COORDINATION (HYDRATION) OF RARE-EARTH IONS IN AQUEOUS CHLORIDE SOLUTIONS FROM X-RAY-DIFFRACTION .2. LACL3, PRCL3, AND NDCL3

The inner sphere water coordination of the rare earth ions La3+, Pr3+, and Nd3+ in concentrated (3.4 to 3.8 m) aqueous chloride solutions have been determined from x-ray diffraction measurements. In each solution the rare earth ion exists as the [RE(H2O) 9]3+ aquo complex as determined from the quantitative resolution of the radial distribution functions. The average RE 3+-H2O distances are 2.580, 2.539, and 2.513 Å for LaCI3, PrCI3, and NdCI3, respectively. The average RE3+⋯CI- ion pair distances are near 5.0 Å. These results, together with the octaaquo complex, [RE(H 2O)8]3+, found previously for the heavy rare earth ions, Tb3+ through Lu3+, establish that the inner sphere water coordination of the rare earth ions in aqueous solutions decreases from nine to eight between Nd3+ and Tb3+. © 1979 American Institute of Physics.