Aqua ions actinides

Table 11.1 Actinide aqua ions - numbers of bound water molecules and metal-water distances"...

E. Galhis, J. Herndndez-Cobos, C. den Auwer, C. L. Naour, D. Guillaumont, E. Simoni, R. R. Pappalardo, and E. Sdnchez-Marcos, Solving the hydration structure of the heaviest actinide aqua ion known the californium(III) case, Angew. Chem. Int. Ed, 49, 3811-3815 (2010). 

Rate Constants and Activation Parameters for Water Exchange on Actinides or Actinyl Aqua Ions... 

Chloride complexation studies of the actinide ions An3+, An4+, An02, and AnO + (An = U, Np, Pu) were reported in several comprehensive reviews [293-295], More recent investigations on aqua and chloro complexes of U02+, NpOz+, Np4+, Pu3+, etc., by x-ray absorption fine structure spectroscopy (XAFS) were reported [296,297]. In particular, it was established for U(IV) and Th(IV) aqua ions and fluoride complexes that both M(IV) aqua ions are 10-coordinate with M — O bond distances for U(IV) and Th(IV) of 2.42 0.01 A and 2.45 0.01 A, respectively [297], Physical and chemical studies of uranium aqueous complexes are reported [298,299a], A series of articles is dedicated to specific sequestering agents for the actinides [299b-e],... 

As for the lanthanides, actinide complexes display high coordination numbers. A study of the aqua ions of early actinides makes an interesting comparison (Table 11.1 lists numbers of water molecules and bond lengths). 

It will be noted that the transitions are often broader than those found in the spectra of lanthanide complexes - and indeed the later actinides, see Section 12.2.4. The 5f energy levels are more sensitive to coordination number than are the corresponding levels in the lanthanides since there are bigger crystal-field effects, one sees pronounced differences between the spectra of 6-coordinate [UCle] and of U" +(aq) (Figure 12.5), leading to the conclusion that the uranium(iv) aqua ion was not six coordinate (most recent EX-AFS results suggest a value of 9 or 10, see Table 11.1). Figure 12.6 displays another example of the difference in spectra between similar complexes of different coordination number. 

In addition to the nitrate system, AUen et al. (1997) reported a structural study of U02, Np02, Np ", and Pu " species in HCl solution by the EXAFS method. They also conducted (Allen et al. 2000) EXAFS experiments on the trivalent lanthanide and actinide ions in HCl solutions. In the dilute HCl solution, on average, the Am and Cm aqua ions are coordinated by 10.3 oxygens at 2.48 A and by 10.2 oxygens at 2.45 A, respectively, while 9.2 water molecules are coordinated to the Pu . The average distance between the Pu " ion and the oxygens of water molecules is 2.51 A. They observed Cl coordination to the Cm above the 10.5 M LiCl concentration. However, Pu does not form inner sphere chloride complexation. 

Note that AG° q has a liquid-phase standard state of 1 moI/L, and we can use a gas-phase standard state of either 1 atm or 1 mol/L, as long as we use the same convention for the oxidized and reduced forms. Often the 1 mol/L standard state is used. There are several sources of uncertainties in the calculations of reduction potentials, and we will comment on them by scanning the published literature on actinide elements, for which the most studied redox systems are actinyl aqua ions, with the exception of one study on Pu(VII)/Pu(VIII) [172], The first comment is that redox potentials are defined with respect to the standard hydrogen electrode corresponding to the following half-equation... 

Aqua species. The coordination number of tetravalent actinide ions and U " " has... 

Excited state luminescence properties of aqua lanthanide and actinide ions. ... 

For a metal cation in aqueous solution, hydrolysis occurs when the first-shell water molecule is sufficiently polarized to promote a proton transfer to a second-shell water molecule. The deprotonated species often forms a mixture of new aqua species and precipitates, depending on the pH and other thermodynamic conditions. Therefore, knowledge of the hydrolysis of actinide ions in aqueous solution enhances our ability to accurately predict the chemistry and thermodynamics of nuclear waste remediation. Hydrolysis occurs easily for highly charged actinide ions in solution, since the highly charged ion weakens the O-H bond in the first-shell water molecule, leading to the release of H (aq). 

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