<jats:p>Intramolecular electron delocalization in discrete mixed-valence transition metal complexes in the condensed phase depends not only on the electronic structure of a single complex but also sensitively on the details of the packing arrangement [D. N. Hendrickson, S. M. Oh, T.-Y. Dong, T. Kambara, M. J. Cohn, and M. F. Moore, Comments Inorg. Chem. 4, 329 (1985)]. The problem of how the cooperative properties of mixed-valence complexes in the solid state depend on the electron localization and/or delocalization in a single complex is studied theoretically. A phenomenological intermolecular interaction which depends on the sense and the magnitude of molecular distortion arising from the electron localization is introduced. A theoretical model is developed based on molecular field theory in order to show what types of phase transitions relating to the electron delocalization are possible in the trinuclear mixed-valence compounds and how the electronic structure of constituent molecules determines the type of phase transition. There are three types of phase transitions: (1) Order–disorder transition with respect to the alignment of the sense of molecular distortion associated with the electron localization; (2) static localization–delocalization transition, where the molecular distortion disappears above the transition temperature and electrons are coherently delocalized on three transition metal ions; (3) dynamical localization–delocalization transition in which the delocalization comes from fast electron transfer between three transition-metal ions and the molecular structure is changed from a static distortion to a dynamical distortion. The theoretical model is used to explain the observed temperature dependencies of heat capacity and Mössbauer spectra for the trinuclear mixed-valence complex</jats:p> <jats:p>[Fe3O(O2CCH3)6(py)3](py), where (py) is pyridine [S. M. Oh. T. Kambara, D. N. Hendrickson, M. Sorai, K. Kaji, S. E. Woehler, and R. J. Wittebort, J. Am. Chem. Soc. 107, 5540 (1985); M. Sorai, K. Kaji, D. N. Hendrickson, and S. M. Oh, ibid. 108, 702 (1986)]. The first-order phase transition at ∼112 K is assigned as an order–disorder transition and the higher-order transition at ∼190 K is assigned as a dynamical localization–delocalization transition.</jats:p>