Chemical Bonding and Molecular Structure

Hint: In trigonal bipyramidal, all bond lengths are not equal.

Explanation:

Species	The hybridisation of the central atom	Bond length
${\mathrm{BF}}_4^{\mathrm{\Theta }}$	sp3 (Tetrahedral)	All bond lengths equal
${\mathrm{XeF}}_4$	sp3d2 (square planar)	All bond lengths equal
${\mathrm{SF}}_4$	sp3d(see-saw)	axial bond length > equatorial length
${\mathrm{SiF}}_4$	sp3 (Tetrahedral)	all bond lengths equal

In SiF₄, axial and equatorial bonds are not equal. The axial bonds are longer than equatorial bonds.

Hence, option third is the correct answer.

Hint: Isostructural species have the same hybridization and structure

Isostructural species have the same hybridization and same shape. 

1. CH₃^– and CH₃⁺ 

Carbon hybridization in CH₃^–  is sp³ and carbon hybridization in CH₃⁺ is sp² hybridized. Hence, they are not isostructural. 

2. CO₂ and NH₃

Carbon hybridization in CO₂ is sp and nitrogen hybridization in NH₃ is sp³ . Hence, they are not isostructural.

3. SnCI₄ and CIO₄^– 

Sn hybridization in SnCl₄ is sp³ and molecular geometry is tetrahedral. Cl hybridization in ClO₄^– is sp³ and molecular geometry is tetrahedral. Hence, both are isostructural.

4. AICI₃   and   SO₂ 

AlCl₃ hybridization of Al is sp² and geometry in trigonal planar but in SO₂ sulphur hybridization is sp² but shape is V shape. 

Thus, option third is the correct answer.