The origin of the distribution of an “excess” electron in the trinuclear mixed-valence cluster from Desulfovibrio gigas is discussed. The model used takes into account Heisenberg exchange and resonance interaction between all centres leading to the double exchange. It is shown that electron delocalization over two centres in the ground state is possible in a high-symmetry trimer due to vibronic effects. The same electron distribution also exists in a slightly distorted trimer. In the last case approximately equal population of all centres in the first excited state is possible. These results agree with experimental data.