Question

The Valence Bond Theory (VBT) explains the formation, magnetic behaviour and geometry of coordination compounds. The Crystal Field Theory (CFT) of coordination compounds is based on the effect of different crystal fields (provided by the ligands taken as point charges), on the degeneracy of d-orbital energies of the central metal atom/ion. The splitting of the d-orbitals provides different electronic arrangements in strong and weak crystal fields. Answer the following questions :

29(a). In octahedral crystal field, energies of which d-orbitals will be raised when ligands approach the central metal atom/ion ? Give reason in support of your answer.

Hint:

Octahedral: $e_g$ is high energy. Tetrahedral: $t_2$ is high energy. They are essentially inverted because of ligand approach directions.

Answer 1

Step 1: Understanding the Concept:
Crystal field splitting occurs due to the electrostatic repulsion between the ligand lone pairs and the metal d-electrons.
Step 2: Detailed Explanation:
In an octahedral field, the six ligands approach the metal along the x, y, and z axes.
The lobes of the \( e_g \) orbitals (\( d_{x^2-y^2} \) and \( d_{z^2} \)) point directly along these axes.
Therefore, electrons in these orbitals experience greater repulsion from the ligands than electrons in the \( t_{2g} \) orbitals (which point between the axes).
This increased repulsion raises the energy of the \( e_g \) orbitals.
Step 3: Final Answer:
The energy of \( e_g \) orbitals is raised due to direct axial repulsion from ligands.