Eight-coordinate actinide complexes

Table5. Geometry of monomeric eight-coordinate actinide complexes with bidentate ligands...

Table III. Geometry of Monomeric Eight-Coordinate Actinide Complexes with Bidentate Ligands...

All early actinides from thorium to plutonium possess a stable +4 ion in aqueous solution this is the most stable oxidation state for thorium and generally for plutonium. The high charge on tetravalent actinide ions renders them susceptible to solvation, hydrolysis, and polymerization reactions. The ions are readily hydrolyzed, and therefore act as Bronsted acids in aqueous media, and as potent Lewis acids in much of their coordination chemistry (both aqueous and nonaqu-eous). Ionic radii are in general smaller than that for comparable trivalent metal cations (effective ionic radii = 0.96-1.06 A in eight-coordinate metal complexes), but are still sufficiently large to routinely support high coordination numbers. 

Octacoordination is now recognized to be common for many lanthanide and actinide complexes. Among the < -transition metal ions, however octacoordination is less frequent. The d, d, d and d configurations are known to produce eight coordinated complexes (97). In an excellent review Lvppard (92) surveyed the chemistry of the octa coordinated complexes, especially stereochemical and bonding aspects, and included most of the published material upto 1967. 

Eight coordination frequently occurs in lanthanide and actinide fluoride complexes. The preponderance of these octacoordinate compounds can be assigned to one of two idealized coordination polyhedra, namely the square antiprism, symmetry (D4, ), or the dodecahedron (Dg ) with triangular faces. Of the remaining idealized eight-coordinate polyhedra, some are not represented and others have but few representatives in this class of compounds. 

The actinide nitrates whose structures are known are listed in Table 20.9, and a review of actinide complexes has been published by Casellato et al [413]. These are limited mainly to tetravalent Th and to uranyl complexes. In the Th(iv) and Pu(iv) compounds the NO3 ions are bidentate, which allows for large coordination numbers compared to the usual eight-fold coordination of these ions with monodentate ligands. In the uranyl complexes the two close actinyl oxygen atoms limit the available space for other bonded atoms, and the maximum number of equatorial oxygen atoms is six, even for bidentate NO3 ions. In Cs2lJ02(N03)4 all four NO3 ions are attached to the U atom even though space allows only two of them to be bidentate. 

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