Nonheme “FeIVO” Models: Ab Initio Analysis of the Low‐Energy Spin State Electronic Structures

Abstract

AbstractThe spin‐dependent geometries, energetics, and electronic structures of the [FeO]2+ motif are investigated on the basis of DFT and perturbative CASPT2 ab initio calculations. The synthetic complex [(L52)FeO]2+ exhibits an S = 1 ground state with a low‐lying (≈2000 cm–1) competing quintet and an open‐shell singlet as the second excited state (≈11000 cm–1 above the ground state). By using the geometry optimization information, it was shown from a valence bond reading of the variational wavefunctions that a FeIII–O· picture dominates and that the number of d electrons is almost five in the model system [Fe(NH3)5O]2+. Analogy with the spin‐crossover transition phenomenon is suggested, as the ground and low‐lying quintet states can be seen as resulting from a doublet → quartet transition of a Fe d5 ion. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)