Four lanthanide-based nitronyl nitroxide radical cyclic molecular clusters of formula [Ln(hfac)(3)(NITPhPO(OEt)(2))](2) (Ln(III) = Gd (1), Tb (2A and 2B), and Dy (3) and NITPhPO(OEt)(2) = 4'-[2-(1-oxyl-3-4,4,5,5-tetramethylimidazoline)phenyl]diethoxylphosphine oxide) have been synthesized. Their X-ray structures have been solved and highlight two different crystal packings. For the particular case of the Tb-III derivative, both of them can be obtained. In 2A, the molecules are well-isolated, while 2B shows short contacts between N-O radical groups. Static magnetic studies on the Gd-III derivative (1) demonstrate that lanthanides and radicals are ferromagnetically coupled (J = 3.46 +/- 0.04 cm(-1)). Dynamic magnetic studies show that both compounds 2A and 2B exhibit single molecule magnet behavior. A comparison of their magnetic behaviors highlights that the crystal packing has a crucial influence on the temperature range in which the SMM behavior is observed. In the case of the well-insulated Tb-III-based derivative (2A), the SMM behavior is observed at higher temperatures and lower frequencies than for the one that presents close-packing between the molecules (2B). Comparisons are then possible only under an applied external magnetic field (0.2 T) with Delta = 27.5(6) and 21.0(5) K and tau(0) = 2.64(25) x 10(-9) and 1.76(20) x 10(-9) s for 2A and 2B, respectively.