A four-center two-electron bond is a type of chemical bond in which four atoms share two electrons in bonding which is unusual because in ordinary chemical bonds two atoms share two electrons (2c-2e bonding). This type of bonding is postulated in certain cluster compounds. For instance, the borane B6H7- anion, is a B6H62- octahedron with an additional proton attached to one of the triangular faces. [1]. As a result the octahedron is distorted and a BBBH 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 [2] and is a relatively new research field, fitting in with the already well established three-center two-electron bonds and three-center four-electron bonds.
An example of a purely organic compound with four-center two-electron bonding is the adamantyl dication.[3]
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