Carbon Monoxide: four resonance structures?

[nobahar]

Hello!

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.
Nobahar.

 

[nate808]

Hello Nobahar,

Thank you for your questions about the Water Gas Shift Reaction and the resonance structures of carbon monoxide. Let me address your questions one by one.

Firstly, the Water Gas Shift Reaction is exothermic because it involves the conversion of carbon monoxide (CO) and water (H2O) into carbon dioxide (CO2) and hydrogen gas (H2). This reaction releases energy in the form of heat, making it exothermic. The overall reaction can be written as:

CO + H2O → CO2 + H2 + heat

Now, onto your questions about the resonance structures of carbon monoxide. You are correct in noting that there are three main resonance structures for CO, as shown in the Wikipedia article. The fourth structure that you described, with a negative charge on carbon and a positive charge on oxygen, is indeed possible but highly unstable. This is because it violates the octet rule, which states that atoms tend to form bonds in such a way that each atom has eight valence electrons. In the case of carbon, it is more stable to have four bonds and no lone pairs, rather than two bonds and two lone pairs. Additionally, having a negative charge on carbon and a positive charge on oxygen would result in a highly reactive species, making it unlikely to contribute significantly to the overall resonance hybrid.

As for predicting the most stable resonance structure, it is not always a straightforward process. In general, the most stable structure is the one with the lowest energy, but this can vary depending on the specific molecule and its surrounding environment. In the case of carbon monoxide, the leftmost structure with three bonds and a negative charge on carbon is the most stable because it has the lowest energy. This is due to the fact that the carbon-oxygen bond is very strong and stable, making it difficult for the electrons to be shifted to other positions. In terms of the maximum occupancy of orbitals, it is generally favored for atoms to have a full octet of electrons, but there are exceptions to this rule, as seen in the resonance structures of CO.

Lastly, regarding the partial charges on carbon and oxygen in carbon monoxide, you are correct that they can be described as such. The calculated polarization towards the oxygen atom indicates that it has a slightly higher electron density than the carbon atom, resulting in a partial negative charge on oxygen and a partial positive charge on carbon. This is due to the electronegativity difference between the