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What's wrong with ionic resonance structures? I asked one professor about them once and all he commented was "I've seen them too, and I think they're wrong."

Another professor rejected an ionic resonance structure showing the "H+" ion and asked me what phase the experiment was run in, in an attempt to figure out why the authors reported a resonance contributor with a H+ proton. He specifically asked if the experiment was run in the gas phase.

I think this guy forgot what resonance structures are because I know that he emphasizes that bare protons do not exist in solution, and he was probably thinking about how there could be a bare proton shown in a resonance structure if the molecule is in solution. The resonance structure I showed him was this (which I could see him picturing in solution, given that it is nitrous acid.

enter image description here

However, resonance structures are not discrete forms of the molecule in question - they're just contributors and show that there is partial ionic character; that there is a highly electronegative atom that is withdrawing electron density.

So I ask:

1) Are ionic resonance structures valid?

2) If so, why might they be valid, or what counterarguments might you present to someone who argues they are invalid?

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    $\begingroup$ Ionic resonance structures are of course valid. You might want to dig into Valence Bond Theory - which is the quantitative theory behind resonance structures - to find out a little more about the importance of these. (However, unbelievers are usually hard to convince.) $\endgroup$
    – Martin - マーチン ♦
    Sep 16, 2014 at 17:31
  • $\begingroup$ Do you have any articles recommended for me? I tried the Wikipedia article on VBT but it was, to say the least, rather uninformative. My next step is to search Google books. $\endgroup$
    – Dissenter
    Sep 16, 2014 at 19:01
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    $\begingroup$ I am sorry I can't help very much with that, I picked it up along the way when I was listening to presentations on conferences. However, there is literature linked in the wikipedia page, that should provide a starting point. $\endgroup$
    – Martin - マーチン ♦
    Sep 17, 2014 at 2:31
  • $\begingroup$ "Coulson's Valence" (1961) is a rather dated but readable introduction (archive.org/details/Valence). $\endgroup$
    – J. LS
    Apr 21, 2015 at 9:59

2 Answers 2

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I can't imagine anyone arguing that ionic resonance structures are invalid. The often contribute significantly to the structure of a molecule and help explain the various physical aspects of the molecule. The following ionic resonance structure contributes significantly to the description of the carbonyl compound. $\ce{NaCl}$ is not totally ionic and is described by a blend of both the ionic (major) and covalent (minor) resonance structures shown below.

enter image description here

Resonance structures with a positive hydrogen (proton) are important in explaining hyperconjugative effects.

enter image description here

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  • $\begingroup$ My professor would agree with your ketone resonance structure, having drawn it himself several times. I think that's what he'd consider a "charge-separated" structure rather than an ionic structure (like the ones involving H+ in your second picture. Those are the ones he says are "invalid." @ron $\endgroup$
    – Dissenter
    Sep 16, 2014 at 19:26
  • $\begingroup$ Also I'm curious as to your carbocation structure; is that structure suggesting that the carbocation is unusually acidic (the last two structures suggest to me that the central C-H bond is rather weak since electron density seems to gravitate toward the carbon with the positive charge and incomplete octet and away from the hydrogen). If anything I guess your last picture shows that the central C-CH2 bond is shorter than the C-CH3 bond due to double bond character, correct? $\endgroup$
    – Dissenter
    Sep 16, 2014 at 19:29
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    $\begingroup$ Yes, I suspect if you could measure the pKa of the n-propyl cation, you'd find the central C-H to be more acidic than the corresponding hydrogen atom in propane (but that's still not very acidic). Yes, CH2-CH2+ bond should be shorter than expected, likewise the central C-H bond should be longer than expected. $\endgroup$
    – ron
    Sep 16, 2014 at 19:42
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    $\begingroup$ @Dissenter, the increased acidity is what makes the E1 reaction work. $\endgroup$
    – jerepierre
    Sep 17, 2014 at 2:16
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    $\begingroup$ @Jan Probably the easiest way to "generate" it would be computationally. Photoelectron detachment in 2-iodopropane might also work. See this earlier answer for some related discussion on the 2-propyl carbocation, it may have a non-classical structure. As to "immediate" rearrangement, that depends on the temperature of the experiment and how much stability is provided by the non-classical structure. $\endgroup$
    – ron
    Jun 6, 2016 at 22:02
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I would like to add another shade to the already quite excellent answer of ron.

I will use donor-acceptor complexes, like Lewis acid base adducts, as case in point. Probably one of the simplest and most popular is the ammonia and borane adduct. I consider the structures below to be the most accepted ones, with probably the widest use.
resonance structures of the borane ammonia adduct
These are all structures (not all of the are Lewis approved) that try to cover the concept of a rather difficult bonding situation. In this special case, how to describe a dative bond in an easy model. This also gives us a lot to think about.
The first structure completely lacks the feature of a bond, therefore it cannot possibly be used to describe this molecule alone. But since we are talking about donor acceptor interactions, a comparatively weaker bond, the separated structure also has to be considered to contribute to bonding.
The second structure is not Lewis approved (there are not arrows in the traditional representation). It is much like the first structure, but the bonding here is at least indicated. In my opinion this is the only structure that comes somewhat close to the actual bonding situation. However, it treats the donor acceptor bond between nitrogen and boron as a second class covalent bond - is that really true?
The third structure is often used to indicate, that there is a real covalent bond between boron and nitrogen. To be perfectly honest, what is the difference between polar covalent and dative covalent anyway; it's shared electrons in both cases (see above). In my opinion this structure is one of the best examples why resonance structures are all too often misleading. Since we are in the Lewis picture, we have to obey the rule of two (octet rule), that means we have to assign formal charges. This is suddenly somewhat suggesting (to the untrained mind), that boron will carry a negative charge, which is of course wrong. Nitrogen still is more electronegative and will obviously have the highest negative charge, the electron sharing will hardly change that.
The last structure is a truly awful hybrid, that I came across. It's neither fish nor flesh, nor good red herring. Don't use it, I just included is for the sake of completeness. It makes all the errors of the other structures combined.
So all of the structures have their flaws and have to be seen in conjunction. Does any of these structures become invalid through the flaws it represents? No. If anyone argues this, then the concept of resonance - as an approximation to the true bonding situation - is not understood.

So the validity of ionic resonance structures basically becomes a question of semantics, within the framework of the model, they work very well. Do they represent something real, then the obvious answer is no. Model systems often help us understand complicated points, but they must not be mistaken for the (sometimes unknown) truth and they should also not be confused with more elaborate models.

I previously stated in the comments, that in the framework of Valence Bond Theory, for each resonance structure it is possible to construct a wave function (= configuration). It is then also possible to obtain the contributions of each configuration to the total wave function. This at least offers a quantum mechanical backbone to the whole model of resonance, and it often is described as the quantitative theory (within it's limitations) of it. The biggest problem we have with this nice theory is, that it is very expensive to calculate.

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  • $\begingroup$ $+1$ for the errors of all the other structures combined =D. On a different note, though: One of my professor’s defined a dative bond as one that will be cleaved heterolyticly when supplied with the bond dissociation energy, while ‘traditional’ covalent bonds would dissociate homolyticly. $\endgroup$
    – Jan
    Jun 3, 2016 at 13:07

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