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Carbon Monoxide: four resonance structures?

  1. Sep 11, 2011 #1

    I was looking at the Water Gas Shift Reaction and I was interested to know why it is exothermic.

    Since it involves carbon monoxide, I looked at the resonance structures for CO, and found three, see the wiki article here: http://en.wikipedia.org/wiki/Carbon_monoxide#Resonance_structures_and_oxidation_state

    However, why isn't there a fourth:
    If you take the CO structure with no charge, the central one in the wiki image, then transfer the pi electrons onto the carbon so that the carbon has two lone pairs, a negative charge and the oxgen has two lone pairs and a negative charge. I know it is an unstable structure, but is it not possible, and would simple contribute a tiny amount to the resonance hybrid? The oxygen can give up an electron (the left structure on wiki).

    Furthermore, how is it possible to predict that the most stable is the leftmost structure (from the wiki article), with three bonds, a negative charge on the carbon and a positive charge on the oxygen? It seems to me that this would be unstable (negative charge on carbon, positive charge on oxygen are both generally seen to be unfavourable and in the case of a carboanion, highly reactive - therefore unstable). The article states that the maximum occuopancy of orbitals is favoured, is this always a general rule? I'm aware of the octet rule, but based on the other resonance structures, it doesn't always prevail:
    the rightmost structure from the wiki article has carbon with 4 electrons.

    One more question!
    The wiki article states that "[t]he calculated polarization towards the oxygen atom is 71 % for the σ-bond and 77 % for both π-bonds."
    In which case, would the charges be better described as partial charges, carbon being d- and oxygen d+.

    Many thanks.
  2. jcsd
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