VOLUME CHANGES IN ION ASSOCIATION REACTIONS . INNER- AND OUTER-SPHERE COMPLEXES

Molar volume changes have been measured dilatometrically for substitution of water in Co(NH3)5H2O3+by various anions. They were found to decrease in the order OH- > CH3CH2COO- > SO42- > Cl- > Br- > N03-. This order corresponds to the degree of structuring of water by the anions. Volume changes were determined as well for complexing of Ce3+ by SO42-, CH3CH2COO-, Cl-, and NO3-, and of Eu3+ by NO3-. Comparison of both volume and entropy changes for the lanthanon complexes with the corresponding Co(III) systems support the view that sulfate and propionate complexes of Ce3+ are inner sphere while the chloride complex is outer sphere. However, the nitrate complexes of the lanthanons show intermediate behavior. The volume changes for outer-sphere complexing of Co(NH3)63+ and Co(NH3)5H2O3+ by sulfate were measured in dilute solution (I = 0.1) and found, unexpectedly, to be of the same magnitude as for inner-sphere substitution of sulfate in Co(NH3)5H2O3+. Again this behavior is paralleled by the entropy changes. Raising the ionic concentration to 1 M NaClO4 markedly lowers both the volume and entropy change on outer-sphere binding of SO42- to Co-(NH3)5H2O3+. The wide variability of both inner-sphere and outer-sphere values militates against the establishment of a general criterion for distinguishing between the two structural types based either on volume or entropy changes. Nevertheless, comparisons for related systems under closely similar conditions may be useful. © 1968, American Chemical Society. All rights reserved.