Inversion in DMO (TMO)

[cogujada]

Hey again. 

Okay so first of all, I don't know if inversion is spanglish, but I have a question about DMO in TMO.

Inversion (at least in Spanish/Spanglish) is when the 2PI(bonding) orbitals have less energy than the 2SIGMA(bonding) orbitals, so they are completed with electrons before the 2SIGMA. I don't know if ' have explained myself, is is quite difficult to translate chemical terminology from English to Spanish. 

This inversion occurs, for example, in B₂, C₂ and N₂ (because the difference in energy between 2s and 2p orbitals is extremely low). 

So hold your nose because here comes the question. Why CO suffers inversion and NO doesn't, I mean, why? Both of them have an atom that, by their own, "get inversed". I don't know if you understand me... 

Thanks btw