A type of molecular magnetism that is the result of electronic instability (see @ET07027@) caused by external constraints (temperature, pressure, or electromagnetic radiation), which induce structural changes at molecular and lattice levels. The phenomenon is most characteristic of first-row transition metal complexes, e.g. those of Fe^II. An example of spin-crossover complexes (the term of spin-state isomers is also used) is [Fe(2-pic)₃]Cl₂·EtOH (2-pic = 2-picolylamine). At the Fe–N distance of \(203.2\ \text{pm}\) (\(115\ \text{K}\)), the complex has an electronic @LT07067@ (¹A₁), whereas stretching the bond up to \(219.9\ \text{pm}\) at \(227\ \text{K}\) induces the transition to a @LT06788@ state (⁵T₂).