What is the most "important" resonance structure of SCN⁻?

Question

Numerous online references say that $\ce{SCN-}$ has two resonance structures:

I am wondering why this structure is not also possible?

I expect structure 3 to be rare because of the high formal charges, but shouldn't it be included as a possible resonance structure?

Furthermore, there is disagreement about whether structure 1 or structure 2 is more common. I would expect structure 2 to be more common because the negative charge is on the more electronegative N atom. However, this worksheet says that structure 1 is more common. On the other hand, this video says that structure 2 is more common. Which should it be?

What do you mean my "more common"? – Raditz_35 Feb 20 '18 at 15:55 — Raditz_35, Feb 20 '18 at 15:55

Martin - マーチン · Answer 1 · 2018-07-27T17:50:08.600

I performed a quick calculation on the DF-BP86/def2-SVP level of theory and analysed it with Natural Resonance Theory (from the Natural Bond Orbital Theory). This results in the following major contributors to this wave function: $$\left[ \underset{(1)}{\overset{67.49\%}{\ce{^-S-C#N}}} \ce{<->} \underset{(2)}{\overset{21.25\%}{\ce{S=C=N^-}}} \right]$$

The third contributor is a weird structure with a 'long-distance-bond' between the sulfur and the nitrogen with $7.22\%$. All other contribution are neglected/discarded.

While your structure 3 is indeed a valid contributor, its actual contribution will be very small. Forcing the program to use it as a structure, it resulted in an error, as it was unable to match the orbitals to that structure. The reason for that is likely that the overlap between the sulfur and the carbon is too poor to actually be considered a good contributor. This would also explain the smaller contribution of the second structure.

Here are the localised (according to NBO) molecular orbitals:

(Colour code: blue/orange - occupied molecular orbital [Lewis]; red/yellow - virtual molecular orbital [unorccupied, non-Lewis])

The total contributions in terms of atomic orbitals to the above:

     (Occupancy)   Bond orbital / Coefficients / Hybrids
 ------------------ Lewis ------------------------------------------------------
   8. (1.98209) LP ( 1) S  1            s( 79.86%)p 0.25( 20.13%)d 0.00(  0.01%)
   9. (1.77474) LP ( 2) S  1            s(  0.00%)p 1.00( 99.91%)d 0.00(  0.09%)
  10. (1.77474) LP ( 3) S  1            s(  0.00%)p 1.00( 99.91%)d 0.00(  0.09%)
  11. (1.96342) LP ( 1) N  3            s( 52.42%)p 0.91( 47.54%)d 0.00(  0.04%)
  12. (1.99743) BD ( 1) S  1- C  2
               ( 44.86%)   0.6698* S  1 s( 20.64%)p 3.81( 78.61%)d 0.04(  0.75%)
               ( 55.14%)   0.7425* C  2 s( 51.41%)p 0.94( 48.47%)d 0.00(  0.12%)
  13. (1.99846) BD ( 1) C  2- N  3
               ( 41.02%)   0.6404* C  2 s( 48.44%)p 1.06( 51.48%)d 0.00(  0.08%)
               ( 58.98%)   0.7680* N  3 s( 48.02%)p 1.08( 51.75%)d 0.00(  0.23%)
  14. (1.99735) BD ( 2) C  2- N  3
               ( 43.98%)   0.6632* C  2 s(  0.00%)p 1.00( 99.91%)d 0.00(  0.09%)
               ( 56.02%)   0.7485* N  3 s(  0.00%)p 1.00( 99.80%)d 0.00(  0.20%)
  15. (1.99735) BD ( 3) C  2- N  3
               ( 43.98%)   0.6632* C  2 s(  0.00%)p 1.00( 99.91%)d 0.00(  0.09%)
               ( 56.02%)   0.7485* N  3 s(  0.00%)p 1.00( 99.80%)d 0.00(  0.20%)
 ---------------- non-Lewis ----------------------------------------------------
  16. (0.01904) BD*( 1) S  1- C  2
               ( 55.14%)   0.7425* S  1 s( 20.64%)p 3.81( 78.61%)d 0.04(  0.75%)
               ( 44.86%)  -0.6698* C  2 s( 51.41%)p 0.94( 48.47%)d 0.00(  0.12%)
  17. (0.01384) BD*( 1) C  2- N  3
               ( 58.98%)   0.7680* C  2 s( 48.44%)p 1.06( 51.48%)d 0.00(  0.08%)
               ( 41.02%)  -0.6404* N  3 s( 48.02%)p 1.08( 51.75%)d 0.00(  0.23%)
  18. (0.22011) BD*( 2) C  2- N  3
               ( 56.02%)   0.7485* C  2 s(  0.00%)p 1.00( 99.91%)d 0.00(  0.09%)
               ( 43.98%)  -0.6632* N  3 s(  0.00%)p 1.00( 99.80%)d 0.00(  0.20%)
  19. (0.22011) BD*( 3) C  2- N  3
               ( 56.02%)   0.7485* C  2 s(  0.00%)p 1.00( 99.91%)d 0.00(  0.09%)

               ( 43.98%)  -0.6632* N  3 s(  0.00%)p 1.00( 99.80%)d 0.00(  0.20%)

On the terminology. Alchimista already explained most of this, however, I cannot stress enough: There is no such thing as a most stable resonance structure. Therefore when you say common, you probably mean large contribution to the wave function, and when you say rare, you probably mean little contribution. None of the resonance structures can be independent from each other, as they are all hypothetical.

Please read more about that here: What is resonance, and are resonance structures real?

One simple way to rationalize this at the student level is that third row elements like S generally form weak pi bonds relative to second row elements like C and N. — Andrew, Apr 01 '22 at 14:32
@Andrew I think that may be an okay rule of thumb, but ultimately it is too simple, and when they come across thioketones you have to start making exceptions to these rules. There's really no winning with simplicity here. — Martin - マーチン, Apr 01 '22 at 18:34
I don’t see why thioketones are an exception. This rule explains why the enethiol tautomer is usually preferred if one is accessible and why they polymerize so readily — Andrew, Apr 01 '22 at 22:19
@Andrew I don't wish to explain the reactivity of thioketones, from a bonding perspective they are still best explained with a double bond and that would include a pi bond. I reject your statement as it does not rationalize any of what I have written, but instead condenses it down in a rule of thumb. If anything your statement could somewhat be rationalised through this example, but not the other way around. You're welcome to contribute an answer, but if your statement is the takeaway of my post, then it, my answer, clearly doesn't get the point across and should rather be deleted. — Martin - マーチン, Apr 02 '22 at 00:31
My comment was not intended as a "take-away" from your answer nor was it criticism. You've given a great answer to the specific question. I was adding to it by offering a rule of thumb with which one can intuit the qualitative answer without resorting to advanced computation. With regard to thioketones, I still do not understand why they are problematic. I didn't say S could not form pi bonds, just that they are weak compared to second row elements, a fact which is consistent with the behavior of thioketones and also consistent with your analysis here of SCN- — Andrew, Apr 02 '22 at 13:22
Off topic but just wanted to say this answer helped me (successfully) challenge the answer key in an Indian Olympiad exam, hence the bounty (after all who can refuse to believe actual computation chemistry calculations :D) — Box Box Box Box, Apr 05 '22 at 11:27

score 5 · Accepted Answer · answered Feb 20 '18 at 18:04

First I must note the improper use of the terms common and rare as we are not to answer which structure most frequently occurs. I consider this as due to not carefully chosen words.

We have to predict which of the above sketched limiting structure is the more stable or, precisely, the most important one,e.g. that entering the molecular orbital with higher weight.

The one you proposed is indeed possible and you also know why is not the major contributor, and even not a major one.

Usually, as you said, discerning between structures with a formal charge is done by placing it according to the elements electronegativity.

In our case this rule points to structure 2, with the negative charge on nitrogen.

However, examining the energy of the corresponding bonds we note that 2 is a cumulene, which is not a particularly stable configuration around a carbon atom.

Opposite in 1, a stable C-N triple bond is attained, with the big sulphur atom still capable to spread electron density over itself.

We are therefore facing a case in which is not very easy to answer, and myself I will have to doubt.

As matter of the fact I remember that 1 is indeed the major contributor. In SCN anion the negative charge is about 50 % on sulphur and 30 % on the nitrogen side. But on the values I can be wrong.

What is the most "important" resonance structure of SCN⁻?

2 Answers2