JEE Advanced 2024 Paper 1 · Q13 · Magnetic Behaviour (CFT)

Correct answer:1

Solution

A complex is diamagnetic only if it has zero unpaired electrons. Determine d-electron count and ligand field strength: • $[Mn(NH_3)_6]^{3+}$: $Mn^{3+}$ is $d^4$. $NH_3$ is borderline, in $Mn^{3+}$ it gives high-spin → 4 unpaired. Paramagnetic. • $[MnCl_6]^{3-}$: $Mn^{3+}, d^4$, $Cl^-$ weak → high-spin, 4 unpaired. Paramagnetic. • $[FeF_6]^{3-}$: $Fe^{3+}, d^5$, $F^-$ weak → high-spin, 5 unpaired. Paramagnetic. • $[CoF_6]^{3-}$: $Co^{3+}, d^6$, $F^-$ weak → high-spin, 4 unpaired. Paramagnetic. • $[Fe(NH_3)_6]^{3+}$: $Fe^{3+}, d^5$, $NH_3$ for $Fe^{3+}$ behaves high-spin, 5 unpaired. Paramagnetic. • $[Co(en)_3]^{3+}$: $Co^{3+}, d^6$, en is strong field → low-spin $t_{2g}^6 e_g^0$, 0 unpaired. Diamagnetic ✓ Only one diamagnetic species → $\mathbf{1}$.

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