Pi Bonds

Orbital Overlap

The overlap of two 2p_y orbitals is shown below. The two nuclei both lie on the z axis. Use the slider to move the two atoms close together. Notice that as the atoms approach each other, the two 2p_y orbitals overlap with the region of overlap occurring above and below the line connecting the two nuclei. The overlap region itself is between the two nuclei but not directly in the middle of the two nuclei.

This type of overlap is designated pi (p). The energy diagram for the interaction is shown at the lower left. The energies of the isolated 2p_y orbitals are shown on the far left and right of the energy diagram. When the orbitals are brought close together, two molecular orbitals are formed and these molecular orbitals are shown in the center of the diagram.

Molecular Orbital Diagram View Along

Pi Bonding Orbital

Of the two molecular orbitals, the lower energy orbital is the bonding orbital. The sigma bonding orbital formed by overlap of the 2p_z orbitals in the N₂ molecule are shown below.

Observe that electron density is concentrated in the region between the two nuclei. This is characteristic of a bonding orbital.

Observe that the electron density is there is no electron density along the line containing the two nuclei; the electron density is concentrated in the region above and below this line. This is characteristic of a pi-type interaction.

Pi Anti-Bonding Orbital

The higher energy molecular orbital is the antibonding orbital. The pi bonding orbital formed by overlap of the 2p_y orbitals in the N₂ molecule are shown below.

Observe that the region between the two nuclei is void of electron density. In fact, there is a nodal surface separating the two nuclei. This is characteristic of an anti-bonding orbital.

Observe that the electron density is concentrated in regions above and below the line passing through the two nuclei. There is no electron density along this line. This is characteristic of a pi-type interaction.