Question

Among the following, the correct statement(s) is are:

• A. $Al(C{H}_3{)}_3$ has the three-center two-electron bonds in its dimeric structure
• B. The Lewis acidity of $BC{l}_3$ is greater than that $AlC{l}_3$
• C. $AlC{l}_3$ has the three-center two-electron bonds in its dimeric structure
• D. $B{H}_3$ has the three-center two-electron bonds in its dimeric structure

Answer 1

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

The correct options are
A

$Al(C{H}_3{)}_3$ has the three-center two-electron bonds in its dimeric structure

B

The Lewis acidity of $BC{l}_3$ is greater than that $AlC{l}_3$

D

$B{H}_3$ has the three-center two-electron bonds in its dimeric structure

$Al(C{H}_3{)}_3$ has the three-center two-electron bonds in its dimeric structure

But this is not the case for $AlC{l}_3$. In this structure, the bridging chlorine atoms each have a coordinate covalent bond with the other aluminium atom.

Diborane also has the three-center two-electron bonds.

The Lewis acidity of $BC{l}_3$ is greater than that $AlC{l}_3$

If you look at Al and Cl, they are in the same period and hence, back-bonding is stronger because the difference in orbital energies is not too much. However, B and Cl belong to different periods. Hence, the back-bonding is weaker since the difference in orbital energies is not conducive for back bonding.