Crystal Field Theory
Definition and meaning of Crystal Field Theory in chemistry.
Crystal field theory (CFT) explains how the degenerate d orbitals of a transition-metal ion split into distinct energy levels when surrounded by ligands modeled as point negative charges, accounting for the color, magnetism, and stability of coordination complexes.
In more detail
In an octahedral complex the five d orbitals split into a lower-energy t2g set (dxy, dxz, dyz) and a higher-energy eg set (dz2, dx2−y2), separated by the crystal field splitting energy, Δo. The magnitude of Δo depends on the metal ion's charge and the ligand's field strength, ranked by the spectrochemical series, and determines whether electrons pair up (low-spin, large Δo) or spread out (high-spin, small Δo). Because Δo often falls in the visible range, electronic transitions between the split orbitals absorb specific wavelengths, giving many transition-metal complexes their characteristic colors.
Key facts
| Field | Inorganic Chemistry |
|---|---|
| Splitting parameters | Δo (octahedral), Δt (tetrahedral) |
| Common geometries | Octahedral, tetrahedral, square planar |
| Predicts | Color, magnetism (high-spin/low-spin), complex stability |
In [Ti(H2O)6]3+, the lone d electron sits in a t2g orbital; absorbing visible light near 500 nm promotes it to an eg orbital, so the complex transmits and appears purple.
Frequently asked questions
How does crystal field theory differ from ligand field theory?
CFT treats ligands as simple point charges producing purely electrostatic repulsion with the metal's d electrons, ignoring covalency. Ligand field theory builds on CFT using molecular orbital theory to include metal-ligand orbital overlap and covalent bonding.
What decides if a complex is high-spin or low-spin?
The size of Δo relative to the electron pairing energy: weak-field ligands (e.g., I−, Cl−) give small Δo and high-spin complexes, while strong-field ligands (e.g., CN−, CO) give large Δo and low-spin complexes.