Question

The correct order of B-F bond length follows the sequence:
(A) $B{F}_3$ < $B{F}_2-OH$ < $B{F}_2-N{H}_2$ < $B{F_4}^{-}$
(B) $B{F}_2-N{H}_2$ < $B{F}_2-OH$ < $B{F}_3$ < $B{F_4}^{-}$
(C) $B{F}_3$ < $B{F_4}^{-}$ < $B{F}_2-OH$ < $B{F}_2-N{H}_2$
(D) $B{F}_3$ < $B{F}_2-N{H}_2$ < $B{F}_2-OH$ < $B{F_4}^{-}$

Answer 1

Hint: $B{F}_3$undergoes back bonding which leads to double bond character, while $B{F}_2-OH$ and $B{F}_2-N{H}_2$have electronegative atoms O and N involved of which O is more electronegative than N.

Complete step by step answer:
> In $B{F}_3$: It has a $s{p}^2$ hybridised structure and so a trigonal planar structure. B has vacant 2p orbital while F atoms have completely filled unutilised 2p orbitals. Since both of these orbitals are of the same energy level (2p), pπ-pπ back bonding occurs between B and F. In this a lone pair of electrons from the unutilised completely filled orbital of F is transported to the vacant 2p orbital of B atom. This type of back bonding causes the bond between B and F to develop a double bond character. This causes the bond length B-F to be shorter than normal.
Hence it has the shortest bond length here.
> In $B{F_4}^{-}$: It is $s{p}^3$ hybridised with a tetrahedral shape and B atom here does not have any vacant 2p orbital left and so there is no scope for back bonding. So, all the B-F bond lengths are purely single bonds.
So, they have a higher bond length than $B{F}_3$.
> Now talking about $B{F}_2-OH$ and $B{F}_2-N{H}_2$: we know that electronegativity of O is much more than that of N.
Electronegativity has a direct effect on the bond length between 2 atoms. A more electronegative atom has a higher tendency to pull the pair of electrons forming the bond towards itself. This causes the bond length to become shorter than usual and since O is more electronegative than N, it attracts the bonding electrons more and so the bond length of B-F is shorter in $B{F}_2-OH$as compared to $B{F}_2-N{H}_2$. But still this bond length is more than double bond length but less than that of single bond.
> So the bond length is smallest in $B{F}_3$, followed by $B{F}_2-OH$, then $B{F}_2-N{H}_2$ and that of $B{F_4}^{-}$ is the longest.

Hence the correct option is: (A).

Note: Back bonding occurs only if one of the atoms has a vacant orbital and other atom has a lone pair of electrons to transfer. Both of these should involve orbitals of the same energy only.
Also a more electronegative atom attracts the bonding electron pair towards it more and so has a shorter bond length as compared to lesser electronegative atoms.