Here's why:

* Nitrogen's valency: Nitrogen typically forms three bonds (N³⁺) or five bonds (N⁵⁺) with other atoms.

* Oxygen's valency: Oxygen typically forms two bonds (O²⁻).

To satisfy the valencies of nitrogen and oxygen, the compound NO5 would require nitrogen to have a +5 charge and each oxygen to have a -2 charge. This leads to an overall charge of +1 for the molecule, which is impossible for a neutral compound.

Therefore, the formula NO5 does not represent a real compound.