Question

How do pi bonds work?

Answer 1

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.

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