Question

In which of the following molecule, vacant orbitals can not participate in bonding?

• A. $B_2{H}_6$
• B. $A{l}_{2}C{l}_6$
• C. $[H_{3}N\cdot B{F}_3]$
• D. $S{i}_2{H}_6$

Answer 1

Answer: (C)

$[H_{3}N\cdot B{F}_3]$

The answer is third part. In this the vacant orbital of boron cannot participate in bonding due to Back Bonding.

Back bonding is a type of resonance.

In compounds like $B{F}_3$, the boron atom has an incomplete octet. The fluorine atom on its side has a lone pair which it can donate to boron. But, fluorine is also a very electronegative element. So, it also has a tendency to take back the electrons that it had donated to boron. This way, the lone pair of electrons keep jumping between fluorine and boron.This is called back bonding. This provides the lone pair of electrons more number of exchange positions (which simply means more space). As a result, the molecule becomes more stable.