Vibronic effects in a model four-centre, one-electron mixed-valence system

Potential-energy surfaces are calculated for a tetranuclear mixed-valence system consisting of closed-shell metal central and an extra electron which can be distributed amongst the sites. The conditions for electron localisation and delocalisation are explored as a function of the vibronic coupling and electronic coupling parameters. Three disting states are observed: a localised state having C 3v symmetry, a fully delocalised state with T d symmetry and a state of dynamical delocalisation in which the electron is transferred rapidly between localised minima connected by low-energy saddle-points