The coordination (hydration) of rare earth ions in aqueous chloride solutions from x-ray diffraction. II. LaCl3, PrCl3, and NdCl3a)

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Abstract
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 RE3+–H2O distances are 2.580, 2.539, and 2.513 Å for LaCl3, PrCl3, and NdCl3, respectively. The average RE3+⋅⋅⋅Cl− ion pair distances are near 5.0 Å. These results, together with the octaaquo complex, [RE(H2O)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+.