Arrange the following complexes in decreasing order of number of unpaired electrons on the metal ion

(A) $[Co(NH_3)_6]^{3+}$
(B) $[CoF_6]^{3-}$
(C) $[FeF_6]^{3-}$
(D) $[Fe(CN)_6]^{3-}$

Choose the correct answer from the options given below:

(C), (B), (D), (A)

The correct answer is Option (3) → (C), (B), (D), (A)

All complexes are octahedral with metal in +3 oxidation state (Co³⁺ d⁶; Fe³⁺ d⁵).

The number of unpaired electrons depends on whether the complex is low-spin (strong-field ligands → pairing occurs) or high-spin (weak-field ligands → maximum unpaired electrons).

• (C) [FeF₆]³⁻: Fe³⁺ (d⁵), F⁻ is weak-field ligand → high-spin → 5 unpaired electrons.
• (B) [CoF₆]³⁻: Co³⁺ (d⁶), F⁻ weak-field → high-spin → 4 unpaired electrons (t₂g⁴ eg²).
• (D) [Fe(CN)₆]³⁻: Fe³⁺ (d⁵), CN⁻ is very strong-field ligand → low-spin → 1 unpaired electron (t₂g⁵).
• (A) [Co(NH₃)₆]³⁺: Co³⁺ (d⁶), NH₃ is strong-field ligand → low-spin (diamagnetic) → 0 unpaired electrons (t₂g⁶).

Summary of unpaired electrons:

• (C): 5
• (B): 4
• (D): 1
• (A): 0

Thus, decreasing order: (C) > (B) > (D) > (A).