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Four-center two-electron bond

A 4-center 2-electron (4c–2e) bond is a type of chemical bond in which four atoms share two electrons in bonding, with a net bond order of . This type of bonding differs from the usual covalent bond, which involves two atoms sharing two electrons (2c–2e bonding).

Four-center two-electron bonding is postulated in certain cluster compounds. For instance, the borane anion is a octahedron 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 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 alkenes 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

(2007). "A new 4c–2e bond in {{chem". [[Chem. Commun.]].
(2004). "Electron-Deficient Bonding in Rhomboid Rings". [[J. Am. Chem. Soc.]].
(1987). "Four-Center Two-Electron Bonding in a Tetrahedral Topology. Experimental Realization of Three-Dimensional Homoaromaticity in the 1,3-Dehydro-5,7-adamantanediyl Dication". Angew. Chem. Int. Ed. Engl..
(2002). "Exceptionally Long (≥2.9 Å) CC Bonding Interactions in {{chem". Chemistry: A European Journal.
(2013). "Evidence for Multicenter Bonding in Dianionic Tetracyanoethylene Dimers by Raman Spectroscopy". Angew. Chem. Int. Ed..

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