CHEMISTRY:

An Unrecognized Type of Chemical Bonding

Most scientists recognize covalent and ionic bonding to be the two types of chemical bonding between atoms. Sason Shaik (The Hebrew University of Jerusalem) and coworkers have recently reviewed a third, and typically unrecognized, classification of chemical bonding.

This third classification is known as "charge-shift" bonding. It is easy to envision it fundamentally transforming the way chemists view the architecture and properties of molecules, and consequently their reactivity.

A puzzle.

A bond in a molecule is typically considered to be ionic if there is a very unequal sharing of electrons between the bonded atoms. Covalent bonds have more equal (though not necessarily exactly equal) sharing of electrons.

However, some molecular bonds defy classification as either ionic or covalent. An example is the fluorine-fluorine bond.

Here, you have two atoms of the same identity. The bond between them would be expected to be covalent.

However, it is not; this bond shares some of the characteristics of an ionic bond, in that there is repulsion between the two atoms. This and other experimental observations have left some chemists scratching their heads.

Certain theoretical predictions of the bonding between atoms have also yielded strange results. Such results have occurred on occasion when chemists have attempted to calculate the energy of a bond that is expected to be covalent.

When one performs this calculation, including parameters important for covalent bonding, the result does not necessarily match similar calculations of bond energy when parameters that are important for ionic bonding are included as well. It is clear that an unrecognized type of bonding is at work in such cases, which has come to be called charge-shift bonding.

Charge-shift bonding.

What is a charge-shift bond? Bonds that exhibit an exchange force will exhibit the charge-shift effect.

Atoms that are electronegative (strong attraction for electrons), possess a lone pair of electrons (two paired electrons in its valence shell), possess a tight positive charge, or molecules that are highly strained (cyclopropane) all exhibit an exchange force. These are all conditions that compress molecular bonds.

Consequently, the energy of the bond increases beyond what may be predicted if it were simply an ionic or covalent bond. It is more accurate to classify these bonds as charge-shift, one that clearly possesses both ionic and covalent character.

Implications.

Chemists do not yet know how to rigorously predict which bonds exhibit the charge-shift effect, because it is a relatively recent discovery. Consequently, they are only beginning to learn how it affects molecular geometry and reactivity.

However, it is easy to envision that charge-shift bonding may have a previously unrecognized and important role in basic chemistry. Even the issue of how atoms link together to form molecules, one that many scientists think was resolved long ago, has been reopened, reinforcing the fundamental scientific principle that no idea or observation is beyond question and revision.

for more information:

Shaik, S., Danovich, D., Wu, W., & Hiberty, P. C. (2009). Charge-shift bonding and its manifestations in chemistry Nature Chemistry, 1 (6), 443-449 DOI: 10.1038/nchem.327





