Cyano-bridged mixed valence compounds have been known to exist since 1704, but a lot of interest in the bi-nuclear and tri-nuclear species has emerged only more recently. The growing interest in these complexes is due mainly to their promising and useful applications in electrochromism, molecular magnetism and molecular electronics. These properties are activated by the excitation of their metal-to-metal charge transfer (MMCT) transition. This book characterizes aqueous solutions of the FeHEDTA / [Fe(CN)6]4- system which form intensely colored solutions that absorb strongly in the visible/NIR region of the electromagnetic spectrum. Typically binuclear (Fe(III)-Fe(II)) and trinuclear (Fe(III)-Fe(II)-Fe(III)) complexes are formed. Information on reorganizational parameters, electronic coupling between metal centers, molar absorptivities, equilibrium constants, and delocalization factors are discussed for binuclear and trinuclear Fe(III)/Fe(II) complexes, and compared to similar systems reported in the literature. This book is ideal for researchers in chemistry, physics, material science, and engineering.