Question

$p\pi -d\pi$ back bonding cannot occur in:

• A. $O(Si{H}_3{)}_2$
• B. $P{F}_3$
• C. $As{F}_3$
• D. $B{F}_3$

Answer 1

The correct option is D $B{F}_3$

For pπ−dπ back bonding, one atom must have a lone pair in its p-orbital and the other must have vacant d-orbitals.

a) In $O(Si{H}_3{)}_2$, oxygen has a lone pair in its $p$-orbital and $Si$ has vacant d-orbitals. So, $O(Si{H}_3{)}_2$ has $p\pi -d\pi$ back bonding.

b) In $P{F}_3$, fluorine has a lone pair in its $p$-orbital and $P$ has vacant $d$-orbitals. So, $P{F}_3$ has $p\pi -d\pi$ back bonding.

c) In $As{F}_3$, fluorine has a lone pair in its $p$-orbital and As has vacant $4d$-orbitals. So, $As{F}_3$ has very poor $p\pi -d\pi$ back bonding.

d) In $B{F}_3$, fluorine has a lone pair in its $p$-orbital and Boron has vacant $p$-orbitals. So, $B{F}_3$ has $p\pi -p\pi$ back bonding and not $p\pi -d\pi$ back bonding.