Consider the following reaction $\mathrm{BCl}_3+\mathrm{NH}_3 \longrightarrow \mathrm{BCl}_3 \cdot \mathrm{NH}_3$ The geometries of $\mathrm{BCl}_3$ and $\mathrm{BCl}_3 \cdot \mathrm{NH}_3$ respectively are

• Correct answer
  A

  trigonal planar and tetrahedral
• B

  tetrahedral and square planar
• C

  tetrahedral and trigonal bipyramidal
• D

  square planar and trigonal pyramidal

Solution

The molecular geometry of $\mathrm{BCl}_3$ is trigonal planar.
In $\mathrm{BCl}_3 \cdot \mathrm{NH}_3$, the central atom $\mathrm{B}$ has 6 valence electrons that makes it an electron deficient molecule. $\mathrm{BCl}_3$ acts as a Lewis acid as it needs 2 electrons to complete its octet. $\mathrm{NH}_3$ can donate its lone pair to boron as it acts as Lewis acid. The resulting structure is tetrahedral.

Hence, the correct answer is option: 'A'.