[en] In this work has been studied in solution trivalent lanthanides cations complexes with two ligands, L^py and L^pz presenting respectively pyridines and pyrazines groups in order to compare the affinities of pyridine and pyrazine towards the cations f in water. Indeed, these two dissymmetric tetrapode ligands present a common acid anchoring site whose affinity is strong for the Ln³⁺ cations and which differ only by the pyridines/pyrazines cycles. Their comparison will allow to reveal the effect of the polarizability of the nitrogen donors on the complexation properties of the f elements in water. The complexation of the trivalent lanthanides cations by the ligand L^py has been studied by different physico-chemical methods (ES-MS, NMR, fluorimetry, potentiometry) and shows the formation of complexes LnL^py(H₂O)₃⁺. The same studies for the ligand L^pz are being studied and show that similar complexes are formed. The comparison of the stability constants of the different studied complexes indicates that the contribution of the fragment N(CH₂pz)₂ is clearly weaker than those of the fragment N(CH₂py)₂. Soon, complementary studies will be carried out with Am(III) in order to compare the affinities ratios β(An)/β(Ln) in terms of the polarizability of the implied nitrogenous donor. (O.M.)