Shaik, Wu and Hiberty have proposed a third bond type, and they have a nice review article in Nature Chemistry.1 Along with the long-standing concepts of the covalent bond and the ionic bond, they add a third category: the charge-shift bond.
The valence bond wavefunction for the diatomic A-B is written as
Ψ(VB) = c1φcov(A-B) + c2φion(A+B–) + c3φion(A–X+)
Typically one of these terms dominates and we call the bond covalent if c1 is the largest coefficient or ionic if either c2 or c3 is the largest term. The bond dissociation energy (De) is the difference in energy of the total VB wavefunction (above) and the energy of the separate radicals A. and B.. One can determine the energy due to just a single component of the total VB wavefunction. One might expect that for a covalent bond, the bond dissociation energy derived from just the c1φcov(A-B) term would be close to De. For many covalent bonds this is true. However, Shaik and co-authors show a number of bonds where this is not true. For example, in the F-F bond, the covalent term is destabilizing. Rather, it is the resonance energy due to the mixing of the 3 VB terms that leads to bond formation. Shaik, Wu and Hiberty call this the “charge-shift bond”. They describe a number of examples of typically understood homonuclear and heteronuclear covalent bonds that are in fact charge-shift bonds, and an example of an ionic bond that really is charge-shift.
They argue that the charge-shift bond manifests as a consequence of the virial theorem. When an atom participates in a bond, its size gets smaller and this results in an increase in its kinetic energy. If the atom gets very small, then a substantial resultant change in the potential energy must occur, and this is the charge-shift bond. This also occurs in bonds involving atoms with many lone pairs; the lone-pair bond-weakening effect also causes a rise in kinetic energy that must be offset.
The authors speculate that many more examples of the charge-shift bond are waiting to be uncovered. It will be interesting if this concept catches hold and how quickly it will incorporated into general chemistry textbooks.
References
(1) Shaik, S.; Danovich, D.; Wu, W.; Hiberty, P. C., "Charge-shift bonding and its manifestations in chemistry," Nature Chem., 2009, 1, 443-449, DOI: 10.1038/nchem.327
Henry Rzepa responded on 01 Oct 2009 at 1:13 am #
This is a fascinating idea by Shaik, Wu and Hiberty. I have two comments, or perhaps questions.
1. The charge-shift bond can be associated with the kinetic energy, and this in turn can map to the Laplacian ∇2ρ. So if I understand correctly, these types of bond can be identified using AIM analysis as well as a decomposition of the VB wavefunction. Certainly, the latter is a relatively specialised form, whereas the former is a boiler-plate calculation. But what I am less certain of is whether having the correct value for the Laplacian is in itself sufficiently diagnostic of a charge-shift bond?
2. If a molecule is identified as exhibiting a charge-shift bond, how might that enable us to comment on any aspect of its reactivity or structure? Put another way, what use is the concept? How might it be related to an experimental observable?
David Gosser responded on 06 Feb 2019 at 10:28 pm #
In the Nature of The Chemical Bond (1960) Pauling discusses HF molecule, and estimates that neither the covalent nor the ionic structure are stable, and that the bonding is entirely due to resonance of the two forms. The discussions I am seeing do not seem to recognize that the concept that resonance could be the primary bonding force is nothing new. The range of molecules attributed in this way is, however.