Correct order of spin only magnetic moment of the following complex ions is: (At. No. Fe: 26, Co: 27) (1) [FeF6]3-…

🧠 Three d⁵/d⁶ Cases

Quick d-count and field-strength check:

• $[\mathrm{FeF_6}]^{3-}$: $\mathrm{Fe^{3+}\,d^5}$ + weak $\mathrm{F^-}$ → high-spin → 5 unpaired → μ ≈ 5.92 BM.
• $[\mathrm{CoF_6}]^{3-}$: $\mathrm{Co^{3+}\,d^6}$ + weak $\mathrm{F^-}$ → high-spin → 4 unpaired → μ ≈ 4.90 BM.
• $[\mathrm{Co(C_2O_4)_3}]^{3-}$: $\mathrm{Co^{3+}\,d^6}$ + oxalate (strong enough on $\mathrm{Co^{3+}}$) → low-spin → 0 unpaired → μ = 0.

Decreasing μ: $[\mathrm{FeF_6}]^{3-} > [\mathrm{CoF_6}]^{3-} > [\mathrm{Co(C_2O_4)_3}]^{3-}$ → option (1).

🗺️ Why Oxalate Beats $\mathrm{F^-}$

In the spectrochemical series, $\mathrm{F^-}$ < $\mathrm{H_2O}$ < oxalate < $\mathrm{NH_3}$ < en. With $\mathrm{Co^{3+}}$ (high charge density), oxalate is just strong enough to push $\Delta_o$ above the pairing energy — flipping d⁶ into low-spin. $\mathrm{F^-}$, sitting at the bottom of the series, leaves the same Co³⁺ in high-spin.

⚡ The "Same Metal, Different Spin" Map

For $\mathrm{Co^{3+}}\,(\mathrm{d^6})$:

• High-spin (4 unpaired, μ ≈ 4.9): with $\mathrm{F^-}$, sometimes $\mathrm{H_2O}$ in special cases.
• Low-spin (0 unpaired, μ = 0): with $\mathrm{NH_3}$, en, oxalate, $\mathrm{CN^-}$.

Halides and bare oxide are the only ligands that consistently keep Co(III) high-spin.

⚠️ Don't Pair Fluoride with Oxalate Reflexively

Both $[\mathrm{FeF_6}]^{3-}$ and $[\mathrm{Co(C_2O_4)_3}]^{3-}$ have charge $-3$, but they sit at opposite ends of the spin spectrum. Charge similarity doesn't predict magnetic similarity. Always check d-count and ligand identity together.

$\boxed{\text{Answer: (1) } [\mathrm{FeF_6}]^{3-} > [\mathrm{CoF_6}]^{3-} > [\mathrm{Co(C_2O_4)_3}]^{3-}}$