A property of main-group atoms in molecular entities to acquire coordination
numbers greater than four (which would comply with the Lewis octet rule
). Hypercoordination may be associated with hypervalency
, but usually is referred to peculiar atomic centres in the electron-deficient species
with multicentre σ-bonding, in which the bonding power
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 cation
, where three
bonds may be regarded as normal two center - two electron bonds and the bonding in
fragment is governed by the three-centre, two-electron bond.
A particular case of a hypercoordinated atom is the hydrogen atom included into a
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. https://doi.org/10.1351/goldbook