We report a series of ab initio quantum-mechanical calculations on M 3 + (M = La, Eu and Yb) and MCl 3 model complexes of O=PR 3 ligands (R = H, Me, Et and Ph) to assess the role of R substituents and Cl - counterions on the intrinsic cation-ligand interaction energy. The calculations reveal a marked selectivity in the ligand series, as well as in the cation series. In the absence of counterions, for a given M 3 + cation, the binding sequence follows the order H < Me < Et < Ph, owing to polarization and charge-transfer effects. For a given O=PR 3 ligand, the cation binding follows the sequence La 3 + < Eu 3 + < Yb 3 + , as expected based on the decrease in ionic radius in this lanthanide series. Geometry optimization shows that, as the M 3 + O=PR 3 interaction increases, the O=P bond lengthens and the O M 3 + distance shortens. Similar trends are observed in the R 3 PO MCl 3 complexes but are less pronounced, because of the ligand-anion repulsive interactions and electron transfer from Cl - to the M 3 + cation. The importance of these results in the context of designing efficient ionophores for lanthanide and actinide cations is discussed.