Question

Which of the following compound(s) can act as Lewis acid in both monomeric and dimeric form?

• A. $B{H}_3$
• B. $BeC{l}_2$
• C. $AlC{l}_3$
• D. $Be{H}_2$

Answer 1

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

$Be{H}_2$

a) The lewis structure of $B{H}_3$ shows that boron has only three bond pairs and no lone pairs of electrons. Hence, it can accept electrons from a donor to complete its octet and act as a Lewis acid. In $B_2{H}_6$ structure there are two electrons shared between three centres, forming a $3c-2e^{-}$ bond. Since this is an electron deficient bond, it can still act as a Lewis acid.
b) $BeC{l}_2$ is also a lewis acid due to the incomplete octet of $Be$ and its ability to accept electron pairs, for which it can accept lone pair of electrons from strong lewis bases. When it polymerises $Be$ has coordinate bonds formed by Cl in a strained polymer and it acts as Lewis acid in presence of a strong Lewis base.

c) In $AlC{l}_3$, $Al$ is trivalent and hence it accepts a lone pair of electons from a lewis base to complete its octet and act as a lewis acid. But when it dimerises, bridged $Cl$ forms coordinate bond and the octet of $Al$ is complete but it will still act as a lewis acid due to the vacant d orbitals and accept the lone pair when a lewis base is around.

d) $Be{H}_2$ is also a lewis acid due to the incomplete octet of $Be$ and its ability to accept electron pairs to complete this octet, for which it can accept lone pair of strong lewis bases. When it dimerises as $B{e}_2{H}_4$, there are two electrons shared between three centres, forming a $3c-2e^{-}$ electron deficient bond. So, the octet of $Be$ centres is still incomplete and hence act as lewis acid in dimerised form.