Spin crossover (SCO) is a phenomenon that occurs in some metal complexes wherein the spin state of the complex changes due to an external stimulus. The stimuli can include temperature or pressure. Spin crossover is sometimes referred to as spin transition or spin equilibrium behavior.  The change in spin state usually involves interchange of low spin (LS) and high spin (HS) configuration.
Spin crossover is commonly observed with first row transition metal complexes with a d through d electron configuration in an octahedral ligand geometry. Spin transition curves typically plot the high-spin molar fraction against temperature. Often a gradual spin transition is followed by an abrupt (ΔT = 10K) transition with hysteresis and a two-step transition. The abruptness with hysteresis indicates cooperativity, or “communication”, between neighboring metal complexes. In the latter case, the material is bistable and can exist in the two different spin states with a different range of external stimuli (temperature in this case) for the two phenomena, namely LS → HS and HS → LS. The two-step transition is relatively rare but is observed, for example, with dinuclear SCO complexes for which the spin transition in one metal center renders the transition in the second metal center less favorable. Several types of spin crossover have been identified; some of them are light induced excited spin-state trapping (LIESST), ligand-driven light induced spin change (LD-LISC), and charge transfer induced spin transition (CTIST).