COORDINATION OF U4-) IN SOLID-STATE AND IN AQUEOUS-SOLUTION( IN THE COMPLEX U(P2W17O61)(2)(16)

The aim of this work is to understand the reasons for the selectivity shown in the complexation by unsaturated heteropolyanions (HPA) of act inides (An) which are oxidized to the number +IV. Different studies ha ve been carried out, both in solution and in solid state, on P2W17O611 0- and its complexes with Zn2+, UO22+, Ce4+, Th4+ and U4+ to character ize the nature of the complexation site offered to the cation. Among t he actinides(IV), the U4+ ion has been selected due to its singular sp ectroscopic and magnetic properties. An initial series of studies, in solution, using (NMRP)-P-31 has thus enabled us to characterize these complexes and to compare them to those formed with the complexing agen t PW11O398-. This body of data allows to identify an identical complex ation site for both ligands. An analysis of the (NMRP)-P-31 spectrum o f U(P2W17O61)(2)(16-) has shown a plane of symmetry passing through th e actinide and enables us to confirm the hypothesis of a transfer of i ntermetallic charge on the U-O-W bonds, observed in the visible absorp tion spectrum of the complex. The second part of the work is the study of U(P2W17O61)(2)(16-) in solid state. Following synthesis, elementar y and thermogravimetric analyses have confirmed the formula K-16[U(P2W 17O61)(2)]. 38 H2O of the compound. This solid, characterized by spect rophotometry of visible and IR absorption, shows absorption bands of e nergy identical to those of the coordination compound in solution. We then measured magnetic susceptibility as a function of temperature, ch aracteristic of the Curie-Weiss law. The Curie constant is characteris tic of a site occupied by the cation U4+ of D-4d symmetry (''Archimede s'' antiprism). This eight coordinate form derived from the cube is en tirely favourable to the stability of the actinide(IV) and, associated with a high charge density, tends towards selective complexation of t hese ions.