PHASE-TRANSITIONS ARISING FROM INTRAMOLECULAR ELECTRON-TRANSFER IN MIXED-VALENCE COMPLEXES

Many of the oxo-centered trinuclear transition-metal acetate complexes with the formula [M(III)2M(II)O(O2CCH3)6L3]S (M: Fe or Mn, L: monodentate ligand, S: solvate molecule) have been shown to convert from a statically valence-trapped state at low temperatures to valence-detrapped states at high temperatures, in which the mixed-valence complex is dynamically interconverting between three or four configurations corresponding to the minima on the ground-state adiabatic potential-energy surface. Our recent heat-capacity measurements have clearly demonstrated that the intramolecular electron-transfer event in these complexes occurs as phase transitions in cooperation with orientational disordering of the S that is not explicitly coordinated to the central metal ions M. Although, depending on S, the transition entropy is widely altered from complex to complex, mechanisms of the phase transitions can be interpreted in terms of intermolecular interactions between the distortion dipole of the complex and the permanent dipole of the solvate molecule S. The present paper reviews thermal properties of seven complexes having different combinations of M, L, and S. Particularly emphasized is the importance of complementary roles between the macroscopic entropic aspects and the microscopic aspects derived from spectroscopic and structural studies.