Polymorphic alternating HNN-cobalt(II) chains both behaving as single-chain magnets (HNN=4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide)

Metal-radical alternating chains [Co(hfac)(2)HNN](n) crystallized in two morphs containing repeating all-cis (alpha) and cis, cis, trans (beta) configurations with respect to the O-HNN-Co-O-HNN geometry (HNN = 4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide). Both phases showed magnetization jumps with hysteresis at 2 K. The R phase has a relatively high activation energy of magnetization reorientation (193 K).