Spin crossover in iron(II)-containing complex compounds under a pressure (Review Article)

Abstract

The spin crossover phenomenon consists in switching between two spin states of the central ion of a transition 3d- metal in a molecular complex. The spin crossover can be induced and controlled by temperature, pressure, light radiation etc. The present review addresses all aspects of the effect of hydrostatic pressure on spin-crossover properties of iron(II) complex compounds. The applied hydrostatic pressure allows us to influence the crystal lattice, revealing the role of elastic fields in the cooperative interaction of spin-crossover complexes, and to change directly the volume characteristics of the complex molecules, the distances between the central ion and ligand atoms, and thus affect the strength of the crystal field. The experimental results for all types of iron(II)-containing spin-crossover structures are discussed in details, and the theoretical consideration of the observed effects is given. The physical principles of spin crossover at the molecular level are adequately described by the ligand field theory while the macroscopic properties are understood on the basis of electron–phonon coupling and elastic properties of the crystal lattice.