STRUCTURAL INVESTIGATIONS OF N,N'-SUBSTITUTED MALONAMIDE CRYSTAL COMPOUNDS AS A BASIS TO SUPPORT TRIVALENT LANTHANIDE EXTRACTION MECHANISMS

The problem of bond energies in the nitrate compounds of trivalent lan thanides, Ln, with malonamides, L, of the type: Ln(NO3)(3)(H2O)(m)L an d Ln(NO3)(3)L-2, (L=CH2(OCNR1R2)(2)) is approached through a semi-empi rical theory connecting thermodynamic with structural properties (STT) . Emphasis is given on the relation between bond energies and solvent extraction of complexes of Ln with L from nitric aqueous solutions. ST T is presented and applied to nitrate compounds with R-1=ethyl, R-2=et hyl, R-1=methyl, R-2=cyclohexyl and R-1=methyl, R-2=phenyl. For the fi rst time, the effect of the regular decrease of the bond energies Ln-O (nitrato) and Ln-O(L) with the increase of the volume of L is evidence d. The enthalpies of extraction of Ln by L from low nitric acid concen tration are calculated. Finally, rules connecting the distribution coe fficients D(Ln) in liquid-liquid extraction of Ln (III) by diamides is formulated. These rules appear as a useful guide to predict D(Ln) var iation along the series. (C) 1998 Elsevier Science S.A.