# Draw the diagrams for "NO"_2^-, "NO"_2^+, and "NO"_2. The HOMO of "NO"_2^- shows it is somewhat anti-bonding. Would you expect the nonbonding electron pairs on nitrogen or oxygen to be more reactive? Discuss

Feb 9, 2018

You've seen the molecular orbital (MO) diagram of ${\text{CO}}_{2}$:

${\text{CO}}_{2}$ and ${\text{NO}}_{2}^{+}$ are isoelectronic and thus have the same electron configuration. Thus, simply add one or two electrons into the $2 {b}_{3 u}$ and $2 {b}_{2 u}$ to get ${\text{NO}}_{2}$ and ${\text{NO}}_{2}^{-}$, respectively.

Nitrogen atom has $2 p$ atomic orbitals lower by $\text{2.52 eV}$, and $2 s$ atomic orbitals lower by $\text{6.13 eV}$ than with carbon atom.

That makes the HOMO ($1 {b}_{2 g} , 1 {b}_{3 g}$) belongs more so to nitrogen in ${\text{NO}}_{2}^{+}$ than to carbon in ${\text{CO}}_{2}$. It also means the LUMO ($2 {b}_{3 u} , 2 {b}_{2 u}$) is less antibonding in ${\text{NO}}_{2}^{+}$ than in ${\text{CO}}_{2}$.

The nonbonding electron pairs on oxygen in ${\text{NO}}_{2}^{-}$ should be more reactive...

• The HOMO on ${\text{NO}}_{2}^{-}$ are the ($2 {b}_{3 u} , 2 {b}_{2 u}$) MOs, which are half-filled. Those are the lone pair on nitrogen.

The $4 {a}_{g}$ is the LUMO then, and it belongs more to nitrogen than oxygen, as it is above nitrogen atom in energy, and nitrogen's atomic orbitals are above oxygen's in energy.

• The $2 {a}_{g}$ and $2 {b}_{1 u}$ nonbonding $\sigma$ orbitals, as well as the ($1 {b}_{2 g} , 1 {b}_{3 g}$) $\pi$ orbitals, belong more to oxygen than to nitrogen. Those are the lone pairs on the oxygens.

The lone pair on nitrogen is in $\pi$ orbitals, meant for making $\pi$ bonds. But the lone pairs on oxygen are in $\sigma$ AND $\pi$ orbitals, meant for making $\sigma$ OR $\pi$ bonds.

Hence, upon protonation, the ${\text{H}}^{+}$ goes onto the OXYGEN via the $\sigma$ orbitals $2 {a}_{g}$ and $2 {b}_{1 u}$, making $\sigma$ bonds.