Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R. Assertion A: The…

🧠 Same Fe(III), Two Different Spin States

Both complexes have Fe(III), $\mathrm{d^5}$. The dramatic μ difference (1.74 vs 5.92 BM) comes from ligand field strength:

• $[\mathrm{Fe(CN)_6}]^{3-}$: CN⁻ is strong-field → low-spin $\mathrm{d^5}$: $\mathrm{t_{2g}^5 e_g^0}$ → 1 unpaired → $\mu = \sqrt{3} \approx 1.73$ BM. ✓
• $[\mathrm{Fe(H_2O)_6}]^{3+}$: H₂O is weak-field → high-spin $\mathrm{d^5}$: $\mathrm{t_{2g}^3 e_g^2}$ → 5 unpaired → $\mu = \sqrt{35} \approx 5.92$ BM. ✓

So Assertion A is true.

Reason R: "In both complexes, Fe is in +3 oxidation state." This is also true. But it does not explain why μ differs — it's the ligand strength, not the OS, that drives the spin-state difference.

A true, R true, R does NOT explain A → option (3).

🗺️ Why "Same OS" Isn't the Cause

If OS alone determined μ, the two complexes would have the same μ — but they don't. The cause is the strong-field vs weak-field ligand difference, which determines whether the d-electrons pair.

⚡ The Spectrochemical Cue

CN⁻ at the strong end, H₂O at the weak end. With Fe(III) $\mathrm{d^5}$, CN⁻ forces pairing (low-spin), H₂O doesn't (high-spin). This is the most-tested example of CFT in JEE.

⚠️ Don't Pick Option (4)

R IS true (Fe is +3 in both) but R is NOT the correct explanation of the μ difference. The right reason is ligand field strength. Reading A and R correctly is half the battle in assertion-reason questions.

$\boxed{\text{Answer: (3) Both true, R not the correct explanation of A}}$