Molar volumes and electrostriction behavior of dicarboxylate, disulfonate, tartrate, and bis(ethylenediamine)-glycinatocobalt(III) ions in water.

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The ionic partial molar volumes at infinite dilution, V∞(ion), were determined for several dicarboxylate ions, disulfonate ions, tartrate ions, and [Co(gly)(en)2]2+ (gly=glycinate ion and en=ethylenediamine) from density measurements at 25°C. The values of V∞(ion) obtained for the divalent anions generally increased with their van der Waals volumes. Significant differences in V∞(ion) were observed between geometrical isomers with dicarboxylate and disulfonate ions and were attributed to those in both of electrostriction volume and the void space volume around the anions. The former volume was decreased and the latter was increased with increasing distance between two negatively charged groups in the anions. A relatively strong electrostriction behavior of the tartrate ions, compared to the dicarboxylate ions, suggested the presence of an extra electrostriction caused by hydroxyl groups. The electrostriction strength of [Co(gly)(en)2]2+ was significantly large compared to that of [Ni(en)3]2+, rather close to those of [Cr(en)3]3+ and [Co(en)3]3+, and was attributed to the dissymmetrically localized charge distribution in the [Co(gly)(en)2]2+ ion. The effective ionic radii and apparent van der Waals volumes were estimated by using Glueckauf's equation from the ionic partial molar volume and were discussed in comparison with those calculated from the van der Waals increments of atoms.
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