Spin transition in iron(II) complexes induced by heat,pressure, light,and nuclear decay

The phenomenon of temperature-dependent spin transition will be introduced and the numerous chemical and physical influences affecting the spin transition characteristics will be discussed. We shall mainly concentrate on the spin crossover system Fe(2-pic)₃]X₂·Sol (2-pic=2-aminomethylpyridine; X=Cl, Br; Sol=C₂H₅OH, CH₃OH) and demonstrate how the behaviour of the spin transition⁵T_2g(O_h)?¹A_1g(O_h) is influenced by substituting the metalion, the non-coordinating anions X, the crystal solvent molecules Sol and by isotopic exchange with H/D and¹⁴N/¹⁵N. It will also be shown that the spin transition is very susceptible to pressure. A quantitative spin state conversion from low spin to high spin can also be achieved by illuminating the crystals of a spin crossover system at sufficiently low temperatures. The metastable quintet state can be trapped with practically infinite lifetimes. Several examples for this “Light-Induced Excited Spin State Trapping” (LIESST) will be given. Finally, the occurrence of short-lived anomalous spin quintet states following the⁵⁷Co(EC)⁵⁷Fe nuclear decay, which have been observed in the Mössbauer emission spectra of⁵⁷Co-doped complex compounds, will be discussed with particular references to the LIESST effect.