Arrange the following complexes in increasing order of their magnetic moment

(A) $[CoF_6]^{3-}$
(B) $[Co(H_2O)_6]^{2+}$
(C) $[FeF_6]^{3-}$
(D) $[Fe(CN)_6]^{4-}$

Choose the correct answer from the options given below:

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

The correct answer is Option (1) → (D), (B), (A), (C)

To arrange the given complexes in increasing order of magnetic moment, we need to consider the spin state of the metal ions in each complex based on ligand field strength.

Given complexes:

1. [CoF₆]³⁻ → Co³⁺ (d⁶), F⁻ is a weak field ligand → high spin → 4 unpaired electrons
2. [Co(H₂O)₆]²⁺ → Co²⁺ (d⁷), H₂O is weak field → high spin → 3 unpaired electrons
3. [FeF₆]³⁻ → Fe³⁺ (d⁵), F⁻ weak field → high spin → 5 unpaired electrons
4. [Fe(CN)₆]⁴⁻ → Fe²⁺ (d⁶), CN⁻ strong field → low spin → 0 unpaired electrons

Magnetic moment (μ) ∝ √(n(n+2)), n = number of unpaired electrons)

• [Fe(CN)₆]⁴⁻ → 0 μB
• [Co(H₂O)₆]²⁺ → 3 unpaired → μ ≈ 3.87 μB
• [CoF₆]³⁻ → 4 unpaired → μ ≈ 4.90 μB
• [FeF₆]³⁻ → 5 unpaired → μ ≈ 5.92 μB

Increasing order of magnetic moment:

$[Fe(CN)_6]^{4-} < [Co(H_2O)_6]^{2+} < [CoF_6]^{3-} < [FeF_6]^{3-}$

 Correct answer: (D), (B), (A), (C)