kristallveldtheorie

Crystal Field Theory (CFT) is a simplified model in inorganic chemistry that describes the electronic structure of transition metal complexes by treating ligands as stationary point charges that generate an electrostatic field at the metal ion. This field causes the degenerate five d-orbitals of the metal to split in energy, influencing the electronic configuration and properties of the complex.

In an octahedral arrangement, the five d-orbitals split into two groups: the lower-energy t2g set (dxy, dxz, dyz) and the higher-energy eg set (dx2−y2, dz2). The energy separation between these groups is denoted Δo, the octahedral crystal field splitting energy. In a tetrahedral field, the ordering is reversed and the splitting magnitude is smaller, with Δt roughly 4/9 of Δo. The magnitude of Δo or Δt depends on the metal, its oxidation state, the geometry, and the ligands.

Ligands influence splitting through the spectrochemical series, which orders ligands by their ability to increase Δo. Strong-field ligands (for example CN−, CO) produce large Δo and often favor low-spin configurations, while weak-field ligands (such as H2O, F−, Cl−) yield smaller Δo and high-spin states. The resulting spin state arises from the competition between Δo and the pairing energy of electrons.

CFT helps interpret colors and magnetic properties of complexes, since d–d electronic transitions fall within the visible region. However, CFT neglects covalent bonding and overestimates the purely ionic nature of metal–ligand interactions. More comprehensive approaches, such as ligand field theory and molecular orbital theory, incorporate covalency and provide a more complete description.