Outer sphere refers to an
electron transfer
Electron transfer (ET) occurs when an electron relocates from an atom or molecule to another such chemical entity. ET is a mechanistic description of certain kinds of redox reactions involving transfer of electrons.
Electrochemical processes ar ...
(ET) event that occurs between chemical species that remain separate and intact before, during, and after the ET event. In contrast, for
inner sphere electron transfer the participating redox sites undergoing ET become connected by a chemical bridge. Because the ET in outer sphere electron transfer occurs between two non-connected species, the electron is forced to move through space from one
redox
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a ...
center to the other.
Marcus theory
The main theory that describes the rates of outer sphere electron transfer was developed by
Rudolph A. Marcus in the 1950s. A major aspect of
Marcus theory is the dependence of the electron transfer rate on the thermodynamic driving force (difference in the redox potentials of the electron-exchanging sites). For most reactions, the rates increase with increased driving force. A second aspect is that the rate of outer sphere electron-transfer depends inversely on the "reorganizational energy." Reorganization energy describes the changes in bond lengths and angles that are required for the oxidant and reductant to switch their oxidation states. This energy is assessed by measurements of the self-exchange rates (see below).
Outer sphere electron transfer is the most common type of electron transfer, especially in
biochemistry
Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology an ...
, where redox centers are separated by several (up to about 11) angstroms by intervening protein. In biochemistry, there are two main types of outer sphere ET: ET between two biological molecules or fixed distance electron transfer, in which the electron transfers within a ''single'' biomolecule (e.g., intraprotein).
Examples
Self-exchange
Outer sphere electron transfer can occur between chemical species that are identical except for their oxidation state.
[R. G. Wilkins Kinetics and Mechanism of Reactions of Transition Metal Complexes, 2nd Edition, VCH, Weinheim, 1991. {{ISBN, 1-56081-125-0] This process is termed self-exchange. An example is the
degenerate reaction between the tetrahedral ions
permanganate
A permanganate () is a chemical compound containing the manganate(VII) ion, , the conjugate base of permanganic acid. Because the manganese atom is in the +7 oxidation state, the permanganate(VII) ion is a strong oxidizing agent. The ion is a t ...
and
manganate
In inorganic nomenclature, a manganate is any negatively charged molecular entity with manganese as the central atom.. However, the name is usually used to refer to the tetraoxidomanganate(2−) anion, MnO, also known as manganate(VI) because it ...
:
:
nO4sup>− +
n*O4sup>2− →
nO4sup>2− +
n*O4sup>−
For octahedral metal complexes, the rate constant for self-exchange reactions correlates with changes in the population of the e
g orbitals, the population of which most strongly affects the length of metal-ligand bonds:
*For the
o(bipy)
3.html"_;"title="bipy.html"_;"title="o(bipy">o(bipy)
3">bipy.html"_;"title="o(bipy">o(bipy)
3sup>+/[Co(bipy)
3.html" ;"title="bipy">o(bipy)
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3">bipy.html" ;"title="o(bipy">o(bipy)
3sup>+/[Co(bipy)
3">bipy">o(bipy)
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3sup>+/[Co(bipy)
3sup>2+ pair, self exchange proceeds at 10
9 M
−1s
−1. In this case, the electron configuration changes from Co(I): (t
2g)
6(e
g)
2 to Co(II): (t
2g)
5(e
g)
2.
*For the [Co(bipy)
3]
2+/[Co(bipy)
3]
3+ pair, self exchange proceeds at 18 M
−1s
−1. In this case, the electron configuration changes from Co(II): (t
2g)
5(e
g)
2 to Co(III): (t
2g)
6(e
g)
0.
Iron-sulfur proteins
Outer sphere ET is the basis of the biological function of the
iron-sulfur protein
Iron–sulfur proteins (or iron–sulphur proteins in British spelling) are proteins characterized by the presence of iron–sulfur clusters containing sulfide-linked di-, tri-, and tetrairon centers in variable oxidation states. Iron–sulfur ...
s. The Fe centers are typically further coordinated by cysteinyl ligands. The
e4S4electron-transfer proteins (
e4S4ferredoxin
Ferredoxins (from Latin ''ferrum'': iron + redox, often abbreviated "fd") are iron–sulfur proteins that mediate electron transfer in a range of metabolic reactions. The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co. and applied ...
s) may be further subdivided into low-potential (bacterial-type) and
high-potential (HiPIP) ferredoxins. Low- and high-potential ferredoxins are related by the following redox scheme:
Because of the small structural differences between the individual redox states, ET is rapid between these clusters.
See also
*
Inner sphere electron transfer
References
Physical chemistry
Electron