We report ab initio quantum mechanical calculations on charged LM3+ and neutral LMCl3 complexes formed by lanthanide M3+ cations (M = La, Eu, Yb) and model ligands L, where L are phosphorous derivatives R3PO (R = alkyl/O-alkyl/phenyl), R3PS and R2PS2- (R = alkyl/phenyl), and amide, pyridine, triazine and anisole ligands. Among all neutral ligands studied, Ph3PO is intrinsically clearly the best. However, the comparison of LM3+ to LMCl3 complexes demonstrates that the concept of 'ligand basicity' is not sufficient to compare the efficiency of cation coordination. Counterions play an important role in the structures of the complexes and for the consequences of substitution in the Ligand. For instance, in the absence of competing interactions, phenyl substituted R3PS or R2PS2- ligands interact better than alkyl substituted ones, but the order is reversed in the presence of counterions. Counterions also amplify the alkyl vs. O-alkyl substituent effect in R3PO complexes. Bidentate anions or more bulky anions are expected to amplify the effects observed with chloride anions. Thus, multiple interactions between counterions and the other species in the first coordination sphere markedly contribute to the 'effectiveness' and stereochemistry of ligand-cation interactions. (C) 2000 Elsevier Science S.A. All rights reserved.