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= Mixed valence salts = Mixed valence, or mixed oxidation state, compounds are a unique class of compounds with an element presenting distinct oxidation states. They exhibit a broad range of fascinating physicochemical properties. They are widely used as dyes in pigments and paints due to their intense color.

History
The first man-made mixed valence inorganic complex is Prussian Blue, FeIII4[FeII(CN)6]3·nH2O. This complex dates back to 1704 and found broad application as a dye. After that, P. Day synthesized and studied thoroughly the century-old mixed valence species, Cs2SbCl6. Since then,many other mixed valence salts saw light and were well classified.

Different metallic mixed valence salts
A large variety of mixed valence ompounds have been known in literature using different metals. A brief summary of different metal-based mixed valence salts are descrived below.

Iron
The mixed valence compounds of iron form by far the largest group presently known for any element. Deussen proved that the colorless iron fluoride, thought to be FeF3.4,5H2O, in reality was the mixed valence salt FeIIFe2IIIF8. 10H2O. Another example or iron mixed valence compounds is known as iron lazulite. It is known to have a shiny, jewel-like black color and contains FeII and FeIII in face-sharing octahedral coordination.

Silver
Silver can also possess different oxidation states in multinuclear complexes. Silver-based mixed valence systems are quite rare in literature. Only 20 mixed valence silver compounds are well-defined, some having both AgI/AgIII oxidation states.

The first reported AgI/AgIII compound is AgO, which was better described later on by McMillan as AgIAgIIIO2 rather than AgIIO. In 1966, McWhan et al. described the first molecular AgI/AgIII complexes and explored their high-temperature superconductivity behavior.

Pioneering work done by Naumann et al. unveiled the synthesis of [AgI][AgIII(CF3)4], which was recently crytsallized by Nebra et al.