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Four Ln( III) complexes based on a new nitronyl nitroxide radical have been synthesized and structurally characterized: {Ln(hfac)3[ NITPh(MeO)2]2} (Ln = Eu( 1), Gd( 2), Tb( 3), Dy( 4); NITPh(MeO)2 = 2-(3′,4′-dimethoxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide; hfac = hexafluoroacetylacetonate). The single-crystal X-ray diffraction analysis shows that these complexes have similar mononuclear trispin structures, in which central Ln( III) ion is eight-coordinated by two O-atoms from two nitroxide groups and six O-atoms from three hfac anions. The variable temperature magnetic susceptibility study reveals that there exist ferromagnetic interactions between Gd( III) and the radicals, and antiferromagnetic interactions between two radicals ( JGd-Rad = 3.40 cm−1, JRad-Rad = −9.99 cm−1) in complex 2. Meanwhile, antiferromagnetic interactions are estimated between Eu( III) (or Dy( III)) and radicals in complexes 1 and 4, and ferromagnetic interaction between Tb( III) and radicals in complex 3, respectively. [ABSTRACT FROM AUTHOR]