COMPLEXATION OF LN(3-CONTAINING O=PR3 LIGANDS - A QUANTUM-MECHANICS STUDY() LANTHANIDE CATIONS WITH PHOSPHORYL)

We report a series of ab initio quantum-mechanical calculations on M3 (M=La, Eu and Yb) and MCl3 model complexes of O=PR3 ligands (R=H, Me, Et and Ph) to assess the role of R substituents and Cl- counterions o n the intrinsic cation-ligand interaction energy. The calculations rev eal a marked selectivity in the ligand series, as well as in the catio n series. In the absence of counterions, for a given M3+ cation, the b inding sequence follows the order H < Me < Et < Ph, owing to polarizat ion and charge-transfer effects. For a given O=PR3 ligand, the cation binding follows the sequence La3+ < Eu3+ < Yb3+, as expected based on the decrease in ionic radius in this lanthanide series. Geometry optim ization shows that, as the M3+...O=PR3 interaction increases, the O=P bond lengthens and the O...M3+ distance shortens. Similar trends are o bserved in the R3PO...MCl3 complexes but are less pronounced, because of the ligand-anion repulsive interactions and electron transfer from Cl- to the M3+ cation. The importance of these results in the context of designing efficient ionophores for lanthanide and actinide cations is discussed. (C) 1998 Elsevier Science B.V.