Inner sphere electron transfer

Inner sphere or bonded electron transfer proceeds via a covalent linkage between the two redox partners, the oxidant and the reductant. In Inner Sphere (IS) electron transfer (ET), a ligand bridges the two metal redox centers during the electron transfer event. Inner sphere reactions are inhibited by large ligands, which prevent the formation of the crucial bridged intermediate. Thus, IS ET is rare in biological systems, where redox sites are often shielded by bulky proteins. Inner sphere ET is usually used to describe reactions involving transition metal complexes and most of this article is written from this perspective. However, redox centers can consist of organic groups rather than metal centers.

The bridging ligand could be virtually any entity that can convey electrons. Typically, such a ligand has more than one lone electron pair, such that it can serve as an electron donor to both the reductant and the oxidant. Common bridging ligands include the halides and the pseudohalides such as hydroxide and thiocyanate. More complex bridging ligands are also well known including oxalate, malonate, and pyrazine. Prior to ET, the bridged complex must form, and such processes are often highly reversible. Electron transfer occurs through the bridge once it is established.

The alternative to inner sphere electron transfer is outer sphere electron transfer. In any transition metal redox process, the mechanism can be assumed to be outer sphere unless the conditions of the inner sphere are met. Inner sphere electron transfer is generally enthalpically more favorable than outer sphere electron transfer due to a larger degree of interaction between the metal centers involved, however, inner sphere electron transfer is usually entropically less favorable since the two sites involved must become more ordered (come together via a bridge) than in outer sphere electron transfer.

Taube's experiment
The discoverer of the inner sphere mechanism was Henry Taube, who was awarded the Nobel Prize in 1983 for his pioneering studies. A particularly historic finding is summarized in the abstract of the seminal publication. “When Co(NH3)5Cl++ is reduced by Cr++ in M {meaning 1M} HClO4, 1 Cl- appears attached to Cr for each Cr(III) which is formed or Co(III) reduced. When the reaction is carried on in a medium contg. radioactive Cl, the mixing of the Cl- attached to Cr(III) with that in soln. is less than 0.5%. This expt. shows that transfer of Cl to the reducing agent from the oxidizing agent is direct…” The paper and the excerpt above can be described with the following equation:
 * [CoCl(NH3)5]2+ +  [Cr(H2O)6]2+ →  [CrCl(H2O)5]2+  +  [Co(NH3)5(H2O)]2+

The point of interest is that the chloride that was originally bonded to the cobalt, the oxidant, becomes bonded to chromium, which in its 3+ oxidation state, forms kinetically inert bonds to its ligands. This observation implies the intermediacy of the bimetallic complex [Co(NH3)5(&mu;-Cl)Cr(H2O)5]4+, wherein "μ-Cl" indicates that the chloride bridges between the Cr and Co atoms, serving as a ligand for both. This chloride serves as a conduit for electron flow from Cr(II) to Co(III), forming Cr(III) and Co(II).

The Creutz-Taube Ion
In the preceding example, the occurrence of the chloride bridge is inferred from the product analysis, but it was not observed. One complex that serves as a model for the bridged intermediate is the "Creutz Taube Complex," [(NH3)5RuNC4H4NRu(NH3)5]5+. This species is named after Carol Creutz, who prepared the ion during her PhD studies with Henry Taube. The bridging ligand is the heterocycle pyrazine, 1,4-C4H4N2. In the Creutz-Taube Ion, the average oxidation state of Ru is 2.5+. Spectroscopic studies, however, show that the two Ru centers are equivalent, which indicates the ease with which the electron hole communicates between the two metals. The significance of the Creutz-Taube ion is its simplicity, which facilitates theoretical analysis, and its high symmetry, which ensures a high degree of delocalization. Many more complex mixed valence species are known both as molecules and polymeric materials.

Mixed-valency
The Creutz-Taube Ion is one member of a large class of compounds that are called "mixed-valence." Well known mixed-valence compounds include Prussian Blue and Molybdenum blue. Many solids are mixed-valency including indium chalcogenides. Mixed-valency is required for organic metals to exhibit electrical conductivity.

Mixed-valence compounds are subdivided into three groups, according to the Robin-Day Classification:
 * Class I, where the valences are "trapped," such as Pb3O4 and antimony tetroxide.
 * Class II, which are intermediate in character, the electron hopping requires thermal activation. These species exhibit an intense Intervalence charge transfer (IT) band, a broad intense absorption in the IR- or visible part of the spectrum.
 * Class III, wherein mixed valence is not distinguishable by spectroscopic methods. The Creutz-Taube Ion is an example.  Such species also exhibit an IT band