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 with a single nodal plane containing the line segment between the two bonded species.

Pi bonds are named after the Greek letter "π", as in p orbitals, since the orbital symmetry of the pi bond is the same as that of the p orbital (when observed down the bond axis). P orbitals usually engage in this sort of bonding. However, d orbitals and even sigma bonds 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. Although the pi bond itself is weaker than a sigma bond, pi bonds are only found in multiple bonds in conjunction with sigma bonds and collectively they are stronger than either single bond.

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.

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

See also

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