How do pi bonds work?

1 Answer
Jun 5, 2016

ORBITAL PERSPECTIVE

Pi (pi) bonds are made by overlapping two atomic orbitals sidelong, as shown below. In contrast, sigma (sigma) bonds are made by overlapping two atomic orbitals head-on.

Either way, this overlap can either be in-phase or out-of-phase.

  • The in-phase one (same colors overlapping) is lower in energy and is called the bonding pi overlap. It generates a pi molecular orbital.
  • The out-of-phase one (opposite colors overlapping) is higher in energy and is called the antibonding pi overlap. It generates a pi^"*" molecular orbital.

A double bond has one sigma and one pi bond, while a triple bond has one sigma and two pi bonds.

MOLECULAR ORBITAL DIAGRAM PERSPECTIVE

The MO diagram depiction is:

where:

  • pi_(npx) is the bonding molecular orbital formed by the in-phase overlap of an np_x with an np_x atomic orbital.
  • pi_(npy) is the bonding molecular orbital formed by the in-phase overlap of an np_y with an np_y atomic orbital.
  • pi_(npx)^"*" is the antibonding molecular orbital formed by the out-of-phase overlap of an np_x with an np_x atomic orbital.
  • pi_(npy)^"*" is the antibonding molecular orbital formed by the out-of-phase overlap of an np_y with an np_y atomic orbital.

We have three common ways that we can occupy the pi and pi^"*" molecular orbitals.

  • When the pi molecular orbitals are filled but the pi^"*" ones are not, we have a pi bond.
  • When both kinds of molecular orbitals are filled, those electrons are nonbonding and are lone pairs.
  • When neither kind of molecular orbital is filled, there is no lone pair or bond.

The sigma overlaps are the ones that are head-on, and are not our focus (though you should know those as well).