# How many resonating structures of Carbon Monoxide are there?

Anywhere I look, I get 3 resonating structures for CO molecule, like in this answer. However, according to the rules stated for drawing resonating structures in this site, I wonder why there can't be this resonating structure as well? Although it will be very unstable (due to lesser number of covalent bonds, positive charge on more electronegative atom), yet a minute contribution of this structure must in present in the resonance hybrid of the CO molecule. Then why is this structure not shown anywhere?

• For exactly the reason you have said. It contributes a very minor component to the overall structure. There are an awful lot of possible resonance structures but very few of them are significant contributors. – bon Jun 10 '16 at 9:17

You have said it yourself. There are many possible resonance structures but most of them have a negligible contribution to the overall structure. Your proposed structure is one of them.

The major resonance contributors of carbon monoxide are: $$\ce{\!\overset{\ominus}{C}#\overset{\oplus}{O}\! ~<->~ \!C=O\! ~<->~\!\overset{\oplus}{C}-\overset{\ominus}{O}\!}$$

• The carbon atoms in the last two contributing canonical forms are not in octet condition. Isn't that a problem? – MrAP Aug 22 '17 at 18:49