Interaction of trivalent lanthanide cations with phosphoryl derivatives, amide, anisole, pyridine and triazine ligands: a quantum mechanics study

We report ab initio quantum mechanical calculations on charged LM3+ and neu tral LMCl3 complexes formed by lanthanide M3+ cations (M = La, Eu, Yb) and model ligands L, where L are phosphorous derivatives R3PO (R = alkyl/O-alky l/phenyl), R3PS and R2PS2- (R = alkyl/phenyl), and amide, pyridine, triazin e and anisole ligands. Among all neutral ligands studied, Ph3PO is intrinsi cally clearly the best. However, the comparison of LM3+ to LMCl3 complexes demonstrates that the concept of 'ligand basicity' is not sufficient to com pare the efficiency of cation coordination. Counterions play an important r ole in the structures of the complexes and for the consequences of substitu tion in the Ligand. For instance, in the absence of competing interactions, phenyl substituted R3PS or R2PS2- ligands interact better than alkyl subst ituted ones, but the order is reversed in the presence of counterions. Coun terions also amplify the alkyl vs. O-alkyl substituent effect in R3PO compl exes. Bidentate anions or more bulky anions are expected to amplify the eff ects observed with chloride anions. Thus, multiple interactions between cou nterions and the other species in the first coordination sphere markedly co ntribute to the 'effectiveness' and stereochemistry of ligand-cation intera ctions. (C) 2000 Elsevier Science S.A. All rights reserved.