Question

Boron cannot form which one of the following anions?

a. BF₆³⁻
b. BH₄⁻
c. B(OH)₄⁻
d. BO₂⁻

Answer 1

Boron typically forms compounds with a coordination number of three or four, such as BF₄⁻ or BH₄⁻. The anion which boron cannot form from the given options, due to an excessively high coordination number, is BF₆³⁻. The correct option is (a)

Step-by-step explanation:

The question pertains to the ability of boron to form certain anions by determining which among the given anions boron cannot form. The options provided are:

1. BF₆³⁻
2. BH₄⁻
3. B(OH)₄⁻
4. BO₂⁻

Boron is known to have three valence electrons and tends to form compounds where it is electron-deficient. BF₃, or boron trifluoride, is a classic example of boron's tendency to function as a Lewis acid, as it has only six electrons in its valence shell, which is less than the octet preferred. BF₃ can accept an electron pair, as in the formation of the tetrafluoroborate anion (BF₄⁻), where a fluoride ion acts as a Lewis base and donates a pair of electrons to boron.

However, for the given options, the anion BF₆³⁻ suggests a coordination number that is too high for boron to achieve. Boron typically forms compounds with a coordination number of three or four, as in BF₄⁻ or BH₄⁻. Therefore, the anion which boron cannot form in the given options is BF₆³⁻.