AIIMS 2006 Chemistry VSEPR Theory MCQ Question

To determine which of the given species has a square planar geometry for the central atom, we will apply the Valence Shell Electron Pair Repulsion (VSEPR) theory. This theory helps predict the molecular geometry based on the number of bonding pairs and lone pairs of electrons around the central atom.

Analysis of Each Species:

1. XeF₄ (Xenon Tetrafluoride):

   • Valence Electrons: Xenon has 8 valence electrons. Each fluorine contributes 1 electron, totaling 4 from 4 fluorine atoms. Thus, the total is $8 + 4 = 12$ electrons.
   • Bonding Pairs and Lone Pairs:
     • Xenon forms 4 bonds with fluorine atoms (4 bonding pairs).
     • The remaining electrons are $12 - 8 = 4$ electrons, which correspond to 2 lone pairs.
   • Geometry: The arrangement of 4 bonding pairs and 2 lone pairs leads to an octahedral electron geometry. However, with 2 lone pairs positioned opposite each other, the molecular geometry is square planar.

2. SF₄ (Sulfur Tetrafluoride):

   • Valence Electrons: Sulfur has 6 valence electrons, and with 4 fluorine atoms, the total is $6 + 4 = 10$ electrons.
   • Bonding Pairs and Lone Pairs:
     • Sulfur forms 4 bonds with fluorine (4 bonding pairs) and has 1 lone pair.
   • Geometry: The arrangement of 4 bonding pairs and 1 lone pair leads to a seesaw molecular geometry (not square planar).

3. [NiCl₄]²⁻ (Nickel(II) Tetrachloride):

   • Valence Electrons: Nickel has 10 valence electrons in its +2 oxidation state, and with 4 chlorine atoms, we have $10 + 4 = 14$ electrons.
   • Bonding Pairs and Lone Pairs:
     • Nickel forms 4 bonds with chlorine (4 bonding pairs) and has no lone pairs.
   • Geometry: The arrangement of 4 bonding pairs leads to a tetrahedral molecular geometry (not square planar).

4. [PdCl₄]²⁻ (Palladium(II) Tetrachloride):

   • Valence Electrons: Palladium has 10 valence electrons in its +2 oxidation state, and with 4 chlorine atoms, we have $10 + 4 = 14$ electrons.
   • Bonding Pairs and Lone Pairs:
     • Palladium forms 4 bonds with chlorine (4 bonding pairs) and has no lone pairs.
   • Geometry: The arrangement of 4 bonding pairs results in square planar geometry due to the d8 configuration of palladium, which prefers this geometry.

Conclusion:

From the analysis, we can conclude:

• Square planar species: XeF₄ and [PdCl₄]²⁻
• Seesaw species: SF₄
• Tetrahedral species: [NiCl₄]²⁻

Thus, the species having square planar geometry for the central atom are (i) XeF₄ and (iv) [PdCl₄]²⁻.

Correct Answer: A) (i) and (iv)

Why Other Options are Incorrect:

• Option B (i) and (ii): Incorrect because SF₄ has a seesaw geometry.
• Option C (ii) and (iii): Incorrect as neither SF₄ (seesaw) nor [NiCl₄]²⁻ (tetrahedral) has a square planar geometry.
• Option D (iii) and (iv): Incorrect because [NiCl₄]²⁻ has tetrahedral geometry, not square planar.

This thorough analysis confirms that the only correct answer is option A.