The synthesis, structural characterization, and magnetic properties of four related heterometallic complexes with formulas [(Dy2CoII)-Co-III(C7H5O2)(8)] 6H(2)O (1), [(Dy2NiII)-Ni-III(C7H5O2)(8)] (C7H6O2)(2) (2), (Tb2CoII)-Co-III(C7H5O2)(8) (3), and (Dy2CdII)-Cd-III(C7H5O2)(8) (4) were reported. Each of complexes has a perfectly linear arrangement of the metal ions with two terminal Ln(III) (Ln(III) = Dy-III, Tb-III) ions and one central M-II (M-II = Co-II, Ni-II, Cd-II) ion. It was found that 13 displayed obvious magnetic interactions between the spin carriers according to the direct current (dc) susceptibility measurements. Alternating current (ac) magnetic susceptibility measurements indicate that complexes 1-4 all exhibit single-molecule magnet (SMM) behavior, while the replacement of the diamagnetic CdII by paramagnetic ions leads to a significant slowing of the relaxation thanks to the magnetic interactions between 3d and 4f ions, resulting in higher relaxation barrier for complexes 1 and 2. Moreover, both Dy2Co and Dy2Ni compounds exhibit dual relaxation pathways that may originate from the single ion behavior of individual Dy-III ions and the coupling between Dy-III and Co-II/Ni-II ions, respectively, which can be taken as the feature of 3d-4f SMMs. The U-eff for 1 of 127 K is a relatively high value among the reported 3d-4f SMMs. The results demonstrate that the magnetic coupling between 3d and 4f ions is crucial to optimize SMM parameters. The synthetic approach illustrated in this work represents an efficient route to design nd-4f based SMMs via incorporating suitable paramagnetic 3d and even 4d and 5d ions into the df system.