Question

Identify the homoleptic complex(es) that is/are low spin.

• A. \([Fe(CN)_5NO]^{2−}\)
• B. \([CoF_6]^{3−}\)
• C. \([Fe(CN)_6 ]^{4−}\)
• D. \( [Co(NH_3 )_6]^{3+}\)

Hint:

For complexes with transition metals, the crystal field splitting is influenced by both the oxidation state and the nature of the ligand. Strong field ligands (like \( \text{CN}^- \)) cause low-spin complexes, while weak field ligands (like \( \text{F}^- \) and \( \text{H}_2\text{O} \)) tend to lead to high-spin complexes.

Answer 1

The Correct Option is C

The complex \([Fe(CN)_6]^{4-}\) is low spin.

Why?

• \(CN^-\) is a strong field ligand (large \(\Delta_{o}\)), so it causes big splitting of the d-orbitals.
• For iron in this complex the large splitting favours electron pairing in the lower set of d-orbitals, giving a low-spin configuration.

Note (comparison)

• \([CoF_6]^{3-}\): F⁻ is a weak field ligand → typically high spin.
• \([Co(NH_3)_6]^{3+}\): NH₃ is a stronger field than F⁻ and can give low spin for some metal/oxidation states (e.g. Co(III) d⁶ often low spin), but the clearest low-spin example among the list is \([Fe(CN)_6]^{4-}\) due to CN⁻ being a very strong field ligand.

Answer 2

To determine which homoleptic complex is low spin, we need to examine the ligand field involved and the metal center. Homoleptic complexes contain ligands of the same type around the central metal ion.

1. Understanding Ligands and Spin States:
- Low spin complexes typically result when strong field ligands (like CN⁻) are present, leading to significant splitting of the d-orbitals. This can occur when the ligand causes a large energy difference between the two sets of split d-orbitals, favoring pairing of electrons in the lower energy set.
- High spin complexes are associated with weak field ligands (like F⁻), resulting in a smaller energy difference and less electron pairing.

2. Evaluate the Given Complexes:
- \([Fe(CN)_5NO]^{2-}\): Mixed ligand complex; NO can cause back bonding, influencing spin state.
- \([CoF_6]^{3-}\): Low field ligand like F⁻ suggests high spin.
- \([Fe(CN)_6]^{4-}\): Strong field ligand CN⁻ indicates low spin.
- \([Co(NH_3)_6]^{3+}\): NH₃ is a moderate field ligand; depends on specific conditions.

3. Complex Analysis:
- \([Fe(CN)_6]^{4-}\) positions CN⁻ as a strong field ligand around Fe, leading to a low spin configuration due to large splitting of the d-orbitals, and electron pairing in the lower energy orbitals is favored.

4. Low Spin Identification:
Given the ligand strength and their influence on d-orbital splitting, the complex \([Fe(CN)_6 ]^{4−}\) with a strong field ligand like CN⁻ promotes electron pairing in the lower energy orbitals, thus, it is low spin compared to the others listed.