OUTER-SPHERE HYDRATION - A FACTOR DIFFERENTIATING TETRAKIS(ACETYLACETONATO)ACTINIDES(IV) IN LIQUID-LIQUID PARTITION SYSTEMS

Thermodynamics of water-heptane partition of tetrakisacetylacetonates of tetravalent metal ions of thorium, plutonium, berkelium, cerium, zi rconium and hafnium was studied. Linear relationship between the logar ithm of partition coefficient and the reciprocal of the metal-to-ligan d distance in the chelates has been found. Outer-sphere hydration of t he chelates in the aqueous phase is responsible for the observed diffe rences which are enthalpic in the origin. Shorter and more covalent is the metal-to-ligand bond in the chelate; stronger is the hydrogen bon d between the water molecules and the donor oxygen atoms in the ligand s. Comparison with actylacetonates of trivalent cations shows that hyd rophilic properties of coordinatively saturated metal chelates increas e with increasing charge of the central metal ion.