An imidazolidine-phenolate ligand HL yields quadruple-bridged (μ-NCNimidazolidine)2(μ-Ophenolate)2 ferromagnetic dinuclear nickel and cobalt complexes. Both kinds of bridges contribute to the ferromagnetic coupling, but the ferromagnetism of these samples is mainly ascribed to the double μ-Ophenolate links, on the basis of density functional theory calculations. These studies demonstrate not only that the short M-O-M angles of the M2O2 cores favors the parallel alignment of the electrons but also that these angles are the optimal ones for maximizing the ferromagnetic contribution in these complexes. And these acute angles, close to 90°, are predetermined by the geometrical constrictions imposed by the ligand itself. Thus, HL is an uncommon polydentate donor that induces ferromagnetism per se in its metal complexes by strict control of one geometric parameter, the M-O-M angle.