Coordination Chemistry of Trivalent Lanthanide and Actinide Ions in Dilute and Concentrated Chloride Solutions

Abstract

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 A; Ce3+, N = 9.3, R = 2.52 A; Nd3+, N = 9.5, R = 2.49 A; Eu3+, N = 9.3, R = 2.43 A; Yb3+, N = 8.7, R = 2.32 A; Y3+, N = 9.7, R = 2.36 A; Am3+, N = 10.3, R = 2.48 A; Cm3+, N = 10.2, R = 2.45 A. 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 A; Ce3+, N = 1.8, R = 2.89 A; Nd3+, N = 1.9, R = 2.85 A; Eu3+, N = 1.1, R = 2.81 A. 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 Clion coordination numbers and An-Cl bond lengths are: Am3+, N = 1.8, R = 2.81 A; 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 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.

A Dynamic Model to Explain Hydration Behaviour along the Lanthanide Series

“…The distances between Ln 3 + and the oxygen atoms of the water molecules in the first hydration shell (Figure 1 a) have an almost linear dependence on the atomic number. The differences in r LnÀO values between our data and experimental data [25,30,31] are no larger than 0.05 , and most of them lie within experimental uncertainty. On the other hand, the average coordination numbers (Figure 1 b), obtained as the Experimental data are also reported for comparison: (*), [25] (^) [30] and (~).…”

Molecular solids of actinide hexacyanoferrate: Structure and bonding

“…These values are in very good agreement with our former result : 8.5 O at 2.42 Å. In comparison, the Am-O aq distance has been reported to be 10.3 O at 2.48 Å [23] in HCl solution. The decrease of 0.06 Å between the aquo forms of Am 3+ and Cf 3+ is comparable to that of 0.05 Å for the nitrogen coordination sphere of the hexacyanometallates described above, although a direct comparison is not rigorous because of the possible changes in coordination numbers that are not discussed here.…”

Molecular Dynamics Study of the Hydration of Lanthanum(III) and Europium(III) Including Many-Body Effects

“…9,10 More recent EXAFS studies are not useful to distinguish unambiguously between a CN of 8 or 9 for Ln 3+ ions as concluded by the authors. 11 A second coordination shell is clearly identified with about 16 water molecules. Experimental data for this shell are very scarce with large uncertainties.…”