Explain why [FeO4]4- (structure 21.33) suffers from a Jahn–Teller distortion. The distortion is particularly strong. Is this expected? Give short and clear answer
The Correct Answer and Explanation is:
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Short Answer:
The [FeO4]4−[FeO_4]^{4-}[FeO4]4− ion suffers from a Jahn–Teller distortion because it contains a high-spin d4d^4d4 iron(IV) center in a tetrahedral geometry, which leads to electronic degeneracy in the eee set of orbitals. This degeneracy is unstable and causes the molecule to distort to lower the overall energy. The distortion is particularly strong and this is expected due to the presence of an unpaired electron in degenerate eee orbitals.
Explanation:
The Jahn–Teller effect occurs in coordination complexes when there is an uneven distribution of electrons in degenerate orbitals, leading to a distortion of the geometry to remove the degeneracy and lower the system’s energy. In the case of [FeO4]4−[FeO_4]^{4-}[FeO4]4−, iron is in the +4 oxidation state, giving it a d4d^4d4 electron configuration.
In a tetrahedral crystal field, the d-orbitals split into two sets: a lower-energy eee set (comprising dz2d_{z^2}dz2 and dx2−y2d_{x^2 – y^2}dx2−y2) and a higher-energy t2t_2t2 set (comprising dxyd_{xy}dxy, dxzd_{xz}dxz, and dyzd_{yz}dyz). For a high-spin d4d^4d4 configuration, the four electrons occupy the orbitals as: two electrons in the lower-energy eee set (paired or unpaired depending on spin pairing energy), and two unpaired electrons in the higher-energy t2t_2t2 orbitals.
However, the Jahn–Teller theorem primarily applies to degenerate electronic states that are asymmetrically occupied. In the high-spin d4d^4d4 configuration in a tetrahedral field, the key point is that the partially filled e orbitals are degenerate and asymmetrically filled. This leads to strong Jahn–Teller distortion.
The distortion is particularly strong because the unpaired electron in the degenerate e orbitals creates significant instability. To alleviate this, the tetrahedral structure distorts (e.g., elongates or compresses bonds), lifting the degeneracy and stabilizing the complex.
This behavior is expected for high-spin d4d^4d4 tetrahedral complexes due to their electronic configuration. The magnitude of distortion is large because the Jahn–Teller effect is strongest when degenerate orbitals are unevenly occupied, especially in the eee set, which directly interacts with ligands.
