Boron cannot form which one of the following anions ?

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Boron cannot form which one of the following anions ?

Engineering

Chemistry

The Boron Family and Properties

Compounds of Boron Family

Application of Electronegativity

Question

Boron cannot form which one of the following anions ?

A

BH₄^–

B

BF₆^3–

C

BO₂^–

D

B(OH)₄^–

Due to non-availability of d-orbitals, boron is unable to expand its octet. Therefore, the maximum covalence of boron cannot exceed 4.

Boron (B) has an atomic number of 5, with an electron configuration of $1 s^{2} 2 s^{2} 2 p^{1}$ . It has 3 valence electrons and typically forms covalent compounds due to its small size and high ionization energy. Boron is electron-deficient and often acts as a Lewis acid by accepting electrons.

Let's analyze each anion:

1. BH₄^– (Tetrahydroborate or Borohydride ion): This is a well-known anion where boron forms $sp^{3}$ hybridized bonds with four hydrogen atoms. Boron achieves a complete octet by accepting electrons from hydrogen, making it stable.

2. BF₆^3–: Boron typically has a maximum covalency of 4 due to the absence of d-orbitals in its valence shell (it only has s and p orbitals). Forming BF₆^3– would require boron to expand its octet to 12 electrons, which is not possible. Fluorine is highly electronegative and small, but boron cannot accommodate six fluorine atoms. In contrast, elements like sulfur or phosphorus can form SF₆ or PF₆^– due to available d-orbitals. Thus, BF₆^3– is not feasible.

3. BO₂^– (Metaborate ion): This anion is common in borates. Boron bonds with oxygen, forming structures where it achieves an octet through covalent bonding, often in polymeric forms like (BO₂)_n^n–, but BO₂^– is a stable species.

4. B(OH)₄^– (Tetrahydroxyborate ion): When boric acid B(OH)₃ acts as a Lewis acid, it accepts an OH^– ion to form B(OH)₄^–, where boron is $sp^{3}$ hybridized and has a complete octet.

Therefore, boron cannot form BF₆^3– due to its inability to expand its octet beyond 8 electrons.

Related Topics and Formulae

Octet Rule: Elements tend to bond in such a way that each atom has eight electrons in its valence shell. Boron is an exception, often having less than an octet (e.g., in BF₃), but it cannot exceed an octet.

Maximum Covalency: The maximum number of covalent bonds an atom can form. For boron, it is 4 (e.g., in BH₄^– or BCl₄^–), as it lacks available d-orbitals.

Lewis Acidity: Boron's electron deficiency makes it a good Lewis acid, accepting electron pairs from Lewis bases to form adducts like B(OH)₄^– or BF₄^– (but not BF₆^3–).