Mixed valence complexes contain an element which is present in more than one

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. ...

. Well-known mixed valence compounds include the

Creutz–Taube complex The Creutz–Taube ion is the metal complex with the formula 5+. This cationic species has been heavily studied in an effort to understand the intimate details of inner sphere electron transfer, that is, how electrons move from one metal complex t ...

Prussian blue Prussian blue (also known as Berlin blue, Brandenburg blue or, in painting, Parisian or Paris blue) is a dark blue pigment produced by oxidation of ferrous ferrocyanide salts. It has the chemical formula Fe Cyanide.html" ;"title="e(Cyanide">CN ...

, and

molybdenum blue Molybdenum blue is a term applied to: *reduced heteropolymolybdate complexes, polyoxometalates containing Mo(V), Mo(VI), and a hetero atom such as phosphorus or silicon *reduced isopolymolybdate complexes, polyoxometalates containing Mo(V), Mo(VI) ...

. Many solids are mixed-valency including

indium chalcogenides The indium chalcogenides include all compounds of indium with the chalcogen elements, oxygen, sulfur, selenium and tellurium. (Polonium is excluded as little is known about its compounds with indium). The best-characterised compounds are the In(III ...

Robin–Day classification

Mixed-valence compounds are subdivided into three groups, according to the Robin–Day classification: *Class I, where the valences are trapped—localized on a single site—such as Pb₃O₄ and

antimony tetroxide Antimony tetroxide is an inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a sub ...

. There are distinct sites with different specific valences in the complex that cannot easily interconvert. *Class II, which are intermediate in character. There is some localization of distinct valences, but there is a low

activation energy In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules p ...

for their interconversion. Some thermal activation is required to induce electron transfer from one site to another via the bridge. These species exhibit an intense

Intervalence charge transfer 192 px, The intense blue color of Prussian blue is a consequence of an intervalence charge transfer band. In chemistry, intervalence charge transfer, often abbreviated IVCT or even IT, is a type of charge-transfer band that is associated with m ...

(IT or IVCT) band, a broad intense absorption in the infrared or visible part of the spectrum, and also exhibit magnetic exchange coupling at low temperatures. The degree of interaction between the metal sites can be estimated from the absorption profile of the IVCT band and the spacing between the sites. This type of complex is common when metals are in different ligand fields. For example,

is an iron(II,III)–

cyanide Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms. In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of ...

complex in which there is an iron(II) atom surrounded by six carbon atoms of six

ligands bridged to an iron(III) atom by their nitrogen ends. In the

Turnbull's blue Prussian blue (also known as Berlin blue, Brandenburg blue or, in painting, Parisian or Paris blue) is a dark blue pigment produced by oxidation of ferrous ferrocyanide salts. It has the chemical formula Fe Cyanide.html" ;"title="e(Cyanide">CN ...

preparation, an iron(II) solution is mixed with an iron(III) cyanide (c-linked) complex. An electron-transfer reaction occurs via the cyanide ligands to give iron(III) associated with an iron(II)-cyanide complex. *Class III, wherein mixed valence is not distinguishable by spectroscopic methods as the valence is completely delocalized. The Creutz–Taube complex is an example of this class of complexes. These species also exhibit an IT band. Each site exhibits an intermediate oxidation state, which can be half-integer in value. This class is possible when the ligand environment is similar or identical for each of the two metal sites in the complex. In fact, Robson type dianionic tetraimino-diphenolate ligands which provide equivalent N2O2 environments for two metal centres have stabilized the mixed valence diiron complexes of class III. The bridging ligand needs to be very good at electron transfer, be highly conjugated, and be easily reduced.

Creutz–Taube ion

The

is a robust, readily analyzed, mixed-valence complex consisting of otherwise equivalent Ru(II) and Ru(III) centers bridged by the

pyrazine Pyrazine is a heterocyclic aromatic organic compound with the chemical formula C4H4N2. It is a symmetrical molecule with point group D2h. Pyrazine is less basic than pyridine, pyridazine and pyrimidine. It is a ''"deliquescent crystal or wax-l ...

. This complex serves as a model for the bridged intermediate invoked in

inner-sphere electron transfer Inner sphere electron transfer (IS ET) or bonded electron transfer is a redox chemical reaction that proceeds via a covalent linkage—a strong electronic interaction—between the oxidant and the reductant reactants. In inner sphere electron tran ...

Mixed valence organic compounds

Organic mixed valence compounds are also known. Mixed valency in fact seems to be required for organic compounds to exhibit electrical conductivity.

References

{{Reflist Physical chemistry Electron