Kristallfeldtheorie – Tetraedrische und quadratisch-planare Komplexe

In tetrahedral complexes, the d orbital splitting pattern is inverted relative to octahedral complexes. The dxy, dyz, and dxz orbitals are higher in energy (t2 set), while dx2−y2 and dz2 orbitals are lower in energy (e set). This inversion occurs because none of the tetrahedral ligands point directly at the d orbital lobes, resulting in weaker interactions and smaller crystal field splitting energy (Δtet).

Tetrahedral complexes have smaller crystal field splitting energy (Δtet) because the ligands are positioned between the Cartesian axes rather than pointing directly at the d orbital lobes. This geometry creates less overlap between the crystal field and the d orbitals, resulting in weaker electrostatic repulsion and reduced orbital destabilization compared to octahedral arrangements.

In square planar complexes, the dx2−y2 orbital is highest in energy, followed by dxy, then dz2, with dxz and dyz lowest. The dx2−y2 orbital has lobes pointing directly at ligands in the xy plane, while dxz and dyz have minimal interaction with the crystal field. This arrangement results from removing two trans ligands from an octahedral geometry along the z-axis.

Crystal field splitting energy in square planar complexes (Δsp) is defined as the energy difference between the highest-energy orbital, dx2−y2, and the lowest-energy orbitals, dyz and dxz. This splitting is larger than in octahedral complexes, reflecting the significant destabilization of orbitals aligned with the xy plane ligands.

The ratio of crystal field splitting energies for tetrahedral, square planar, and octahedral complexes is 0.44:1.7:1, assuming the same metal ion and ligand molecules. This demonstrates that square planar complexes have the largest splitting, octahedral complexes are intermediate, and tetrahedral complexes have the smallest splitting energy.

The dz2 orbital has lower energy in square planar complexes because there is minimal overlap between this orbital and the crystal field in the xy plane where the ligands are positioned. Since the dz2 orbital extends primarily along the z-axis, it experiences reduced electrostatic repulsion from the square planar arrangement of ligands.

Crystal field theory applies to tetrahedral and square planar complexes by replacing ligands with negative point charges and analyzing resulting d orbital interactions. The theory predicts orbital energy diagrams based on orbital overlap with the crystal field geometry. Understanding these geometries requires recognizing how ligand positioning relative to d orbital lobes determines destabilization patterns.