Pi bond

File:Electron orbitals.png

Electron atomic and molecular orbitals, showing a Pi-bond at the bottom right of the picture

In chemistry, pi bonds (π 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 π 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.

Pi bonds are usually weaker than sigma bonds because their orbitals go 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 previously p-orbitals due to their parallel orientation. The pi bond by itself is weaker than a sigma bond, but pi bonds are only found in multiple bonds together with sigma bonds and the combination is stronger than either bond by itself.

two p-orbitals forming a π-bond

Atoms with double bonds or triple bonds 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.

Pi bonds do not necessarily have to connect atoms; pi interactions between the metal atom and the sigma bond of molecular hydrogen play critical roles in the reduction of some organometallic compounds. Alkyne and alkene pi bonds often bond with metals in a bond that has significant pi character.

See also

• Aromatic interaction
• Chemical bond
• Delta bond
• Molecular geometry
• Sigma bond