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A 4-center 2-electron (4c–2e) bond is a type of 
An example of a purely organic compound with four-center two-electron bonding is the adamantyl dication. The bond joins the four bridgehead atoms in a tetrahedral geometry.
Tetracyanoethylene forms a dianionic dimer in which the two
chemical bond
A chemical bond is the association of atoms or ions to form molecules, crystals, and other structures. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds or through the sharing of electrons a ...
in which four atom
Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...
s share two electron
The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
s in bonding, with a net bond order of . This type of bonding differs from the usual covalent bond
A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...
, which involves two atoms sharing two electrons (2c–2e bonding).
Four-center two-electron bonding is postulated in certain cluster compound
Nanoclusters are atomically precise, crystalline materials most often existing on the 0-2 nanometer scale. They are often considered kinetically stable intermediates that form during the synthesis of comparatively larger materials such as semic ...
s. For instance, the borane anion
An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
is a octahedron
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 ...
with an additional proton attached to one of the triangular faces. As a result, the octahedron is distorted and a B–B–B–H rhomboid
Traditionally, in two-dimensional geometry, a rhomboid is a parallelogram in which adjacent sides are of unequal lengths and angles are non-right angled.
The terms "rhomboid" and "parallelogram" are often erroneously conflated with each oth ...
ring can be identified in which this 4c–2e bonding takes place. This type of bonding is associated with electron deficient rhomboid rings in general and is a relatively new research field, fitting in with the already well established three-center two-electron bond.
An example of a purely organic compound with four-center two-electron bonding is the adamantyl dication. The bond joins the four bridgehead atoms in a tetrahedral geometry.
Tetracyanoethylene forms a dianionic dimer in which the two alkene
In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins.
The Internationa ...
s are joined face-to-face by a rectangular four-center two-electron bond. Various solid salts of this dianion have been studied to determine bond strengths and vibrational spectroscopic details.
References
{{DEFAULTSORT:Four-Center Two-Electron Bond Chemical bonding