Spin Transitions in Octahedral Iron(II) Compounds with Heterocyclic Ligands

Abstract

Spin transitions (ST) in transition-metal complexes are a well-known phenomenon in coordination chemistry. Most studied are the systems with a d 6 electron configuration, where the transition goes between essentially diamagnetic and paramagnetic states. In many cases the transition temperature is different when heating or when cooling the sample, and the resulting hysteresis can be applied for storage. So, the colour change would allow possible applications such as in storage devices. Our approach has been the development of systems with Fe(II) and heterocyclic ligands, such as the triazoles and tetrazoles. An overview of recent results is given below, with a focus upon structures and changes in the physical properties. Most structures have the general formula FeL 6 (anion) 2 , with L is a monodentate, or half of a didentate ligand. A special case deals with the compound Fe(btr) 2 (NCS) 2 (H 2 O), the first compound with a trans geometry for the NCS ligands that shows a spin transition. The role of the water molecule in the lattice is crucial, as no ST is observed when water is absent or removed in vacuo. iron coordination spin transition Mössbauer magnetism