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)2HNN]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 alpha phase has a relatively high activation energy of magnetization reorientation (193 K).