These terms describe the arrangement of ligands around a central metal ion in Crystal Field Theory (CFT) and how that arrangement causes the splitting of the metal’s d-orbitals.
1. Octahedral Crystal Field
- Arrangement: Six ligands placed symmetrically around the metal, along the x, y, and z axes (like at the corners of an octahedron).
- Effect on d-orbitals:
- Orbitals pointing directly at the ligands (along axes) feel strong repulsion → go higher in energy.
- Orbitals pointing between ligands feel less repulsion → stay lower in energy.
- Common in: Transition metal complexes like [Fe(H₂O)₆]²⁺.
- Properties: Can be high-spin or low-spin depending on ligand strength.
2. Tetrahedral Crystal Field
- Arrangement: Four ligands placed between the axes, at the corners of a tetrahedron.
- Effect on d-orbitals:
- Orbitals pointing between axes feel stronger repulsion → higher in energy.
- Orbitals pointing along axes feel less repulsion → lower in energy.
- Key difference: Splitting pattern is opposite to octahedral and the energy gap is smaller.
- Common in: Complexes like [CoCl₄]²⁻.
- Properties: Usually high-spin because the energy gap is small.
3. Square Planar Crystal Field
- Arrangement: Four ligands placed in one flat plane, along the x and y axes.
- Effect on d-orbitals:
- The orbital lying in the plane and pointing directly at ligands rises very high in energy.
- The other orbitals are pushed less, depending on how they align with ligands.
- Common in: d⁸ metals like Ni²⁺, Pd²⁺, Pt²⁺.
- Properties: Strong splitting, usually low-spin and diamagnetic.
In short:
- Octahedral (6 ligands): 2 groups of orbitals, large splitting.
- Tetrahedral (4 ligands): Opposite splitting pattern, smaller gap.
- Square planar (4 ligands in a plane): One orbital much higher, leading to strong preference for certain electron arrangements.