Question:

[Fe(CN)6]3– should be an inner orbital complex. Ignoring the pairing energy, the value of crystal field stabilization energy for this complex is (–) _____ Δ0. (Nearest integer)

Updated On: Dec 30, 2025
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Correct Answer: 2

Approach Solution - 1

The problem involves calculating the crystal field stabilization energy (CFSE) for the inner orbital complex [Fe(CN)6]3–. To solve this, follow these steps: 
1. Identify the oxidation state of iron in [Fe(CN)6]3–. Given that CN is a ligand with a charge of –1, the overall charge of the complex is –3: 
    Fe + 6(-1) = -3 
    Fe = +3. Thus, iron is Fe3+.
2. Determine the electronic configuration of Fe3+: The ground state of Fe is [Ar] 3d6 4s2. After losing three electrons, Fe3+ becomes [Ar] 3d5.
3. Arrange the electrons in d-orbitals under octahedral coordination: Cyanide is a strong field ligand causing pairing of electrons in the t2g orbitals:
    (t2g) 3d5: (↑↓, ↑↓, ↑) and (eg): (0, 0).
4. Calculate CFSE: CFSE = [(Number of electrons in t2g) × (–0.4)Δ0] + [(Number of electrons in eg) × (+0.6)Δ0    = [(5) × (–0.4)Δ0] + [(0) × (+0.6)Δ0    = –2Δ0.
5. Verify that the calculated CFSE falls within the given range (2, 2): The calculated CFSE of –2Δ0 matches exactly with the required nearest integer value –2. Thus, it is valid.
Conclusion: The crystal field stabilization energy for [Fe(CN)6]3– is –2Δ0, confirmed within the expected range.

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Approach Solution -2

In [Fe(CN)6]3–, Fe is present in (+3) oxidation state Fe(III)
\(⇒\) inner orbital complex
\(⇒\) 5(with pairing)
Configuration \(⇒\)\(t_{2g}^5\)
\(\text{CFSE} = \frac{5 \times (-2)}{5} \Delta_0\)
\(=\text{CFSE} = -2\Delta_0\)
So, the answer is 2.

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Concepts Used:

Coordination Compounds

A coordination compound holds a central metal atom or ion surrounded by various oppositely charged ions or neutral molecules. These molecules or ions are re-bonded to the metal atom or ion by a coordinate bond.

Coordination entity:

A coordination entity composes of a central metal atom or ion bonded to a fixed number of ions or molecules.

Ligands:

A molecule, ion, or group which is bonded to the metal atom or ion in a complex or coordination compound by a coordinate bond is commonly called a ligand. It may be either neutral, positively, or negatively charged.