To this point in the chapter, we've only encountered single bonds, where orbitals that lie along the internuclear axis contribute to bonding. These are known as sigma bonds. In this section we will look at a case where additional bonding can contribute to the overall strength of the bonding beween two atoms. Other cases with mutiple bonding will be encountered in Chapter 3.

If two nitrogen atoms encounter each other as shown in the figure below, there are a lot of singly occupied 2p orbitals—a total to six.

16.1

It's straightforward to think that a bond could form between the two singly occupied 2p orbitals along the N-N axis, but what happens to the off-axis 2p orbitals that lie at 90° angles from the N-N axis? The next figure shows two possibilities that can happen when two N atoms approach each other:

16.2

There is a bound potential energy curve (turquoise) and a repulsive one (magenta).

The correct answer is (a), as we've been on several previous occasions. The repulsive curve corresponds to parallel electron spin pairing.

The figure below shows orbital animations for the on-axis and off-axis 2p oritals of the bound form of N₂ shown in Figure 16.2.

16.3

The sigma bond along the N-N axis (which we are taking as the z-axis) is very similar to other bonding situations we've looked at. The orbitals look like atomic 2p orbitals when the N atoms are far apart. As the separation approaches the minimum separation, each orbitals is attracted toward the other nucleus. The two orbitals can overlap between the N nuclei because the spins of the two electrons are different.

Something similar but a little different is happening to the 2p_x¹ and 2p_y¹ orbitals on each N atom. The electrons in the orbitals are attraction toward the other nuclei, and the orbitals delocalize toward and onto the other nuclei as a results. As with all covalent bonding, overlap occurs between the nuclei that is allowed by the Pauli principle for electrons with opposite spins. The only thing that is really different is that the delocatization is as much off-axis as the orbitals.

The correct answer is (b). For both types of bond orbitals showin in Figure 16.3, the orbitals on N1 and N2 move the same amount toward the other nucleaus, so the bonding is non-polar. The off-axis bonding is known as pi bonding.

The correct answer is (c). There are one sigma bond and two pi bonds. Remember, there are two sets of off-axis 2p orbitals that are parallel to the x-axis and y-axis. Here are the 2D and 3D bonding diagrams for N₂:

16.4

Note that there are couplers drawn between each pair of 2p orbitals. Everything is colored gray in the 3D model because there are no unpaired electrons remaining in N₂.

Let's look at one more pair of orbital animations:

16.5

The 2s² pairs on each N atom move away from the nuclei parallel to the bond axis. They move this way to avoid the triple bonding occurring between the nuclei.