Question

Aluminium forms[ alf6]3- but boron does not form [bf6]3- ion . why

Answer 1

Aluminium forms (AlF6)3- because it have d orbital in its valence shell but Boron does not have d orbital in its valence electron.
Hence,maximum covalency of boron can not exceed 4.
Thus, boron can not form (BF6)3-

Answer 2

Explanation:

The electronic configuration of Al is as follows.

        $1s^{2}2s^{2}2p^{6}3s^{2}3p^{1}$

Whereas the electronic configuration of boron (B) is as follows.

        $1s^{2}2s^{2}2p^{1}$

It can be seen that Al can expand its octet due to the availability of vacant 3d orbital. Hence, the valency of aluminium becomes 6 so, it can easily accommodate six fluorine atoms around it.

Whereas boron does not have a 2d orbital so it cannot expand its octet. Therefore, maximum covalency for boron is 4 so, it cannot accommodate six fluorine atoms around it.