The magnetic coupling interactions of the nitronyl nitroxide radicals bound to diamagnetic (YIII) and paramagnetic (GdIII) rare earth ions in two model magnetic systems based on novel rare earth organic radical complexes Ln(hfac)3(NITPhOCH3)2 (Ln = YIII 1, GdIII 2; hafc = hexafluoroacetylacetonate; NITPhOCH3 = 4′-methoxyo-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) have been investigated by density functional theory (DFT). The magnetic coupling mechanisms were also explored from the viewpoint of molecular orbital and spin density populations. DFT calculations show that the empty 4d-orbitals of YIII and 5d-orbitals of GdIII play an important role in the antiferromagnetic coupling between the two nitronyl nitroxide radical ligands, and that the ferromagnetic coupling between the GdIII ion and the radical magnetic centers can be attributed to the nearly complete localization of the isotropic 4f-shell and singly occupied magnetic orbital (Π*) of the nitronyl nitroxide.