Three-center four-electron bond
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The 3-center-4-electron bond is a model used to explain bonding in hypervalent molecules such as phosphorus pentafluoride, sulfur hexafluoride, and the xenon fluorides. The model considers bonding of the three colinear atoms in main group compounds.
For example in XeF2, the linear F-Xe-F subunit is described by a set of three molecular orbitals (MOs) derived from colinear p-orbitals on each atom. The Xe-F bonds result from the combination of a filled p orbital in the central atom (Xe) with two half-filled p orbitals on the axial atoms (F), resulting in a filled bonding orbital, a filled non-bonding orbital, and an empty antibonding orbital. The two lower energy MO's are doubly occupied. The HOMO is localized on the two terminal atoms. This localization of charge is accommodated by the fact that the terminal ligands are highly electronegative in hypervalent molecules. The molecules PF5 and SF4 are described, according to this model, as having one 3-center-4-electron bond as well as three and two other more conventionally described bonds, respectively. In SF6 and in the xenon fluorides, all bonds are described with the 3-center-4-electron model.
The bonding in XeF2 can also be shown qualitatively using resonant Lewis structures as shown below:
In this representation, the octet rule is not broken, the bond orders are 1/2, and there is increased electron density in the fluorine atoms. These results are consistent with the molecular orbital picture discussed above.
Older models for explaining hypervalency invoked d orbitals; however, quantum chemical calculations suggest that d-orbital participation is negligible due to the large energy difference between the relevant p (filled) and d (empty) orbitals. The 3-center-4-electron bonding model has the advantage of dispensing with the need for d orbitals, which has lead to its acceptance.
One difficulty with 3-center-4-electron bonding model is the implication that "normal" bonds are localized, 2-center-2-electron bonds. In fact, bonding is highly delocalized.
[edit] References
- Greenwood, N. N.; A. Earnshaw (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4. p. 897.
- Weinhold, F.; Landis, C. Valency and bonding, Cambridge, 2005; pp. 275-306.
