Outer sphere refers to an
electron transfer
Electron transfer (ET) occurs when an electron relocates from an atom, ion, or molecule, to another such chemical entity. ET describes the mechanism by which electrons are transferred in redox reactions.
Electrochemical processes are ET reactio ...
(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
Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is t ...
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 or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is t ...
center to the other.
Marcus theory
The main theory describing the rates of outer sphere electron transfer was developed by
Rudolph A. Marcus in the 1950s, for which he was awarded the
Nobel Prize in Chemistry
The Nobel Prize in Chemistry () is awarded annually by the Royal Swedish Academy of Sciences to scientists in the various fields of chemistry. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895, awarded for outst ...
in 1992.
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
An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or "Electron acceptor, accepts"/"receives" an electron from a (called the , , or ''electr ...
and
reductant
In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ).
Examples of substances that are common reducing agents include hydrogen, carbon ...
to switch their
oxidation state
In chemistry, the oxidation state, or oxidation number, is the hypothetical Electrical charge, charge of an atom if all of its Chemical bond, bonds to other atoms are fully Ionic bond, ionic. It describes the degree of oxidation (loss of electrons ...
s. 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, a ...
, where redox centers are separated by several (up to about 11)
angstrom
The angstrom (; ) is a unit of length equal to m; that is, one ten-billionth of a metre, a hundred-millionth of a centimetre, 0.1 nanometre, or 100 picometres. The unit is named after the Swedish physicist Anders Jonas Ångström (1814–18 ...
s by intervening
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
. In biochemistry, there are two main types of outer sphere ET: ET between two separate 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
In chemistry, the oxidation state, or oxidation number, is the hypothetical Electrical charge, charge of an atom if all of its Chemical bond, bonds to other atoms are fully Ionic bond, ionic. It describes the degree of oxidation (loss of electrons ...
.
[R. G. Wilkins Kinetics and Mechanism of Reactions of Transition Metal Complexes, 2nd Edition, VCH, Weinheim, 1991. ] This process is termed self-exchange. An example is the
degenerate reaction between the
tetrahedral
In geometry, a tetrahedron (: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular Face (geometry), faces, six straight Edge (geometry), edges, and four vertex (geometry), vertices. The tet ...
ions
permanganate
A permanganate () is a chemical compound with the manganate(VII) ion, , the conjugate base of permanganic acid. Because the manganese atom has a +7 oxidation state, the permanganate(VII) ion is a strong oxidising agent. The ion is a transition ...
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 c ...
:
:
nO4sup>− +
n*O4sup>2− →
nO4sup>2− +
n*O4sup>−
For
octahedral
In geometry, an octahedron (: octahedra or octahedrons) is any polyhedron with eight faces. One special case is the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex. Many types of i ...
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">bipy.html" ;"title="o(bipy">o(bipy)
3sup>+/
o(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
o(bipy)3sup>2+/
o(bipy)3sup>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 proteins. The Fe centers are typically further
coordinated by
cysteinyl ligands
In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's ...
. 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 t ...
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
{{Reaction mechanisms
Physical chemistry
Electron