Molecular Orbital Model of Electron Delocalization in Mixed-Valence Trimeric Clusters

A simple extended Hückel type molecular orbital model is used to study electronic distributions in mixed-valence iron−oxygen clusters containing three octahedral subunits. Two different structures were considered: one-atom bridged octahedra and octahedra with common faces. The stability of symmetric structures relative to different non-totally symmetric distortions leading to the vibronic charge trapping was examined. It appears that the distribution of the excess electron cannot be described by the simple alternative “localization−delocalization”. The type of electronic distribution depends on the arrangement of octahedral units, the metal−metal distances, and the electronic configurations. The molecular orbital approach gives some insights into the peculiarities of the intramolecular electron delocalization in mixed-valence iron−oxygen trimers which are important for their catalytic activity.