Three-center bond

The three-center bond, the simplest multicenter bonds is a special type of atomic bond ( covalent bond ), in which no two atoms, but three atoms share a pair of electrons. According to the modern atomic theory ( orbital theory) describes three-center bonds by overlap of three atomic orbitals, with three molecular orbitals arise. Of these, one is always bound by and one antibonding. The third molecular orbital can have weak binding, weakly antibonding or non-bonding character. Prime examples of three-center bonds are found in the H3 ion, in diborane ( B2H6 ), in Hydrogendifluorid ion ( FHF ) or in the nitrite ion ( NO2 - ). One speaks in connection with three- or multicenter bonds by a delocalization of the bonding electrons. Two-center bonds - the normal covalent bonds - and lone pairs are, however, referred to as localized.

Three-center bond, HF2 - Ion

Three-center bond, NO2 - ion

Open and closed three-center bonds

A distinction is made between open three-center bonds - how to in FHF - ion is - and three-center bonds, such as that present in the H3 ion. In the first case to show two of the three atoms no appreciable orbital overlap ( here the two fluorine atoms ), in the second case one finds a pairwise overlap of all three atomic orbitals - it forms a ring. By increasing angulation, an open pass in a closed three-center bond. While open three-center bonds to be able to contain four electrons - up to two of which can be found in a non- bonding orbital - is a closed three-center bond stable only if no more than two electrons are accommodated in the molecular orbitals.

3-center -2- electron bond and 3- center-4 - electron bond

One finds in the literature: the term 3-center -2- electron bond (3c -2e ) or 3- center-4 - electron bond ( 3c- 4e). These terms indicate how many electrons are at a given molecule in the three molecular orbitals, which result from the three-center interaction housed. But since only one of the three molecular orbitals can be considered significant and a maximum of binding two electrons can be accommodated in the orbital, only the term three -center two - electron bonds is used throughout. In four electrons and the nonbonding orbital is doubly occupied. Since this orbital has only the two outer atoms substantial contributions (coefficients) " similar ", this electron lone pairs on these atoms. The bonding in these 3- center-4 - electron bond can be accordingly formulate two resonance structures. Herein is seated at each of the two outer atoms of a free electron pair, which points in the direction of the central atom, while the other is formulated from a bond to the central atom ( bond / no -bond interface structure ) (see HF2 - ion).

Symbols

For the adequate representation of a three-center bond with Lewis formulas you need at least two resonance structures. In addition, special symbols have naturalized, but these are not consistently used and accepted as they are partly to be confused with the skeletal formula.

Intramolecular Interactions: Ionic bond | Covalent bond | Metallic bond | Coordinative binding

Intermolecular interactions: hydrogen bonds | Dipole -dipole interactions | Van der Waals forces

Delocalized π - bond | delocalized σ - bond ( three-center bond, four-center bond)

• Chemical Bonding