Molecular Geometries of Fe (II) Spin-Crossover Complexes

Spin-crossover (SCO) complexes are a class of inorganic compounds in which external stimuli facilitate a change in geometry and spin state from a diamagnetic low spin state to a paramagnetic high spin state or vice versa. Due to this bistability, they are an important class of molecules for new energy technologies and molecular electronics¹. The multiconfigurational nature and large number of electrons of these molecules make it difficult to provide accurate theoretical descriptions of the electronic structure. To date, density functional theory (DFT) has been the foundation for the study of these complexes’ geometries². Recent advances in computing has made it now possible to produce geometries with methods beyond DFT.
Octahedral Fe(II) (d⁶) molecules are among the most well studied of this class of compound. Here, an accurate description of the multiconfigurational electronic structure of several Fe(II) SCO complexes using the CASPT2 method is presented. The convergence of geometry with respect to active space and basis set size as well as differences with the traditional DFT approach and the refined CASPT2 structures are noted.
1. P. Gutlich, H. A. Goodwin (2004) Spin Crossover in Transition Metal Compounds I. Springer Berlin
2. K. P. Kepp, Inorganic Chemistry 55, 2717 (2016).