Pi bond
In
chemistry, pi bonds are
chemical bonds of the
covalent type, where two lobes of one involved electron
orbital overlap two lobes of the other involved electron orbital. Of the orbital's node planes, one only goes through both atoms.
The Greek letter p in their name refers to
p orbitals, since the orbital symmetry of the pi bond is the same as that of the p orbital when seen down the bond axis. P orbitals usually engage in this sort of bonding. However, d orbitals can engage in pi bonding also.
Pi bonds are usually weaker than
sigma bonds because their electron density is further from the positive charge of the
atomic nucleus, which requires more energy.
Encyclopedia
In
chemistry,
pi bonds are
chemical bonds of the
covalent type, where two lobes of one involved electron
orbital overlap two lobes of the other involved electron orbital. Of the orbital's node planes, one only goes through both atoms.
The Greek letter
p in their name refers to
p orbitals, since the orbital symmetry of the pi bond is the same as that of the p orbital when seen down the bond axis. P orbitals usually engage in this sort of bonding. However, d orbitals can engage in pi bonding also.
Pi bonds are usually weaker than
sigma bonds because their electron density is further from the positive charge of the
atomic nucleus, which requires more energy. From the perspective of quantum mechanics, this bond weakness is explained by significantly less overlap between the component p-orbitals due to their parallel orientation.
Although the pi bond by itself is weaker than a
sigma bond, pi bonds are most often found in multiple bonds together with sigma bonds and the combination is stronger than either bond by itself. This can be seen from comparison of the carbon-carbon
bond lengths in
ethane ,
ethylene and
acetylene .
[
Atoms with
double bonds or
triple bonds usually have one sigma bond and the rest are usually pi bonds. Pi bonds result from parallel orbital overlap: the two combined orbitals meet lengthwise and create more diffuse bonds than the sigma bonds. Electrons in pi bonds are sometimes referred to as pi electrons. Molecular fragments joined by a pi bond cannot rotate about that bond unless the pi bond breaks because rotation involves destroying the parallel orientation of the constituent p orbitals.
Special cases
Pi bonds do not necessarily have to connect a pair of atoms that are also sigma-bonded. In agostic complexes, pi interactions between a metal atom an the sigma bond of molecular
hydrogen play a critical role in the reduction of some organometallic compounds.
Alkyne and
alkene pi bonds often bond "side-on" with metal atoms in a bond that has significant pi character.
In some cases of multiple bonds between two atoms, there is no sigma bond at all, only pi bonds. Examples are diiron hexacarbonyl , dicarbon and the borane B
2H
2. In these compounds the central bond consists only of pi bonding, and in order to achieve maximum orbital overlap the bond distances are much shorter than expected.
See also
References
