Bent bond

Bent bond, also known as banana bond, is a term in organic chemistry that refers to a type of covalent chemical bond with a geometry somewhat reminiscent of a banana. The term itself is a general representation of electron density or configuration resembling a similar "bent" structure within small ring molecules, such as cyclopropane (C3H6), three-center two-electron bonds found in diborane (B2H6), or as a representation of double or triple bonds within a compound that is an alternative to the sigma and pi bond model.

Small cyclic molecules
Bent bonds are a special type of chemical bonding in which the ordinary hybridization state of two atoms making up a chemical bond are modified with increased or decreased s-orbital character in order to accommodate a particular molecular geometry. Bent bonds are found in strained organic compounds such as cyclopropane, oxirane and aziridine.

In these compounds it is not possible for the carbon atoms to assume the 109.5° bond angles with standard sp³ hybridization. Increasing the p character makes it possible to reduce the bond angles to 60°. At the same time the carbon to hydrogen bonds gain more s-character and shorten. In cyclopropane the maximum electron density between two carbon atoms does not correspond to the internuclear axis hence the name bent bond. In cyclopropane the interorbital angle is 104°. This bending can be observed experimentally by X-ray diffraction of certain cyclopropane derviatives: the deformation density is outside the line of centers between carbons. The carbon carbon bond lengths are unusually short: 151 pm versus 154 pm for a regular bond.

Cyclobutane is a larger ring but still has bent bonds. In this molecule the carbon bond angles are 96° for the planar conformation and 88° for the puckered one. Contrary to cyclopropane the CC bond lengths actually increase and not decrease and this is mainly due to 1,3-nonbonded steric repulsions. In terms of reactivity cyclobutane is relatively inert and behaves like ordinary alkanes.

Other models
For cyclopropane, the bent bond model continues to have support despite the emergence of other theories such as Walsh orbitals, which aimed to do a better job fitting molecular orbital theory in light of spectroscopic evidence and group symmetry arguments. Critics of the Walsh orbital theory argue that this model does not represent the ground state of cyclopropane. There have been attempts to "repair" the walsh orbital theory but this has largely met with resistance; Walsh orbitals may still explain bonding in other molecules of interest.

Double and triple bonds
Two different explanations for the nature of double and triple covalent bonds in organic molecules were proposed in the 1930s. Linus Pauling proposed that the double bond results from two equivalent bonds, which later came to be called banana bonds. Erich Hückel proposed a representation of the double bond as a combination of a sigma bond plus a pi bond. The Hückel representation is the better-known one, and it is the one found in most textbooks since the late-20th century. There is still some debate as to which of the two representations is better, although some theoretical chemists consider both models to be practically equivalent. In a 1996 review, Kenneth B. Wiberg concluded that "although a conclusive statement cannot be made on the basis of the currently available information, it seems likely that we can continue to consider the σ/π and bent-bond descriptions of ethylene to be equivalent."

Other applications
The bent bond theory can also explain other phenomena in organic molecules. In fluoromethane (CH3F) for instance the experimental F-C-H bond angle is 109° but should be less. This is due to the fact that according to Bent's rule, the C-F bond gains p-orbital character leading to high s-character in the C-H bonds and H-C-H bond angles approaching that of sp2 orbitals e.g. 120° leaving less for the F-C-H bond angle. The difference is again explained in terms of bent bonds.

Bent bonds also come into play in the gauche effect explaining the preference for gauche conformations in certain substitutend alkanes and the cis effect associated with some unusually stable alkene cis isomers.