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While studying resonance from this Chemistry LibreTexts article, I found the rules to follow in order to decide which structure is the most stable. Among them:

  1. The structure with the least number of formal charges is more stable
  2. The structure with the least separation of formal charges is more stable.

Firstly I'm not sure how to interpret the "number of formal charges" in rule 1: does it refer to the number of atoms with non-zero formal charge or to the magnitude of each formal charge? It confuses me a bit also because the total number of formal charge (the sum of them) is clearly constant (and equal to the total charge or molecule).

Secondly I don't understand which of these two rules (1 or 2) is more important and can dominate the other one.

For example two possible structures of $\ce{CNS^{-}}$ are $$\ce{C=N=S}\qquad\ce{C#N-S}$$

The formal charge for the first one are (in order) -2, 1, 0 while for the second one are -1, 1, -1.

The first is preferable according to rule 1 (interpreting it as the number of atoms with non-zero formal charge) while the second is preferable according to rule 2 since the separation of charge is not as strong as in the first structure (I hope I'm understanding correctly rule 2).

Am I reading incorrectly the two rules or is there one that is actually "stronger" than the other?

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    $\begingroup$ Choose some other structure with reasonable stability. This cleared, It seems you get the rules the right way. $\endgroup$ – Alchimista Jan 5 '18 at 14:10
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    $\begingroup$ Mesomeric structures aren't "stable", but more or less significant contributors. Also such few rationalisation are only supposed to prevent you from writing structures which are't significan't contributors at all. $\endgroup$ – Mithoron Jan 5 '18 at 16:56
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You will encounter situations such as these when the two rules seem to contradict each other, or they confuse you. However, when this happens, you must use another criteria to determine the best resonance structure. Specifically, you must take electronegativity into account.

What this means is that the most electronegative element should have the lowest formal charge, and the most electropositive element should have the highest formal charge. This is because the electronegativity "rules" explained above are what are likely to happen (that is how thiocyanate is likely to appear). This makes sense because the higher EN and element has, the more likely it is to "hog" the electrons and have a lower formal charge. When you use these rules to evaluate the resonance structures of the $\ce{CNS^{-}}$ ion, you can figure out why the first resonance structure is preferable.

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  • $\begingroup$ Before upvoting you should consider correcting for the OP proposed an exotic structure - without significant contributing forms -- that is not thiocyanate nor isothiocyanate. $\endgroup$ – Alchimista Jan 6 '18 at 14:00
  • $\begingroup$ Sorry, can you clarify? $\endgroup$ – H. Khan Jan 6 '18 at 19:15
  • $\begingroup$ The structures are not limiting forms for thiocyanate $\endgroup$ – Alchimista Jan 6 '18 at 19:18
  • $\begingroup$ Sorry, I don’t understand. If you want you can edit the answer to reflect what you’re saying feel free to do so. $\endgroup$ – H. Khan Jan 6 '18 at 20:01
  • $\begingroup$ There's not a stable anion with a N-C-S linkage no matter the bonds / formal charges. Thiocyanate and isothiicyanate have structures with N-C-S linkage. $\endgroup$ – Alchimista Jan 6 '18 at 20:20
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The hypothetical ion you discuss must have 16 electrons.

I do not see how one accommodates them in a meaningful way about a proposed structure with linkage C-N-S.

Beside that I see question marks instead of bonds, lone pairs and in case unpaired electrons (it is likely a problem of browser, I am using SE app), as the formal charges suggest you had hard time to find a way leading to something just remotely approaching a stable structure. You ended in limiting forms with high formal charges both in number as well separation, not to mention a C atom with 10 electrons around! This should have pointed you to firstly ask the stability of such an anion.

As the comments to the Q suggestively already pointed out, there is little if no meaning to compare the stability of hypothetical limiting forms for a not existing unstable structure.

Besides this extreme case, an attempt to rationalised the weight of limiting forms for very unstable species based on simple rules will likely fail. Or, at least, it should be attempted just if experience show that such a structure can indeed be observed if not isolated. Likely advanced criteria and calculations would elucidate the situation.

This state, your Q has been probably dictated by the habit of writing CNS- for thiocyanate.

CNS- or less confusedly written NCS- has a structure with, in fact, N-C-S linkage for which you will easily obtain two more or less equally important limiting forms.

One with formal charge -1 on nitrogen and one with a FC -1 on sulphur.

Indeed this is an example of bond isomerism (thiocyanate and isothiocyanate for S linkage and N linkage to other fragments, respectively).

For a general answer useful in other realistic cases I point you to the first part of the A by H. Khan here in this thread.

The second part should be omitted as the author try to follow your line and ended in some "forced"assertion regarding thiocyanate ion which, as said above, is not the subject in case or identifying an hypothetical limiting form with a C -2 as the major contributor.

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