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

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⁺.