Spin crossover

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, pressure or radiation. Spin crossover is referred to as spin transition if takes place suddenly,or spin equilibrium, when is gradual. However, this terminology is not strict, the "cross-over" and "transition" being often considered equivalent. The change in spin state usually involves transformation from a low spin (LS) ground state (at low temperatures and/or high pressure) towards a high spin (HS) ground configuration (at high temperature or reduced/normal pressure).

Spin crossover is commonly observed with first row transition metal complexes with a d⁴ through d⁷ electron configuration in an octahedral ligand geometry. Spin transition curves typically plot the high-spin molar fraction against temperature. The abruptness with hysteresis indicates cooperativity, or "communication", between neighboring metal complexes, throughout the whole lattice. A transparent account of cooperative lattice effects is the mechano-elastic model of spin transition, developed in terms of volume changes of molecular units, along the spin state swap and intermolecular effects assumed as harmonic oscillators. The spin crossover, or spin transition is a neat example of bi-stability. A material is bistable when it exists in the two different states, with distinct properties, the form being tunable by external stimuli (e.g. temperature). 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).