Question

The compounds in which the mentioned bond angle in parenthesis is found to be greater than expected, not due to back bonding is:

• A. $H_{3}SiNCS(\angle Si-N-C)$
• B. $B{I}_3(\angle I-B-I)$
• C. $MeNCS\left(\angle CNS\right)$
• D. None of these

Answer 1

The correct option is C $MeNCS\left(\angle CNS\right)$

In the compound, the $C-N-S$ bond angle is greater than expected. But this is not due to back bonding. This is because $MeNCS$ is a resonance hybrid of two structures, one is linear and the other is bent. In the linear structure, the $CNC$ bond angle is ${180}^o$ whereas in the bent structure, the $CNC$ bond angle is ${120}^o$. Hence, the average bond length is ${142}^o$.

While in $B{I}_3,\angle I-B-I$ is greater because of back bonding due to the presence of lone pair of iodine atoms. While in $H_{3}SiNCS,\angle Si-N-C$ is also different than expected due to the back bonding of the lone pair of electrons of nitrogen.