hypercoordination

https://doi.org/10.1351/goldbook.HT07051
A property of main-group atoms in molecular entities to acquire @C01329@ numbers greater than four (which would comply with the @LT07065@). Hypercoordination may be associated with @HT07054@, but usually is referred to peculiar atomic centres in the electron-deficient species with multicentre σ-bonding, in which the bonding @P04792@ 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 @C00907@, 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 @H02899@.
HT07051.png
Source:
PAC, 1999, 71, 1919. (Glossary of terms used in theoretical organic chemistry) on page 1945 [Terms] [Paper]