Question

Apply the crystal field theory to the tetrahedral complex.

Answer 1

In a tetrahedral complex, a central metal ion is surrounded by four ligands positioned at the corners of a tetrahedron.

1. d-Orbital Splitting Pattern: The five d-orbitals split into two energy levels but in reverse compared to octahedral complexes:
   • Higher energy: d_xy, d_yz, d_xz
   • Lower energy: d_x²−y², d_z²
2. Cause of Splitting: In a tetrahedral arrangement, ligands approach the metal ion between the coordinate axes. As a result, the d-orbitals orientated between the axes (d_xy, d_yz, d_xz) experience greater repulsion and are raised to a higher energy level.
3. Crystal Field Splitting Energy (Δt): The energy gap in tetrahedral complexes is denoted as Δ_t. Because Δ_t is relatively small, electron pairing is less favorable, and most tetrahedral complexes are high-spin, with more unpaired electrons.
4. Electron Filling in Tetrahedral Fields: Electrons fill the t₂ (lower) set first, then the e (higher) set. Due to small Δ_t, electrons tend to avoid pairing. As a result, electrons remain unpaired as much as possible, leading to maximum unpaired electrons and paramagnetic behavior.
5. Example: [MnCl₄]²⁻ in this complex, manganese is in the +2 oxidation state (Mn²⁺), with an electronic configuration of 3d⁵. Chloride (Cl⁻) is a weak-field ligand, resulting in a small crystal field splitting energy (Δ_t). Due to the small Δ_t, the complex is high-spin, and all five d-electrons remain unpaired. Therefore, [MnCl₄]²⁻ is strongly paramagnetic and coloured.