The bond dissociation energy of `-F` in `BF_(3)` is `646 kJ mol^(-1)` whereas that of `C-F` in `CF_(4)` is `515kJ mol^(-1)`. The correct reason for higher `B-F` bond dissociation energy as compared to that of `C-F` is

Options

A. lower degree of `p_(pi)-p_(pi)` interaction between `B` and `F` in `BF_(3)` than that between `C` and `F` in `CF_(4)`
B. significant `p_(pi)-p_(pi)` interaction between `B` and `F` and `BF_(3)` whereas there is no possibility of such interaction between `C` and `F` in `CF_(4)`
C. stronger `sigma` bond between `B` and `F` in `BF_(3)` as compared to that between `C` and `F` in `CF_(4)`
D. smaller size of `B` atom as compared to that of `C` atom

Answer

B

Explanation

In `BF_(3)`, boron atom has a vacant `2p` orbital and each fluorine has fully-filled unutilized `2p` orbitals. Any of the `F` atoms can share two `e^(-)`s with the vacant `2p` orbital of `B`, thus, forming `p_(pi)-p_(pi)` bond. This type of bond (known as dative or back bonding) imparts some double bond character to the `B-F` bond. This is not possible in `CF_(4)` as there is no empty `p`-orbital on `C` atom.
Note that `C` atom `(77 p m)` is smaller than `B` atom `(85 p m)`.