pibond

A pi bond, commonly written as pibond in informal usage, is a covalent bond formed by the side-by-side overlap of adjacent p orbitals on two atoms. It typically accompanies a sigma bond in double and triple bonds, contributing extra electron sharing between the atoms.

In a molecule such as ethene, the carbon atoms form a sigma bond using hybridized sp2 orbitals and a pi bond from the remaining unhybridized p orbitals. The pi bond arises from the sideways overlap of these p orbitals, producing electron density above and below the plane of the bonded atoms. This arrangement contrasts with the head-on overlap that creates a sigma bond.

Pi bonds are generally weaker than sigma bonds and are more easily disrupted during chemical reactions. They restrict rotation about the bond axis, which leads to cis/trans (geometric) isomerism in alkenes. In extended systems, pi orbitals can overlap over several adjacent atoms, creating conjugation and, in aromatic rings, a delocalized pi electron system that stabilizes the structure.

A triple bond consists of one sigma bond and two pi bonds, while a double bond consists of one sigma and one pi bond. In benzene and other aromatic compounds, the six p orbitals around the ring participate in a continuous delocalized pi system, rather than forming discrete localized pi bonds, contributing to unique chemical properties and resonance stabilization.

From a molecular orbital perspective, pi bonds correspond to bonding pi molecular orbitals formed by the in-phase combination of p atomic orbitals, with corresponding antibonding pi* orbitals at higher energy. The formation and strength of pi bonds depend on orbital energy compatibility and proper orientation.