# Stretching frequency of nitro and nitrito

I got the values of the stretching frequency of nitro and nitrito ligands from this webpage. I understand why stretching frequencies are close in nitro and well separated in nitrito. How to explain why nitrito has slightly higher stretching frequency?

• Please consider adding a comment when you down vote – Archisman Panigrahi Apr 6 '18 at 17:19

The stretching frequency of nitro ($\ce{<-NO_2}$ )corresponds to the stretching of the bonds between $\ce{N}$ and $\ce{O}$, which are in between double and single bond, due to resonance, and the bond order between these $\ce{N}$ and $\ce{O}$ becomes almost equivalent in both the two bonds.
Thus the stretching frequency of $\ce{-NO_2 }$ corresponds to partial double bond character between N and O and almost equivalence of two bonds.
On, the other hand, the two streching frequency of nitrito ($\ce{<-O^ -N=O)}$ corresponds to one $\ce{N -O}$ pure single bond and one $\ce{N=O}$ pure double bond as there is no delocalisation in this case. So, due to that pure double bond stretching frequency of $\ce{N=O}$, one streching frequency of nitrito is higher than Nitro, which is consisting of only partial double bonds.