]3+; the difference in KA was greatest for the iodide. The entropy and enthalpy changes in the ion association of [Co(en)3]3+ with the halide ions were smaller than those of [Co(NH3)6]3+. These results suggested that the hydration of [Co(en)3]3+ was relatively weak and its hydration water molecules in the vicinity of the methylene groups could be more easily substituted by the structure-breaker anions, compared with the water molecules around the amino groups. The KA and tmin values for the nitrate and perchlorate of [Co(en)3]3+ were smaller than those of [Co(NH3)6]3+, and both the entropy and enthalpy changes of ion association for the former salts were more positive. This was attributed to the decrease in the fraction of the contact ion pairs formed by the hydrogen bonding between the polar hydrogen atoms of the complexes and the oxygen atoms of the anions.