Is There a Need for New Models of the Spin Crossover?

The existing models of the low-spin to high-spin transition (spin crossover) are briefly reviewed. Experimental data pointing to a need of new models are displayed. A statistical model with the distribution of the solid-state cooperativeness is outlined. A modeling is shown as well as its application to a spin crossover system [Fe(bzimpy)₂](C1O₄)₂] · 0.25H₂O. This shows an abrupt spin crossover at temperature as high as 403 K with a hysteresis width of 12 K. The angled walls of the hysteresis loop can be followed by the outlined statistical model.