Two isostructural ID complexes, [M(hfac)(2)NaphNN](n) [M = Mn-II (1) or Co-II (2); NaphNN = 1-naphthyl nitronylnitroxide], were synthesized and exhibit very strong antiferromagnetic metal radical exchange coupling. Compound 2 shows slow magnetic relaxation behavior with a high blocking temperature (T-B approximate to 13.2 K) and a very high coercive field of 49 kOe at 4.0 K.