Four-center two-electron bond explained

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.^[1] 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^[2] 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.^[3] 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.^[4] Various solid salts of this dianion have been studied to determine bond strengths and vibrational spectroscopic details.^[5]

Notes and References

1. A new 4c–2e bond in . K. . Hofmann . M. H. . Prosenc . B. R. . Albert . . 2007 . 2007 . 3097–3099 . 10.1039/b704944g .
2. Electron-Deficient Bonding in Rhomboid Rings . M. M. . Balakrishnarajan . R. . Hoffmann . Roald Hoffmann . . 2004 . 126 . 40 . 13119–13131 . 10.1021/ja0467420 .
3. Matthias. Bremer. Paul von Ragué. Schleyer. Karl. Schötz. Michael. Kausch. Michael. Schindler. 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.. 1987. 26. 761–763. 10.1002/anie.198707611.
4. Exceptionally Long (≥2.9 Å) CC Bonding Interactions in Dimers: Two-Electron Four-Center Cation-Mediated CC Bonding Interactions Involving π* Electrons . Del Sesto . R. E. . Miller . J. S. . Lafuente . P. . Novoa . J. J. . Chemistry: A European Journal . 2002 . 8 . 21 . 4894–4908 . 12397591 . 10.1002/1521-3765(20021104)8:21<4894::AID-CHEM4894>3.0.CO;2-2 .
5. Casado. J. . Burrezo . P. M. . Ramírez . F. J. . Navarrete . J. T. L. . Lapidus . S. H. . Stephens . P. W. . Vo . H.-L. . Miller . J. S. . Mota . F. . Novoa . J. J. . Evidence for Multicenter Bonding in Dianionic Tetracyanoethylene Dimers by Raman Spectroscopy . Angew. Chem. Int. Ed. . 10.1002/anie.201207813 . 2013. 52. 25. 6421–6425 .