Spin crossover — An unusual chemical equilibrium

Abstract

The nature and theoretical models of spin crossover equilibrium between high-spin and low-spin forms of transition metal complexes are reviewed. Spin crossover compounds are promising materials for information storage and display devices. In the solid state spin crossover is accompanied by several phenomena related to phase transitions. A critical analysis of theoretical models proposed for the explanation of these phenomena is given. The paper mainly focuses on two models that provide for adequate descriptions of the majority of experimental data, viz. the model of the Ising-like Hamiltonian and the molecular statistical model. Descriptions of spin crossover yielded by these two models are formally similar but not identical and they are based on fundamentally different concepts of molecular interactions and ordering, the latter being the origin of the two-step spin crossover. The Ising-like Hamiltonian model approaches spin crossover from the point of view of properties of lattices whereas the molecular statistical model explains this phenomenon starting from molecules. The latter approach provides for the elucidation of the molecular nature of cooperative phenomena observed in spin crossover which is important for developing the synthetic strategy of promising spin crossover compounds.