A bi-stable spin crossover complex can exist in two electronic states with different physical properties. Switching back and forth between the two states can be achieved by changing the temperature, pressure or by irradiating the compound with light. At t he core of such a complex sits a transition metal ion, most often Fe2+, bonded to several ligand atoms. When individual complexes and other components, like counterions and solvate molecule, form an ordered crystalline solid, the spin crossover behaviour of a macroscopic sample depends on the collective behaviour of all the components of the crystal. Changing a component of the system will also change the spin crossover behaviour. The change of spin state also produces a colour change that is envisaged to be of technical use in temperature sensors and optical switches. The abrupt transition with hysteresis confers a memory effect to the crystalline solid that may be used for the development of memory devices. This project aims at synthesising new such sys tems and studying their crystalline products in detail with temperature using X-ray diffraction, Squid (magnetisation), calorimetry, optical spectroscopy and laser irradiation over a temperature range from 2 K to room temperature. It implies investigating collective behaviour of iron complexes in terms of short- and long-range correlations as well as the role of the other magnetically passive crystal structure components involved in the spin crossover process and developing theoretical models for the cros sover process and associated structural transitions. The latter modelling is primarily based on Landau theory but also includes combination with Ising-like models of theory.