We herein reported the syntheses, structures, and magnetic properties of three dinuclear heterometallic m(II)Ln(III) complexes, namely, [M(II)Ln(III)(H2L)-(CH3OH)(2)(NO3)(2)](NO3)center dot S (M = Co, Ln = Dy, S = MeOH (1(CoDy)); M = Zn, Ln = Dy, S = MeOH (2(ZnDy)); M = Co, Ln = Y, S = MeNO2 (3(CoY)), H4L = 2,6-diacetylpyridine bis[2-(semicarbazono) propionylhydrazone]. Synthesized from the predesigned multidentate ligand H4L, which has two different coordination pockets (smaller N3O2 and larger N2O4 pockets) suitable for either a 3d or a 4f metal center, all these complexes have very similar structures, where the M-II centers possess a pentagonal bipyramidal (PBP) geometry and the Ln(III) sites have a tetradecahedron geometry. Magnetic measurements on these compounds revealed the existence of weak ferromagnetic coupling between the Co2+ and Dy3+ centers and the field-induced slow magnetic relaxation of all three complexes. Furthermore, theoretical calculation on all these complexes indicates that although the change of the diamagnetic Zn2+ ion to the paramagnetic Co2+ ion only slightly modifies the local magnetic anisotropy of the Dy3+ ion, the weak Co-Dy magnetic interaction decreases the energy barrier. These compounds are the first systematic results of a heterometallic 3d-4f single-molecule magnet containing predesigned PBP 3d metal ions.