Stretching frequency of nitro and nitrito

I got the values of the stretching frequency of nitro and nitrito ligands from [this webpage](https://socratic.org/questions/linkage-isomers-nitropentaamminecobalt-iii-chloride-shows-2-bands-at-1065-and-14). 
I understand why stretching frequencies are close in nitro and well separated in nitrito. How to explain why nitrito has slightly higher stretching frequency?




First of all, in the webpage, I think there is some shuffling between Nitro and Nitrito first in the headline. But, according to the contents, **Nitro has closer stretching frequency, but Nitrito has distinguished stretching frequency.Let's take that one as the correct data**.
  

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. 


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