A crystallographic phase transition involving changes in the solvate molecule has been found for mixed-valence [Fe3O(O2CCH3)(6)(3-Cl-py)(3)]. 3-Cl-py (1), where 3-Cl-py is 3-chloropyridine. Single-crystal X-ray structures were determined at 300, 228, 200, 169, and 122 K for complex 1. At 300, 228, and 200 K the crystal is monoclinic, space group P2(1)/c, whereas at 169 and 122 K it is triclinic, space group P (1) over bar. Determinations of the unit cell parameters at several temperatures shows that a reversible crystallographic phase transition between the monoclinic and triclinic forms occurs at similar to 200 K. Complex 1 crystallizes in the monoclinic space group P2(1)/c at 300:K, having a unit cell with a = 21.212(8) Angstrom b = 8.434(2) Angstrom, c 23.676(3) Angstrom, and Z = 4. Refinement with 5702 observed [F-o > 4 sigma(F-o)] reflections gave R = 0.0542 and R-w = 0.0937. Complex 1 crystallizes in the triclinic space group P (1) over bar at 122 K, having a unit cell with a = 20.983(11) Angstrom, b = 8.360(4) Angstrom, c = 23.293(10) Angstrom, and Z = 4. At 300 K there is one somewhat asymmetric Fe3O complex in the structure. The core dimensions in the Fe3O complex at 300 K indicate that the complex is becoming almost valence-detrapped. At 122 K there are two different Fe3O complexes in the unit cell, both of which are similar in dimensions. As the temperature is decreased from 300 to 122 K, each Fe3O complex becomes more and more distorted in an equilateral triangle. At 122 K one iron ion in each Fe3O complex clearly is a high-spin. Fe-II ion and the other two are high-spin Fe-III ions. There are significant changes in the nature of the 3-Cl-py solvate molecules above and below the phase transition that are likely important in controlling the valence detrapping. At 122 K there are two different Fe3O complexes,; each with their nearby 3-Cl-py solvate molecules in one position. There are three different phases: a monoclinic one with all solvate molecules disordered, a second triclinic phase at 169 K with half of the solvate molecules disordered, and a third triclinic phase at 122 K with all solvate molecules statically ordered. Fe-57 Mossbauer spectra taken in the 110-293 K range show that complex 1 converts from valence-trapped at 110 K to become detrapped by 293 K, where a single quadrupole-split doublet is seen. Throughout the 140-230 K range it was necessary to employ one Fe-III doublet and two Fe-II doublets to fit each Mossbauer spectrum. It is shown that the two Fen doublets likely arise from Fe3O complexes experiencing the different disordered solvate environments described above. Thus, while the similar to 200 K structural phase transition involving the solvate molecules does not precipitously lead to an increase in the rate of electron transfer in Fe3O complexes in 1, it is clear that the changes seen in the solvate molecules from X-ray structures do play a major role in the valence detrapping in complex 1.