F-ELEMENT CROCONATES .2. THORIUM(IV) AND DIOXO-URANIUM(VI) CROCONATES - SYNTHESIS, CRYSTAL-STRUCTURE AND THERMAL-BEHAVIOR

Two complexes of actinide croconates, thorium(IV) and dioxo-uranium(VI) were prepared: Th(H2O)7(C5O5)2 and UO2(H2O)K2(C5O5)2. The crystal structure of these complexes was determined by X-ray single crystal technique. The thorium croconate hydrated crystallizes in the orthorhombic system, space group Pnma. It is made of discrete, neutral entities. In this complex the croconate ligand is only monodentate. The uranyl hydrated potassium croconate crystallizes in the monoclinic system, space group C2/c. This complex has a 3D structure presenting tunnels in which are localized the water molecules bound to the uranium atom. The outstanding feature of this structure is the high number (seven) of metals bound to each croconate ligand. This latter is bis-monodentate towards the uranium atom, bis-chelating towards the potassium atom and the two oxygen atoms are bound to one supplementary potassium atom. The thermal behaviour of these complexes was studied. For the thorium croconate hydrated dehydration occurs first, followed by the decomposition of the croconate ligand which yields thorium oxide, ThO2. A more complex and interesting behaviour is noticed for the complex UO2(H2O)K2(C5O5)2. The products formed at the outset of the decomposition seem to influence the further stages.