The octahedral diamagnetic low spin complex among the following is:

🧠 Three Filters: Octahedral + Diamagnetic + Low-Spin

🗺️ Test Each

• (1) $[\mathrm{Co(NH_3)_6}]^{3+}$: Co(III) $\mathrm{d^6}$ + strong N-donor NH₃ → octahedral, low-spin, 0 unpaired → diamagnetic ✓
• (2) $[\mathrm{CoF_6}]^{3-}$: Co(III) $\mathrm{d^6}$ + weak F⁻ → octahedral high-spin → 4 unpaired (paramagnetic) ✗
• (3) $[\mathrm{CoCl_6}]^{3-}$: Co(III) $\mathrm{d^6}$ + weak Cl⁻ → octahedral high-spin → paramagnetic ✗
• (4) $[\mathrm{NiCl_4}]^{2-}$: Ni(II) tetrahedral, paramagnetic — not octahedral ✗

⚡ The "Strong vs Weak Field on Co(III)" Switch

| Ligand on Co(III) | Spin | Magnetic | |---|---|---| | NH₃, en, CN⁻, NO₂⁻ | low-spin | diamagnetic | | F⁻, Cl⁻, H₂O (sometimes) | high-spin | paramagnetic |

Co(III) is the textbook case of spin-state crossover driven by ligand field.

⚠️ Don't Pick (4)

NiCl₄²⁻ is tetrahedral, not octahedral. Even though Ni²⁺ is sometimes diamagnetic, that's only in square-planar geometry with strong field ligands. Tetrahedral Ni²⁺ has 2 unpaired → paramagnetic.

$\boxed{\text{Answer: (1) } [\mathrm{Co(NH_3)_6}]^{3+}}$