The synthesis and characterization of three isomorphous complexes [NiII(L)LnIII(DBM)3] (Ln = Gd (1·2.5CH3CN·0.5H2O), Tb (2·2CH3CN·0.5MeOH), and Dy (3·2CH3CN·0.5MeOH·0.5H2O)) are reported (H2L = N,N′-dimethyl-N,N′-(2-hydroxy-3-methoxy-5-methylbenzyl)ethylenediamine, DBM− = anion of 1,3-diphenylpropane-1,3-dione). The flexible ligand L2− contains an N2O2-inner and an O4-outer coordination site. There are diphenoxo bridges between NiII and LnIII ions in these complexes. The remaining coordination sites are occupied by DBM− anions. Direct current (dc) magnetic susceptibility measurements indicate the presence of intramolecular ferromagnetic interactions in desolvated complexes 1–3. The magnetic coupling constant JNiGd in complex 1 is estimated to be 1.11 cm−1 (H = −2JNiGdSNiSGd). Alternating current (ac) magnetic susceptibility studies reveal that complexes 2 and 3 show frequency-dependent out-of-phase signals, which indicate that they exhibit SMM behavior. The energy barriers for complexes 2 and 3 under a 2 kOe applied direct current (dc) magnetic field are estimated from Arrhenius plots to be 14.4 and 11.3 K, respectively.