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+ c ations (La3+, Eu3+, Yb3+) with model ligands L (L = amide, urea, thioamide and thiourea derivatives). The role of counterions and stoichiometry on lig and binding energies is investigated by a comparison of charged ML3+ comple xes with the neutral MCl3L and MCl3L2 ones. The calculations show that all ligands display strong interactions with the cation. Trends in binding ener gies in ML3+ (urea > thiourea > amide > thioamide) are found to differ From those of calculated protonation energies (thiourea > urea > thioamide > am ide). Adding counterions or increasing the coordination number may also mod ify the relative affinities. Changes in structural parameters, electron tra nsfer and polarization effects are analysed. The calculations reveal a stri king difference in the binding mode of sulfur compared to oxygen ligands, a nd the role of steric repulsions in the first coordination sphere, due to c ounterions and increased coordination number. The results are discussed in the context of modelling complexes of lanthanide and actinides.