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

We have used EXAFS spectroscopy to investigate the inner sphere coordinatio n of trivalent lanthanide (Ln) and actinide (An) ions in aqueous solutions as a function of increasing chloride concentration. At low chloride concent ration, 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- complexati on 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 acro ss 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+, s how the same structural effects as the early Ln(3+) ions, i.e., Cl(-)ion re placement of the H2O at high chloride thermodynamic activities. The Cl(-)io n 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 pr eviously for Pu3+ which showed no significant chloride complexation in 12 M LiCl, these results suggest that the extent of chloride complexation is in creasing across the An(3+) series. The origin of the differences in chlorid e complex formation between the Ln(3+) and An(3+) ions and the relevance to earlier work is discussed.