A property of main-group atoms in molecular entities to acquire @[email protected] numbers greater than four (which would comply with the @[email protected]). Hypercoordination may be associated with @[email protected], but usually is referred to peculiar atomic centres in the electron-deficient species with multicentre σ-bonding, in which the bonding @[email protected] of a pair of electrons is spread over more than two atoms. An example of a hypercoordinated atom is the five-coordinate carbon atom in the methanium @[email protected], where three C–H bonds may be regarded as normal two center - two electron bonds and the bonding in the remaining CH2 fragment is governed by the three-centre, two-electron bond. A particular case of a hypercoordinated atom is the hydrogen atom included into a @[email protected]
PAC, 1999, 71, 1919. (Glossary of terms used in theoretical organic chemistry) on page 1945 [Terms] [Paper]