Concept: Isostructural Species

Isostructural species are those that have the same shape (molecular geometry) and the same hybridization of the central atom. To determine if two species are isostructural, we need to find the hybridization of the central atom and the number of lone pairs and bond pairs around it, which dictates the molecular geometry.

Why (D) is correct:

• BF₄⁻: The central atom is Boron (B). Boron has 3 valence electrons. It forms 4 single bonds with 4 Fluorine atoms, and there is a -1 charge, meaning an extra electron. So, total electrons around B = 3 (valence) + 1 (from charge) + 4 (from F atoms) = 8 electrons. This means 4 bond pairs and 0 lone pairs. The hybridization is sp³, and the geometry is tetrahedral.
• NH₄⁺: The central atom is Nitrogen (N). Nitrogen has 5 valence electrons. It forms 4 single bonds with 4 Hydrogen atoms, and there is a +1 charge, meaning one less electron. So, total electrons around N = 5 (valence) - 1 (from charge) + 4 (from H atoms) = 8 electrons. This means 4 bond pairs and 0 lone pairs. The hybridization is sp³, and the geometry is tetrahedral.

Since both BF₄⁻ and NH₄⁺ have sp³ hybridization and a tetrahedral geometry, they are isostructural.

Option Analysis:

• A) BCl₃ and BrCl₃: BCl₃ is trigonal planar (sp², 3 bond pairs, 0 lone pairs). BrCl₃ is T-shaped (sp³d, 3 bond pairs, 2 lone pairs). They are not isostructural.
• B) NH₃ and BH₃: NH₃ is trigonal pyramidal (sp³, 3 bond pairs, 1 lone pair). BH₃ does not exist as a stable monomer; it dimerizes to B₂H₆. If it were to exist, it would be trigonal planar (sp², 3 bond pairs, 0 lone pairs). They are not isostructural.
• C) NF₃ and BF₃: NF₃ is trigonal pyramidal (sp³, 3 bond pairs, 1 lone pair). BF₃ is trigonal planar (sp², 3 bond pairs, 0 lone pairs). They are not isostructural.

Correct Answer: (D)