Boron cannot form which one of the following anions?
- AB(OH)₄⁻
- BBH₄⁻
- CBO₂⁻
- DBF₆³⁻
Show answer and explanation
Correct answer: (d) BF₆³⁻
Explanation
Boron is in the second period. It has no d-orbitals in its outer shell, so it can hold at most four electron pairs (an octet) around it; its largest covalency is 4.
- B(OH)₄⁻, BH₄⁻ (borohydride) and BO₂⁻ (metaborate) all keep within this limit.
- BF₆³⁻ would need six bonds to boron, which is not possible.
Aluminium, below boron in Group 13, has d-orbitals and does form [AlF₆]³⁻, as in cryolite.