Actinide bond distances

Table 5 Mulliken Populations (Per Atom), Bond Distances, and Bond Energies (Do) for the Early di-Actinides.

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],... 

For those actinides stable in the pentavalent state, ions of the form An02(H20)x+ (An = Np, Pu x = 4, 5, 6) have been postulated. Pa is not included in this list since it readily hydrolyzes in aqueous solutions to form Pa205-xH20. All of the structural data for these complexes come from XAFS. The bond distances for both An=0 (1.83 A for both Np and Pu) and An-OH2 (2.51 and 2.5 A, respectively) expand in the... 

The best-studied aqueous actinide halide systems are the fluorides and the chlorides. Fluoride and chloride ions are added to the actinyl centers in a stepwise fashion (Scheme 4). The end member in the actinyl fluoride system is the pentafluoride, An02F5 . For the uranyl analog, the U = O and U-F bond distances were found to be 1.79 and 2.18A respectively. The uranium tetrafluoride ion, U02F4 , has been isolated as a dimer with two bridging fluoride ligands. The U = O, U-Fterminai, uud U-Fbridging distauces for this complex were found to be 1.79, 2.15 to 2.20 A, and 2.30 A, respectively. 

Protactinium tetrafluoride, like the other actinide tetrafluorides, possesses the 8-coordinate UF4-type of structure (Table III) but no bond distances are available. It is easily the most stable tetravalent halide of protactinium and can be handled in the atmosphere, at least for a limited period, without hydrolysis or oxidation occurring. As mentioned earlier it is the usual starting material for the preparation of protactinium metal. Tetrafluoride hydrates have not been fully characterized, but a mixed fluorosulfate, PaF2S04 2H20 can be precipitated from aqueous solution (131). Protactinium tetrafluoride is soluble in aqueous ammonium fluoride solutions, for which some spectral properties have been recorded (4, 83). 

Unit cell dimensions for the tetravalent protactinium fluoro complexes are listed in Table VIII. LiPaFs is a member of an isostructural series of 1 1 complexes formed by the actinide elements thorium to curium inclusive (91). Structural details are available for LiUP5 (5S), but bond distances have not been reported for the protactinium complex. The 7 6 complexes, like their actinide(IV) analogs (Th-Cm with Na, Th-Cm with K, and Th-Pu with Rb) are all 4, 114) isostructural with Na7Zr F3 (60) in which each zirconium atom is 8-coordinate and the... 

Analogous and carbonates can be assumed to be isostructural with the well-characterized uranyl compounds, albeit with slightly shorter bond distances to reflect the actinide contraction. For the triscarbonato complexes of Np this has been confirmed by the structures of K4Np02(C03)3 by single-crystal structure determinationof [(CH3)4N]Np02(C03)3. " ... 

The study of the U(IV)-chloride system is relevant for the discussion of chloride complexes also for other tetravalent actinides, viz. Th". The study describes the use of EXAFS methods to determine the composition and bond distances in the first coordination sphere of uranium. The complexes studied are weak and the anthors have used factor analysis to separate the spectra for the two species present in the EXAFS test soln-tions. The number of coordinated water and chloride ligands was determined from these spectra. The authors report two alternative interpretations of their EXAFS results ... 

Table 9 Important bond distances in actinide complexes containing O- or N- or S-donor ligands...

In general, M-C distances in transition metal complexes vary in the 190-240 pm range. Greater distances are encountered in the case of alkyl compounds of the lanthanides and the actinides. For a given metal the bond distance decreases according to the series ... 

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