The synthesis, structural determinations and magnetic properties of an original dinuclear Cu-Gd and two trinuclear Cu-Ln-Cu complexes (Ln = Gd or Dy) made with a Schiff base ligand and carboxylate ligands are reported. In each complex, the copper and gadolinium or dysprosium ions are linked by a double phenoxo bridge and a eta1:eta1:mu trifluoroacetato bridge. The dinuclear Cu-Gd complex exhibits the usual ferromagnetic interaction while the trinuclear (Cu-Gd-Cu) complexes can not be fitted with a simple Hamiltonian that only takes into account the Cu-Gd interaction. Introduction into the Hamiltonian of a Cu-Cu interaction between the terminal copper ions gives a nice fit in the case of a ferromagnetic Cu-Cu interaction (J'(Cu-Cu) = 8.3 and 9.8 cm(-1)). For the first time, the occurrence of a ferromagnetic interaction between the copper ions of the trinuclear entities is evidenced and supported by the field dependence of the magnetization. As there is no direct proof for the existence of an interaction between two centers through a third magnetic center located in between them, we can conclude that the existence of pi-pi stacking interactions through the phenyl rings of the ligands are responsible for the presence of these interactions.