Coordination chemistry of trivalent lanthanide and actinide ions in dilute and concentrated chloride solutions

We have used EXAFS spectroscopy to investigate the inner sphere coordination of trivalent lanthanide (Ln) and actinide (An) ions in aqueous solutions as a function of increasing chloride concentration. At low chloride concentration, the hydration numbers and corresponding Ln,An-O bond lengths an as follows: La3+, N = 9.2, R = 2.54 Angstrom; Ce3+, N = 9.3, R = 2.52 Angstrom; Nd3+, N = 9.5, R = 2.49 Angstrom; Eu3+, N = 9.3, R = 2.43 Angstrom; Yb3+, N = 8.7, R = 2.32 Angstrom; Y3+, N = 9.7, R = 2.36 Angstrom; Am3+, N = 10.3, R = 2.48 Angstrom; Cm3+, N = 10.2, R = 2.45 Angstrom. In ca. 14 M LiCl, the early Ln(3+) ions (La, Ce, Nd, and Eu) show inner sphere C1- complexation along with a loss of H2O. The average chloride coordination numbers and Ln-CI bond lengths are as follows: La3+, N = 2.1, R = 2.92 Angstrom; Ce3+, N = 1.8, R = 2.89 Angstrom; Nd3+, N = 1.9, R = 2.85 Angstrom; Eu3+, N = 1.1, R = 2.81 Angstrom The extent of Cl- ion complexation decreases going across the Ln(3+) series to the point where Yb3+ shows no Cl- complexation and no loss of coordinated water molecules. The actinide ions, Am3+ and Cm3+, show the same structural effects as the early Ln(3+) ions, i.e., Cl(-)ion replacement of the H2O at high chloride thermodynamic activities. The Cl(-)ion coordination numbers and An-CI bond lengths are: Am3+, N = 1.8, R = 2.81 Angstrom; Cm3+, N = 2.4, R = 2.76 A. When combined with results reported previously for Pu3+ which showed no significant chloride complexation in 12 M LiCl, these results suggest that the extent of chloride complexation is increasing across the An(3+) series. The origin of the differences in chloride complex formation between the Ln(3+) and An(3+) ions and the relevance to earlier work is discussed.