Why is the N-N bond length greater in dinitrogen trioxide than in dinitrogen tetroxide?


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> Why is the N-N bond length greater in dinitrogen trioxide than in dinitrogen tetroxide?
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If the lone pair on $\ce{N}$ in $\ce{N2O3}$ delocalises, then the $\ce{N-N}$ bond will have a double bond character, so it should be shorter. 


From data on Wikipedia, the $\ce{N-N}$ bond length in [dinitrogen trioxide](https://en.wikipedia.org/wiki/Dinitrogen_trioxide?oldformat=true) is $\pu{186.4pm}$ while that of [dinitrogen tetroxide](https://en.wikipedia.org/wiki/Dinitrogen_tetroxide) is $\pu{178pm}$.


