This article describes research from our laboratory on the chemistry and spectroscopic properties of inorganic mixed valence complexes. After a brief review of the seminal work of Taube, Creutz, Day, Robin, Hush, and others in the 1960s and the confounding efforts to identify the borderline between class II and III mixed valence systems in the 1990s and early 2000s, we describe our first experiments to observe and analyze the coalescence of v(CO) band shapes in the 1D IR spectra of mixed valence complexes of the type {[Ru3O(OAc)(6)(CO)(L)](2)-BL}(-), where L = a pyridyl ligand and BL = pyrazine or 4,4'-bipyridine, to estimate rate constants of intramolecular electron transfer (ET). The strong involvement of the bridging ligands in mixed valence complexes of this type was first identified in the appearance of totally symmetric vibrational modes of pyrazine bridging ligands in the IR because of strong vibronic coupling within a three state metal cluster-bridge-metal cluster model Application of the Brunschwig-Creutz-Sutin semiclassical three state model of mixed valency accounts well for the appearance of two intervalence charge transfer bands that are observed in the near-IR region of the electronic absorption spectra of these mixed valence ions. The direct spectroscopic observation of "mixed-valence isomers", the two alternate charge distributions of a mixed valence ion, are described. The equilibrium constants of mixed valence isomers provide quantitative thermodynamic estimates of the electronic coupling, H-ab. The extent of delocalization in many of the mixed-valent bridged dimers of triruthenium clusters produces unusual behavior, especially rate constants for ET that are independent of normal solvent reorganization energies but do depend on solvent dynamical dipolar reorientation times. The strong dependence of rates on solvent dynamics is also found to produce a non-Arrhenius dependence of ET rate constants on temperature, faster rates in frozen solutions compared to fluid solutions. The studies of these mixed valence systems have provided generalized guidelines for establishing where a particular mixed valence system lies along the class II/III delocalization transition, and they have increased our understanding of the ET dynamics at the delocalization threshold.