Coordination Chemistry of Trivalent Lanthanide and Actinide Ions in Dilute and Concentrated Chloride Solutions
journal contributionposted on 20.01.2000, 00:00 by P. G. Allen, J. J. Bucher, D. K. Shuh, N. M. Edelstein, I. Craig
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 are as follows: La3+, N = 9.2, R = 2.54 Å; Ce3+, N = 9.3, R = 2.52 Å; Nd3+, N = 9.5, R = 2.49 Å; Eu3+, N = 9.3, R = 2.43 Å; Yb3+, N = 8.7, R = 2.32 Å; Y3+, N = 9.7, R = 2.36 Å; Am3+, N = 10.3, R = 2.48 Å; Cm3+, N = 10.2, R = 2.45 Å. In ca. 14 M LiCl, the early Ln3+ ions (La, Ce, Nd, and Eu) show inner sphere Cl- complexation along with a loss of H2O. The average chloride coordination numbers and Ln−Cl bond lengths are as follows: La3+, N = 2.1, R = 2.92 Å; Ce3+, N = 1.8, R = 2.89 Å; Nd3+, N = 1.9, R = 2.85 Å; Eu3+, N = 1.1, R = 2.81 Å. The extent of Cl- ion complexation decreases going across the Ln3+ 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 Ln3+ ions, i.e., Cl- ion replacement of the H2O at high chloride thermodynamic activities. The Cl- ion coordination numbers and An−Cl bond lengths are: Am3+, N = 1.8, R = 2.81 Å; Cm3+, N = 2.4, R = 2.76 Å. 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 An3+ series. The origin of the differences in chloride complex formation between the Ln3+ and An3+ ions and the relevance to earlier work is discussed.