JEE Advanced · 2017 · 21 May 2017 · Paper 2 · Chemical Bonding and Molecular Structure

1. (A)$\mathrm{Al}{\left({\mathrm{CH}}_3\right)}_3$ has the three-centre two-electron bonds in its dimeric structure✓ Correct
2. (B)${\mathrm{BH}}_3$ has the three-centre two-electron bonds in its dimeric structure✓ Correct
3. (C)${\mathrm{AlCl}}_3$ has the three-centre two-electron bonds in its dimeric structure
4. (D)The Lewis acidity of ${\mathrm{BCl}}_3$ is greater than that of ${\mathrm{AlCl}}_3$✓ Correct

Answer: (A), (B), (D)

Solution

[A] Correct: Trimethylaluminum forms a dimer with 3-center-2-electron (3c-2e) bridging bonds involving the methyl groups.

[B] Correct: Diborane (${\mathrm{B}}_2{\mathrm{H}}_6$) has two 3c-2e bridging B-H-B bonds.

[C] Incorrect: ${\mathrm{Al}}_2{\mathrm{Cl}}_6$ has normal 2-center-2-electron coordinate bonds through chlorine bridges, not 3c-2e bonds.

[D] Correct: ${\mathrm{BCl}}_3$ is a stronger Lewis acid than ${\mathrm{AlCl}}_3$ because boron is smaller and has a greater tendency to accept electron pairs, despite back-bonding from chlorine.

JEE Advanced · 2017 · 21 May 2017 · Paper 2 · One or more correct options