Chemistry - What is the correct lewis structure of SCH⁻?
Solution 1:
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.
Solution 2:
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:
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.
3
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
