Structural aspects of spin crossover.: Example of the [FeIILn(NCS)2] complexes

The interplay between the spin crossover and the structural properties of the complexes in the solid state is still under investigation. In particular the following questions may be asked. What are the structural modifications of the metal coordination sphere at the spin crossover? How are the dimensions and the symmetry of the crystallographic unit cell affected by the spin crossover? Conversely, how may structural properties influence the spin crossover behavior? Do intramolecular parameters account for the features of the spin crossover? What are the relevant characteristics of the crystal packing for the cooperativity? Do the above questions have general answers that can be used for all the spin crossover compounds? This contribution tries to give answers to these questions. The discussion is based on a large structural data set provided in the literature for the six-coordinated iron(II) mononuclear complexes of general formula [FeLn(NCS)(2)]. The effects of temperature, light and pressure on the X-ray diffraction crystal structures are reviewed. The structural modifications due to the spin crossover are first estimated, these include the expansion and the distortion of the FeN6 octahedron, the isotropic and the anisotropic changes of the unit cell. The influence of the structural properties on the features of the spin crossover is then discussed. For example, intramolecular properties such as Fe-N bond lengths are in general not relevant to account for the spin crossover features. In contrast, hydrogen bonds play a paramount role in the propagation of the spin conversion throughout the crystal lattice.