Interaction of M3+ lanthanide cations with amide, urea, thioamide and thiourea ligands:: a quantum mechanical study

We report an ab initio quantum mechanical study on the interaction of M3+ cations (La3+, Eu3+, Yb3+) with model ligands L (L = amide, urea, thioamide and thiourea derivatives). The role of counterions and stoichiometry on ligand binding energies is investigated by a comparison of charged ML3+ complexes with the neutral MCl3L and MCl3L2 ones. The calculations show that all ligands display strong interactions with the cation. Trends in binding energies in ML3+ (urea > thiourea > amide > thioamide) are found to differ From those of calculated protonation energies (thiourea > urea > thioamide > amide). Adding counterions or increasing the coordination number may also modify the relative affinities. Changes in structural parameters, electron transfer and polarization effects are analysed. The calculations reveal a striking difference in the binding mode of sulfur compared to oxygen ligands, and the role of steric repulsions in the first coordination sphere, due to counterions and increased coordination number. The results are discussed in the context of modelling complexes of lanthanide and actinides.