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The reaction of Cu(hfac)2 with a spirocyclic nitronyl nitroxide 2-(1-ethylpyrazol-4-yl)-4,5-bis(spirocyclopentane)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (LEt) gave a heterospin complex [Cu(hfac)2LEt]-I, whose solid state is formed by polymer chains with a head-to-tail motif (the bridging function inside the chains is performed by the paramagnetic organic ligand). At 295 K the distance from the Cu atom to the N atom of the pyrazole ring and to the O atom of the NO group is 2.333(2) and 2.451(2) Å, respectively. The cooling of the compound to 110 K leads to a structural transition, as a result of which the unit cell volume decreases by more than 7%. The elongated axis of the octahedron changes from N–Cu–ONO to Ohfac–Cu–Ohfac in all coordination units as a result of a significant decrease in the Cu–ONO and Cu–N bond lengths to 2.002(4) and 2.034(5) Å, respectively, and an increase in the two Cu–Ohfac distances from 1.950(2) and 1.955(2) Å to 2.263(4) and 2.213(4) Å, respectively. The [Cu(hfac)2LEt]-I crystals are elastic and stable during the thermally induced spin crossover. The transition from the high-temperature to low-temperature phase is accompanied by a spin transition at 125 K at 1.0 × 10−4 GPa, as a result of which [Cu(hfac)2LEt]-I transforms from a paramagnetic compound to a diamagnetic one due to the complete spin coupling in the {Cu(ii)–OṖ–N＜} exchange clusters inside the polymer chains. The reversible phase transformation is accompanied by a reversible contrastive change of color from blue to dark brown upon cooling and vice versa from dark brown to blue upon heating. The spin crossover temperature is very sensitive to the change of external pressure: when pressure increases from 1.0 × 10−4 GPa to 0.48 GPa, it increases to 325 K. The increase of pressure provokes a transition from a high-spin to low-spin state, which again transforms into the high-spin state when the external load is removed. The [Cu(hfac)2LEt]-I crystals are the first examples of polymer-chain breathing crystals whose paramagnetic properties are suppressed by the lowering of temperature and/or increase of pressure. [ABSTRACT FROM AUTHOR]