The research lies in the interdisciplinary field of Molecular Magnetism and concerns the study of polynuclear 3d/4f metal complexes with large Magnetocaloric Effect (MCE), aiming at the preparation of Molecular Magnetic Refrigerants. Magnetic cooling is an environmentally friendly cooling technology that does not use ozone-depleting chemicals, hazardous chemicals or greenhouse gases and has a higher cooling efficiency than conventional refrigerants. The He depletion and its rising price requires new solutions. Molecular Magnetic Materials have been proposed as candidate materials for achieving magnetic cooling at low temperatures as an alternative to the use of liquid He, mainly due to helium depletion and its high cost. Magnetic cooling is based on the magnetocaloric effect, i.e. the change in magnetic entropy during the application of a magnetic field. The larger the MCE of a magnetic material, the greater the potential temperature change and the more ideal it would be for magnetic refrigeration applications. The molecular magnetic materials developed to date do not exhibit sufficiently large MCE values to be proposed for practical magnetic cooling applications. With the proposed research we tried to contribute to the synthesis and characterization of new 3d / 4f polynuclear complexes that meet the basic conditions for the occurrence of a large MCE, i.e. high spin value in the ground state, S, as a result of ferromagnetic coupling, negligible magnetic anisotropy, as a result of isotropic metal ions, low molecular weight and large metal-ligands molar ratio. We synthesized Cu / Ln heterometallic complexes, studied the structures by X-ray crystallography and their magnetic and magnetocaloric properties.