We have isolated and characterized a new paramagnetic bidentate ligand, tert-butyl 5-phenyl-2-pyridyl nitroxide (phpyNO). The spin distribution onto the pyridine ring was clarified from the Electron Spin Resonance spectrum. Complexation of phpyNO with nickel(II) and copper(II) ions gave ML2-type chelated compounds. The magnetic measurements of [Ni(phpyNO)(2)(H2O)(2)]center dot(ClO4)(2) and [Cu(phpyNO)(2)(H2O)]center dot(ClO4)(2)center dot C6H4Cl2 revealed that the metalradical exchange couplings were ferromagnetic with 2J/k(B) = +409(10) and +434(12) K, respectively. The torsion angle (phi) around M-O-N-C-2py can be regarded as a reliable indicator for the plane geometry of chelates; namely, highly planar chelates defined by small phi exhibit ferromagnetic coupling. An approximate linear relation. was found in the J versus phi plot using the data of the present complexes and. related known compounds. The critical angle of phi, at which the sign of the metal-radical exchange changes from positive to negative, was 12.6(9)degrees. This finding could be almost reproduced by density-functional theory calculation on a model copper(II)-nitroxide dyad. The exchange couplings in equatorially coordinated copper(II)-and nickel(II)-nitroxide complexes are very strongly ferromagnetic by nature.