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 nitrato compounds of trivalent lanthanides, Ln, with malonamides, L, of the type: Ln(NO3)3(H2O)mL and Ln(NO3)3L2, (L=CH2(OCNR1R2)2) is approached through a semi-empirical 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. STT is presented and applied to nitrato compounds with R1=ethyl, R2=ethyl, R1=methyl, R2=cyclohexyl and R1=methyl, R2=phenyl. For the first 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 evidenced. The enthalpies of extraction of Ln by L from low nitric acid concentration are calculated. Finally, rules connecting the distribution coefficients D(Ln) in liquid-liquid extraction of Ln (III) by diamides is formulated. These rules appear as a useful guide to predict D(Ln) variation along the series. (orig.)