Methods for the synthesis of binuclear [Cu(hfac)₂L^Et]₂ and tetranuclear [[Cu(hfac)₂]₄(L^Et)₂] heterospin compounds based on copper hexafluoroacetylacetonate [Cu(hfac)₂] and 2-(4-ethylpyridin-3-yl)-4,5-bis(spirocyclopentyl)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (L^Et), were developed. The crystals of the complexes are elastic and do not crash during repeated cooling–heating cycles. It was found that a singlet–triplet conversion occurred in all of the {Cu(II)–O•–N<} exchange clusters in the molecules of the binuclear [Cu(hfac)₂L^Et]₂ which led to spin coupling with cooling. The transition occurred in a wide temperature range with a maximum gradient ΔχT at ≈180 K. The structural transformation of the crystals takes place at T < 200 K and is accompanied by the lowering of symmetry from monoclinic to triclinic, twinning, and a considerable shortening of the Cu–O_NO distance (2.19 and 1.97 Å at 295 and 50 K, respectively). For the tetranuclear [[Cu(hfac)₂]₄(L^Et)₂], two structural transitions were recorded (at ≈154 K and ≈118 K), which led to a considerable change in the spatial position of the Et substituent in the nitronyl nitroxyl fragment. The low-temperature process was accompanied by a spin transition recorded as a hysteresis loop on the χT(T) curve during the repeated cooling–heating cycles (T½↑ = 122 K, T½↓ = 115 K). This transition is unusual because it causes spin coupling in half of all of the {>N–•O–Cu²⁺} terminal exchange clusters, leading to spin compensation for only two paramagnetic centers of the six centers in the molecule.

Changes in the hydrostatic pressure may significantly affect the spin dynamics of magnetically active compounds. Hence, objects exhibiting spin transitions have attracted attention of researchers in different fields. Since these transitions are reversible and do not cause the chemical decomposition of the compounds, spin state conversions can be responsible for the functional activity of materials as data storage media, external stimulus sensors or actuators for certain solid-state processes. Moreover, multispin compounds can demonstrate specific structural dynamics that is not inherent in diamagnetic compounds, which is of independent interest. The review presents the analysis of recent data on the effect of hydrostatic pressure on the crystal structure and magnetic properties of metal complexes with organic diamagnetic and paramagnetic ligands, exhibiting spin transitions and valence tautomerism, organic paramagnets and single-molecule magnets. The bibliography includes 179 references.