The magnetic relaxation and anisotropy of 3- and 4-coordinate lanthanide complexes were systematically investigated, and the change of SMM behavior originating from the equatorially coordinating ligand field was successfully elucidated through combined experimental and theoretical studies. Remarkably, a novel approach taking into account the different contributions of atomic charges, dipole moments, and quadrupole moments was used to map the electrostatic potential around the metal center in the Dy-III derivatives, revealing the key role played by the ligands as a whole and not just by the coordinating donor atoms as often considered.