Which one of the following species responds to an external magnetic field?

🧠 "Responds to Magnetic Field" = Paramagnetic = Has Unpaired e⁻

Walk each:

| Complex | M | d-count | Spin | Unpaired | |---|---|---|---|---| | (1) $[\mathrm{Fe(H_2O)_6}]^{3+}$ | Fe(III) | d⁵ | weak H₂O → high-spin | 5 ✓ | | (2) $[\mathrm{Ni(CN)_4}]^{2-}$ | Ni(II) | d⁸ | sq-planar, dsp² | 0 | | (3) $[\mathrm{Co(CN)_6}]^{3-}$ | Co(III) | d⁶ | low-spin | 0 | | (4) $[\mathrm{Ni(CO)_4}]$ | Ni(0) | d¹⁰ | – | 0 |

Only (1) has unpaired electrons → only (1) is paramagnetic.

🗺️ The "Three Diamagnetic Decoys"

Options (2), (3), (4) are all carefully chosen to be diamagnetic:

• (2) Square-planar d⁸ + strong field.
• (3) Octahedral d⁶ + strong field, low-spin.
• (4) d¹⁰ → no unpaired possible.

Only the lone weak-field, high-spin d⁵ complex responds.

⚡ Fe(III) + Weak Field = Maximum Magnetism

$[\mathrm{Fe(H_2O)_6}]^{3+}, [\mathrm{FeF_6}]^{3-}, [\mathrm{FeCl_4}]^-$ — all high-spin d⁵ → 5 unpaired → $\mu \approx 5.92$ BM. Strongest paramagnetism among first-row transition metals.

⚠️ Diamagnetic ≠ "No Magnetic Property"

Diamagnetic substances are weakly repelled by a magnetic field (they have orbital magnetic moments). The question asks "responds to" the field — meaning attracted (paramagnetic). All four "respond" weakly, but only (1) is attracted.

$\boxed{\text{Answer: (1) } [\mathrm{Fe(H_2O)_6}]^{3+}}$