The Variation in the Standard Entropies of Aqueous Tripositive Lanthanide Ions

Previous published entropies of solid lanthanide trichloride hydrates are reviewed and revised by taking account of the Kramers degeneracy of ground states with half-integral values of J. The revised values necessitate corrections to the standard entropies of the aqueous tripositive lanthanide ions which display an irregular variation with both atomic number and ionic radius. These irregularities can be smoothed out by taking account of contributions from the electronic or magnetic entropy, from structural changes in the coordination that have been revealed by EXAFS and XANES measurements and X-ray crystallography, and from an increasing intrusion of eight coordination across the series. When these contributions are removed, the entropies of the aqueous ions refer to a hypothetical nine-coordinate state in trigonal triprismatic coordination. Within experimental error, they then vary uniformly across the lanthanide series and there is a good linear correlation with the reciprocal of the metal-oxygen distance in the nine-coordinate aqua complexes.