Spin Transitions in Iron(II) Complexes

Abstract

Iron(II) has the 3d6 outer electronic configuration and hence its octahedral complexes may be either high-spin or low-spin. The high-spin configuration, t2g4eg2, is adopted when relatively weak-field ligands are coordinated while the low-spin, t2g6eg0, is obtained when strong-field ligands are involved. In Oh symmetry the ground states are 5T2g and lA1g, for the high-spin and low-spin configurations, respectively. While in most iron(II) six-coordinate complexes the symmetry is lower than Oh, these designations are generally adopted in the following sections for convenience. [Fe(H2O)6]2+ and [Fe(en)3]2+ (en = ethylenediamine) are typical examples of high-spin complexes, while [Fe(CN)6]4− and [Fe(bpy)3]2+ (bpy = 2,2{\textasciiacutex}-bipyridine) contain strong-field ligands and are low-spin. The two configurations, represented in Fig. 1, differ in the number of unpaired electrons and hence they give rise to marked differences in all those properties of complexes which depend on electronic configuration. Important among these are magnetic and electronic spectral properties. In addition, the Mössbauer spectra for the two spin states are quite distinct and the spectral parameters are clearly diagnostic of the spin state, as is discussed in section 3.4. Further, the absence of electrons in the antibonding eg orbitals in the low-spin configuration allows closer approach of the donor atoms and hence stronger metal --- donor atom interaction.