# What is the CNC bond angle in methyl isothiocyanate?

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?

• 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? Jul 5, 2020 at 11:06
• Actually a typing mistake. It would be CH3NCS. Sorry for the inconvenience. Jul 5, 2020 at 14:04
• 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. Jul 5, 2020 at 14:05
• @andselisk the answers aren't really good though. To all who are tempted, I would let this question open for the time being. Jul 5, 2020 at 18:46
• Jul 8, 2020 at 16:53

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: $$\ce{H3C-N=C=S <-> H3C-N^+\bond{3}C-S^-}$$
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.
• 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. Jul 8, 2020 at 21:01