According to VSEPR theory, for $\mathrm{sp^2}$ hybridisation the bond angle is approximately $120^\circ$. In $\ce{H3CNCS}$, the nitrogen has one lone pair. As lone-pair-bond-pair repulsion is greater than bond-pair-bond-pair repulsion, the bond angle of $\ce{CNC}$ should be less than $120^\circ$, but it is actually greater. How can this be rationalised?
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3$\begingroup$ I took a liberty to correct formatting, notations and added compound's name. But where did you find a C−N−C in $\ce{\ce{CH3CNS}}?$ What angle are you talking about, exactly? Have you seen doi.org/10.1071/CH10303, especially its SI available for free? $\endgroup$– andselisk ♦Jul 5, 2020 at 11:06
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1$\begingroup$ Actually a typing mistake. It would be CH3NCS. Sorry for the inconvenience. $\endgroup$– ShivaJul 5, 2020 at 14:04
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$\begingroup$ No prob, I corrected the name and bond order accordingly. I'm pretty sure answers from Bond angle in CH3NCO would apply here as well: partial delocalization of the lone pair on N into a π orbital on C. This effect is even more pronounced in silyl isothiocyanate, which is almost linear. $\endgroup$– andselisk ♦Jul 5, 2020 at 14:05
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2$\begingroup$ @andselisk the answers aren't really good though. To all who are tempted, I would let this question open for the time being. $\endgroup$– Martin - マーチン ♦Jul 5, 2020 at 18:46
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$\begingroup$ Related: What is the Si–N–C bond angle in silyl isothiocyanate? $\endgroup$– orthocresolJul 8, 2020 at 16:53
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1 Answer
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I have just answered What is the CNC bond angle in methyl isocyanate? Pretty much everything written there also applies to methyl isothiocyanate, just a bit more extreme.
Here is the structure calculated on the RI-BP86/def2-SVP level of theory:
The bond angle is $\angle(\ce{CNC}) = 156^\circ$, so we expect a much larger contribution of the right side in the following resonance description: $$\ce{H3C-N=C=S <-> H3C-N^+\bond{3}C-S^-}$$
Reasons for this are not trivial, but that would exceed the scope of this answer.
TL;DR From VSEPR we would expect a trigonal coordination and a $120^\circ$ angle. Refining with resonance, we'll expect a widening of the bond angle. Everything beyond that needs much more involved models.
answered Jul 8, 2020 at 16:41
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$\begingroup$ Similar to an other comment, not about methyl isothiocyanates, but $\ce{R-N=C=S}$ there are at least these 15 entries in the COD with coordinates to compare with: crystallography.net/cod/7116329.html and (just substitute the numbers), COD7116328, 7116327, 7116326, 7116325, 7103098, 4117347, 4108942, 4108356, 4075734, 4024672, 4024671, 2234166, 2233483, and 2214501. $\endgroup$ Jul 8, 2020 at 21:01