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So I was listening to an online chemistry lecture the other day, and the theme was "bond".

In the 'coordinate covalent bond' part, the teacher said "Examples of the coordinate covalent bond can be $\ce{SO_2}$, $\ce{HNO_3}$, etc." and I could figure out that both molecules have resonance structures: between $\ce{N}$ and $\ce{O}$ atoms in $\ce{HNO_3}$, between $\ce{O}$ and $\ce{S}$ atoms in $\ce{SO_2}$. Also I am aware of the resonance structures in the $\ce{C_6H_6}$(benzene) molecule, so I looked up the Wikipedia page and could find this sentence:

"..., while the VB description involves a superposition of resonance structures."

So I became curious about this: Is there any close correlation between resonance and coordinate covalent bond? In other words, is it safe to view a molecule as having coordinate covalent bond when it has resonance structure in it?

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  • $\begingroup$ Correlation? Safe? Your question doesn't make much sense... $\endgroup$ – Mithoron Aug 17 '17 at 13:54
  • $\begingroup$ I think what you mean by resonance is formal charges. Not resonance. I once thought of this too. $\endgroup$ – Tan Yong Boon Aug 17 '17 at 22:53
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Coordinate covalent bonds and Resonance are very very vastly different things. You may have coordinate compounds without resonance, and you may have resonance involving compounds without any coordinate covalent bonds. There's no connection.

Coordinate Covalent Bonds (a.k.a Dative Bonds):

These bonds are just like any other covalent bond. They involve sharing of electrons in between the two atoms that make up the bond. The only difference between a regular covalent bond and a coordinate one is that both the electrons belong to one atom in case of coordinate covalent bonds. Head over to Wikipedia's page on Coordinate Covalent Bonds to learn more, if you're interested.

Resonance:

This is, as I mentioned, vastly different from dative bonds. This is simply a delocalization of $\pi$-electrons along a larger stretch of the molecule as opposed to being confined to being in-between two atoms. Resonance occurs when two $\pi$-systems are in conjugation with each other.

For example, consider butadiene:

There are two $\pi$-systems that are in conjugation with each other. As a result, the two systems kind of overlap and become one bigger system.

You might wonder, why do these molecules undergo resonance anyway? Well it's all based on a very simple fact: Like charges repel. Electrons in the $\pi$ molecular orbitals are no exception either. As opposed to being confined to being between two atoms, they prefer to spread over the molecule when given the chance. If you put a 100 misanthropes into a large hall, would they all form two clumps or evenly spread throughout the hall?


Anyway, now you can see, there isn't any relationship between dative bonds and resonance. There are compounds which have no dative bonds, but possess resonance, eg. benzene, butadiene, pyridine, et cetera. There are compounds that have dative bonds, and yet no resonance, eg. ammonia-boron adduct, tetraaquacopper(II), et cetera.

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  • $\begingroup$ Thanks for the detailed explanation! As many pinpointed my question as unclear, I also felt the same and you fixed it. $\endgroup$ – PenPoint Aug 18 '17 at 2:09
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Based on the examples you have given, that does appear to be the case but if you look for more examples you'll find that this is not particularly true. Two well known compounds can be used to demonstrate:

(1) Carbon Monoxide: Coordinate Covalent Bond from O -> C, but no resonance structure

(2) Benzene: Resonance Structure but no Coordinate Covalent Bonds

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