Conductivity and Ion Association of Some Diastereoisomeric Salts in Water. II. Ion Association between the Δ(−)589- or Λ(+)589-cis-Bis(ethylenediamine)dinitrocobalt(III) Ion and the Δ(+)589-(Ethylenediamine)bis(malonato)cobaltate(III) Ion and Its Temperature Dependence

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Abstract
The electric conductivities of aqueous solutions of the diastereoisomeric salts, Δ(−)589-cis-[Co(NO2)2(en)2]Δ(+)589-[Co(mal)2(en)] and Λ(+)589-cis-[Co(NO2)2(en)2]Δ(+)589-[Co(mal)2(en)](en=ethylenediamine and mal=malonate ion), have been measured at several temperatures from 15 to 35 °C. From the analysis of the conductivity data using the Pitts equation, it was found that the Δ- or Λ-cis-[Co(NO2)2(en)2]+ and Δ-[Co(mal)2(en)]− ions are appreciably associated in spite of the large monovalent ions and that the ion association is almost nonstereoselective and is endothermic in contrast to the exothermic ion association between Δ- or Λ-cis-[Co(NO2)2(en)2]+ and Δ-[Co(edta)]− (edta=ethylenediaminetetraacetate ion). By considering the obtained values of thermodynamic parameters of ion association and the significantly large Stokes radius of Δ-[Co(mal)2(en)]− compared to the effective radius estimated from its partial molar volume, this complex anion was presumed to have two hydration water molecules and to release one of them by associating with the complex cations. Possible short-range interactions between the complex ions were also discussed by considering the structures of the complex ions and the chemical properties of the ligands.

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