The coordination ability of tris(pyridyl)aluminates can be modified by the steric and electronic character of substituents at the 6-positions of their pyridyl rings. Whereas [EtAl(6-Me-2-py)3]- (1) coordinates strongly to lanthanide (II) ions [Eu(II) and Yb(II)], [EtAl(6-Br-2-py)3]- (2) forms much weaker complexes, and [EtAl(6-CF3-2-py)3]- does not coordinate at all. The modification of the donor ability of these ligands is investigated by solid-state studies of the Ln(II) sandwich compounds and by competitive coordination studies in solution.