The instability of states with full electron (hole) delocalisation in mixed-valence Fe ₄ S ₄ clusters. the role of symmetry distortions

The electronic states and patterns of delocalisation of [Fe₄S₄]³⁺ and [Fe₄S₄]¹⁺ clusters as function of their topology are studied. It is shown that the states with equal charge distribution over the whole cluster are unstable. Already minor deviations from high cluster symmetry favour pair delocalisation of the 'excess' electron (hole). However, when two or more channels for delocalisation are nearly equivalent also other patterns of delocalisation are possible. The effect of delocalisation is different with respect to the sign of transfer parameters and exhibits different trends for the [Fe₄S₄]³⁺ and [Fe₄S₄]¹⁺ clusters.