Question

Which one of the following is the electron deficient molecule?

• A. $Si{H}_4$
• B. $P{H}_3$
• C. $B_2{H}_6$
• D. $C_2{H}_6$

Answer 1

The correct option is C $B_2{H}_6$

$B_2{H}_6$ atom has three half-filled orbitals in an excited state. $B_2{H}_6$is electron deficient molecule because boron atom has three half-filled orbitals in an excited state. Two electrons of boron-hydrogen bond are involved in the formation of a three-centre bond also called the hydrogen bridge.
The chemical structure of $B_2{H}_6$ can be observed as,


The valency of boron is 3, but in this particular structure, the two electrons of boron atom are shared with hydrogen atoms forming covalent bonding.

If we look at ethane - $C_2{H}_6$ . There are two carbon atoms bonded with each other and each carbon atom is bonded with another three hydrogen atoms. Thus, there is no sharing of electrons and the structure is electron sufficient.


While in phosphine, one phosphorus atom is bonded with three hydrogen atoms, the valency of phosphorus being 3 is satisfied.

In silane, one silicon atom is bonded to four hydrogen atoms. The valency of silicon is 4 which has been satisfied with the adequate number of electrons in bonding.
Option (a) is correct.