Substituent Effects on the Spin-Transition Temperature in Complexes with Tris(pyrazolyl) Ligands

Iron (II) complexes with substituted tris(pyrazolyl) ligands, which exhibit a thermally driven transition from a low-spin state at low temperatures to a high-spin state at elevated temperatures, have been studied by M\"ossbauer spectroscopy and magnetic susceptibility measurements. From the observed spectra the molar high-spin fraction and the transition temperature have been extracted. All substituents, except for bromine, lead to a decrease of the transition temperature. Density functional calculations have been carried out to compare the experimentally observed shifts of the transition temperature with those derived from theory.