Hello,

Aluminium has vacant d-orbitals. So, it can expand its coordination number from 4 to 6. Thus, it forms octahedral [AlF6]3- ion in which Al undergoes sp3d2 hybridisation.

On the other hand, Boron does not have d-orbitals. Therefore, it can have a maximum coordination number of 4. So, boron can not form [BF6]3- ion.

Hope it helps!

Good luck!

Aluminum is formed (AlF6) 3- because it has a d orbital in its valence shell, but boron does not have a d orbital in its valence electron. Therefore, the maximum boron covalence cannot exceed 4. Thus Aluminum can form [AlF6]3 but Boron can not form [BF6]3.

I hope this answer helps. All the very best for your future endeavors!