Transition metal based molecular systems have long been recognised as promising candidates for molecular devices. Possible applications include light-harvesting systems, data storage devices and molecular switches. Current research trends aim at replacing the originally employed transition metals (e.g. Ru, Os, Rh, Ir) with more abundant, less expensive and less toxic alternatives, such as iron. Light induced switching between the singlet ground state and the metastable quintet excited state has been demonstrated for several Fe(II) complexes at low temperature. . Unfortunately, in the complexes investigated so far the short room temperature lifetime of the quintet state makes their application in practical switching devices impossible. Increasing the stability of the metastable quintet excited state in Fe(II) complexes is clearly a well-suited task for theory based ligand engineering, as in this approach a large number of compounds can be rapidly screened and evaluated at acceptable cost.