The reaction of copper(II) hexafluoroacetylacetonate [Cu(hfac)(2)] with the stable nitronyl nitroxide 2-(1-ethyl-3-methyl-1H-pyrazol-4-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (L-a) resulted in a paired heterospin complex [[Cu(hfac)(2)](3)(mu-O,N-L-3)(2)][Cu(hfac)(2)(O-L-a)(2)]. The crystals of the compound were found to be capable of a reversible single-crystal-to-single-crystal (SC-SC) transformation initiated by the variation of temperature. At room temperature, the molecular structure of [[Cu(hfac)(2)](3)(mu-O,N-L-a)(2)][Cu(hfac)(2)(O-L-a)(2)] is formed by the alternating fragments of the pair complex. Cooling the crystals of the complex below 225 K caused considerable mutual displacements of adjacent molecules, which ended in a transformation of the molecular structure into a polymer chain structure. A reversible topotactic polymerization depolymerization coordination reaction actually takes place in the solid during repeated cooling heating cycles: [[Cu(hfac)(2)](3)(mu-O,N-L-a)(2)][Cu(hfac)(2)(O-L-a)2] reversible arrow [Cu(hfac)(2)(mu-O,N-L-a)](infinity). Polymerization during cooling is the result of the anomalously great shortening of intermolecular distances (from 4.403 angstrom at 295 K to 2.460 angstrom at 150 K; Delta d = 1.943 angstrom) between. the terminal Cu atoms of the trinuclear fragments {[[Cu(hfac)(2)](3)(mu-O,N-L-a)(2)]} and the noncoordinated N atoms of the pyrazole rings of the mononuclear {[Cu(hfac)(2)(O-L-a)(2)]} fragments. When the low-temperature phase was heated above 270 K, the polymer chain structure was destroyed and the compound was again converted to the pair molecular complex. The specifics of the given SC-SC transformation lies in the fact that the process is accompanied by a magnetic anomaly, because the intracrystalline displacements of molecules lead to a considerable change in the mutual orientation of the paramagnetic centers, which, in turn, causes modulation of the exchange interaction between the odd electrons of the Cu2+ ion and nitroxide. On the temperature curve of chi T, this shows itself as a hysteresis loop. The nontrivial character of the recorded spin transition during the cooling of the sample below 225 K lies in the fact that the magnetic moment abruptly increased. In contrast, heating the sample above 270 K led to a drastic decrease in chi T. This behavior of chi T is caused by a stepwise change in the character of the exchange interaction in the {>N-O-center dot-Cu2+-O-center dot-N<} fragments. The lengthening of distances between the paramagnetic centers on cooling below 225 K led to a transition from antiferromagnetic to ferromagnetic exchange and, vice versa, the shortening of distances between the paramagnetic centers during the heating of the heterospin polymer above 270 K led to a transition from ferromagnetic exchange to antiferromagnetic exchange.