Actinide complexes seven-coordination

Ureas. Urea adducts (and those of the closely related A-alkylated derivatives) may be prepared from nonaqueous solvents alternatively, preparation in aqueous alcoholic solution leads to the formation of hydrates. In contrast to the carbamides discussed above, there is relatively little variability in the coordination number of reported urea adducts of tetravalent actinides. Most complexes are either six- or seven-coordinate higher coordination numbers are observed for the larger thorium ion (Table 15). 

The trivalent transplutonium halides have been extensively studied. Several reviews deal specifically with actinide halides. " In aqueous solution the mono- and bis-complexes have been characterized, with the formation of the latter decreasing down the halide series. For example, AmF and AmF2" have been studied, but only a very weak monochloride complex AmCP has been reported. These species are reported to have coordination numbers as high as 11, although recent EXAFS studies show that the hydration number decreases with increasing halide concentration (and ionic strength) at concentrations below which the halo complexes form. These data suggest the coordination numbers of the mixed aquo halo complexes are probably seven to ten. 

---