Which of the following complexes will exhibit maximum attraction to an applied magnetic field?

🧠 Maximum Attraction = Maximum Unpaired Electrons

Magnetic attraction scales with the number of unpaired electrons. Compute $n$ for each:

| Complex | Metal | d-count | Field | Unpaired | |---|---|---|---|---| | (1) $[\mathrm{Ni(H_2O)_6}]^{2+}$ | Ni(II) | d⁸ | weak | 2 | | (2) $[\mathrm{Co(en)_3}]^{3+}$ | Co(III) | d⁶ | strong → low-spin | 0 | | (3) $[\mathrm{Zn(H_2O)_6}]^{2+}$ | Zn(II) | d¹⁰ | (any) | 0 | | (4) $[\mathrm{Co(H_2O)_6}]^{2+}$ | Co(II) | d⁷ | weak → high-spin | 3 |

Maximum unpaired: $[\mathrm{Co(H_2O)_6}]^{2+}$ with 3 unpaired → strongest attraction.

🗺️ Walk the Spin States

For Co(II) $\mathrm{d^7}$ with weak H₂O: $\mathrm{t_{2g}^5 e_g^2}$ → 3 unpaired. This dominates the others.

⚡ The "d¹⁰ = 0 Unpaired" Reflex

Zn(II), Cu(I), Cd(II), Hg(II), Ag(I) — all $\mathrm{d^{10}}$, all diamagnetic. Recognise this and skip them when looking for maximum paramagnetism.

⚠️ Co(III) vs Co(II)

Co(III) with strong field = 0 unpaired (low-spin d⁶). Co(II) with weak field = 3 unpaired (high-spin d⁷). Same metal, very different magnetism — always check both the OS and the ligand.

$\boxed{\text{Answer: (4) } [\mathrm{Co(H_2O)_6}]^{2+}}$