[en] Full text of publication follows: Actinide and lanthanide ions in solution present a wide variety of coordination environments which are very often unknown, even though a detailed knowledge of their molecular structures, dynamics and energetics in a given media is essential to predict their behaviour and to control a ligand affinity toward the metal ions. Quantum chemistry calculations on heavy elements ions in solution remain challenging but they can be an important tool to complete experimental measurements for obtaining structural and energetic information and for probing the nature of the metal-ligand bonds. In the present study, quantum chemistry calculations have been performed on lanthanide and actinide ions in the trivalent oxidation state in the presence of water molecules and nitrogen donor ligands. The geometrical structures of the complexes and the binding strength of the ligands have been determined in the gas phase and in solution. The calculations were first performed on lanthanide and actinide molecular compounds which have been well characterized experimentally in order to validate a theoretical approach and then extended to a larger number of actinide ions. The nature of the metal-ligand bonds within the series of cations and the specificity of the actinide-ligand bond will be discussed from a geometric, energetic and electronic standpoint and the role played by solvation effects will be analyzed by comparing gas phase and solution calculations. (author)