Which among the following species has unequal bond lengths?

🧠 Unequal bond lengths arise when the central atom has lone pairs that create two non-equivalent bonding positions (as in trigonal bipyramidal or octahedral derivatives with LP).

• $\ce{SiF4}$: tetrahedral, all 4 Si–F bonds equivalent → equal ✗
• $\ce{SF4}$: see-saw (sp³d, 1 LP). Trigonal bipyramidal electron geometry → 2 equatorial S–F (shorter, ~154 pm) + 2 axial S–F (longer, ~164 pm) → unequal ✓
• $\ce{XeF4}$: square planar (sp³d², 2 LP), all 4 Xe–F bonds equivalent → equal ✗
• $\ce{BF4^-}$: tetrahedral, all 4 B–F bonds equivalent → equal ✗

In $\ce{SF4}$, lone pair occupies an equatorial position of the trigonal bipyramidal framework. The equatorial S–F bonds are different from the axial S–F bonds — equatorial bonds are in the plane and shorter, axial bonds are perpendicular and longer.

$\boxed{\text{Answer: (b) } \ce{SF4}}$