Which one of the following species doesn't have a magnetic moment of 1.73 BM (spin only value)?

🧠 $\mu = 1.73$ BM corresponds to 1 unpaired electron (spin-only formula: $\mu = \sqrt{n(n+2)} = \sqrt{1 \times 3} = 1.73$ BM). Find which species does NOT have exactly 1 unpaired electron.

• $\ce{O2+}$ (15e): \pi^*_{2p}^1 → 1 unpaired → μ = 1.73 BM ✓
• $\ce{O2-}$ (17e): \pi^*_{2p}^3 (one orbital full, one half-filled) → 1 unpaired → μ = 1.73 BM ✓
• $\ce{[Cu(NH_3)_4]Cl_2}$: $\ce{Cu^{2+}}$ is d⁹ → 1 unpaired → μ = 1.73 BM ✓
• CuI: $\ce{Cu+}$ is d¹⁰ (fully filled d shell) → 0 unpaired → μ = 0 BM ✗

CuI does not have a magnetic moment of 1.73 BM.

$\boxed{\text{Answer: (b) CuI}}$