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I need to find the Lewis structure of the ion $\ce{SCH^-}$.

I think the Lewis structure should be: $\ce{S=C-H}$, with one lone pair on the carbon and two lone pairs surrounding the sulfur.

Is this correct?

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This model seems reasonable, as you can see from the formal charges carbon has the -1 value while the rest are 0. One thing that you have forgot is to add brackets around the structure to show that it is a ion, include its overall charge.

$$\ce{[\overset{..}{\underset{..}{S}}=\overset{..}{C}-H]-}$$

Although, I suppose a structure of $\ce{[\overset{..}{C}#\overset{..}{S}-H]-}$ would work with the same formal charge, except that sulfur has the expanded octet, which is possible but I am not sure if that is more reasonable or not, it probably is not.

It is a very odd ion, to say the least, probably very unstable. Perhaps you have mistaken the hydrogen with nitrogen? It would be more reasonable to be asked to draw the lewis structure for $\ce{[SCN]-}$, thiocyanate ion. I am not even know how to pronounce or name the $\ce{[SCH]-}$ion you are required to draw, or if it even exists. Maybe that is why you need to draw it though.

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    $\begingroup$ That second structure of $\ce{CSH-}$ you propose should be purged into hell and it is about as unlikely. $\endgroup$ – Jan Oct 31 '17 at 6:13
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The ion in question is the deprotonated form of thioformaldehyde. While Lewis structures are in general not a very accurate way to depict the molecular structure, it should be made clear, that the molecule is not linear.* It is also necessary, that the formal charges within the molecule add up to the overall charge of the ion. Hence the probably best Lewis structure is the following:

deprotonation equilibrium of thioformaldehyde

Please note that the expanded octet around sulfur, which would involve the d orbitals of sulfur, has been disproved, therefore an alternative structure as proposed in another answer is not correct.


Footnote: Just to make sure, I have run a conformational analysis using CREST and GFN2-xTB, see GitHub. It yielded one bent structure, which I have further optimised with Orca 4.2.0 on the RI-BP86-D3(BJ)/def2-SVP level of theory. Normal coordinate analysis confirmed no imaginary frequencies. If you would like to visualise the ion, e.g. with Avogadro, below is a simple Xmol file, including the absolute energy on the given level of theory, and a rendered image.

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FINAL SINGLE POINT ENERGY      -436.706625474783
S        0.024944000      0.000000000      0.050790000
C       -0.062532000      0.000000000      1.757406000
H        1.046395000      0.000000000      2.108467000

representation of the molecular structure of SCH⁻ at RI-BP86-D3(BJ)/def2-SVP

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  • $\begingroup$ I had a feeling but could never be sure. Do we have any references here, provided this is a bit rare to come across? And do we have some basic sites that are mostly accepted on Chem SE? Many high-rep users include reliable sources, but are mostly out of our search results, but would be nice if one could cross-check facts with those references. $\endgroup$ – Sir Arthur7 Feb 22 at 17:24
  • $\begingroup$ @SirArthur7 I'm not entirely sure what you mean, i.e. what kind of references would you like. For this kind of homework exercise, I really do not want to go into too much detail about the electronic structure. $\endgroup$ – Martin - マーチン Feb 22 at 20:34
  • $\begingroup$ I'm sorry. I just kinda wanted to know if this ion finds a mention in some article or so. But you went far more than what I could ask for. Thanks for your time :) this got a solid response now. I was actually asking something else too, maybe kind of a meta post, where we could list down some sites that are mostly considered reliable sources here on this site. I saw some meta discussion going on about this, we have one for books and resources for learning chem at different levels, another could be useful too. $\endgroup$ – Sir Arthur7 Feb 23 at 6:32

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