A series of mononuclear transition metal complexes of the tetradentate (N2O2) 6,6'-bis(3 "-oxide-1 "-oxyl-4 ",4 ",5 ",5 "-tetramethylimidazoline-2 "-yl)-2,2'-bipyridine (NIT-bpy) has been prepared and characterized by spectroscopic, magnetic, and single-crystal X-ray diffraction studies. The free NIT-bpy ligand is planar with an anti-conformation, whereas in each complex this ligand is tetracoordinated to the metal in a syn-conformation comprising the equatorial plane of a distorted octahedron. Complexes differ by the nature or the ancillary ligands occupying the trans-axial positions, these being two water molecules in [Ni-II(NIT-bpy). 2H(2)O](ClO4)(2), 1, one molecule of water and one perchlorate anion in [M(NIT-bpy). H2O . ClO4]ClO4 (M = Ni(II), 2; Mn(II), 3; Co(II), 4), or two perchlorate anions in [Cu-II(NIT-bpy). 2ClO(4)], 5. The ferromagnetic metal-nitroxide coupling found for the nickel complex 1 (J(12) = + 39.6 cm(-1)) arises from near-coplanarity between the pi-conjugated radical plane and the equatorial plane of the trans-diaqua{Ni(N-2O2)(H2O)(2)} octahedron. Deviations from this geometry lead to antiferromagnetic components that are dominant in complex 1 (J(23) = - 48.7 cm(-1)) and tend to reduce the extent of the ferromagnetic interactions in complex 2 (J(12) = +27.8 cm(-1) and J(23) = +6.9 cm(-1)). For the Mn2+ complex 3, variable temperature studies indicate antiferromagnetic behavior J = - 65.3 cm(-1), but with weak intermolecular ferromagnetic interactions zJ' = + 0.22 cm(-1). For the Co2+ complex 4. experimental data were best explained as a weak ferromagnetic interaction J(12) = + 0.40 cm(-1) and a relatively strong antiferromagnetic interaction J(23) = - 28.0 cm(-1). The Cu2+ complex 5 exhibits strong antiferromagnetic behavior J = -166.8 cm(-1). The magnetic data of the [M'(NIT-bpy)](ClO4)(2) (M' = Zn(II), 6; Fe(II) low-spin, 7) complexes were fit to a Curie-Weiss law (theta = -2.43 cm(-1) for 6 and theta = -0.51 cm(-1) for 7). No intramolecular radical-radical interaction is apparent in any oi these complexes.