Four-center two-electron bond
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 boranea B6H7- anion, is a B6H62- octahedron with an additional proton attached to one of the triangular faces. . 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  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.
- A new 4c–2e bond in B6H7– Kathrin Hofmann, Marc H. Prosenc and Barbara R. Albert Chem. Commun., 2007, 3097 - 3099, doi:10.1039/b704944g
- Electron-Deficient Bonding in Rhomboid Rings Musiri M. Balakrishnarajan and Roald Hoffmann J. Am. Chem. Soc.; 2004; 126(40) pp 13119 - 13131; (Article) DOI: 10.1021/ja0467420