Crystal Field Theory

Crystal Field Theory (CFT) was developed to cover the colour and magnetic behaviour that VBT cannot explain. It assumes the metal-ligand bond is purely ionic, arising from electrostatic attraction between the electron-rich ligand and the electron-deficient metal; the central metal atom/ion and the ligands are modelled as point charges (for charged species) or electric dipoles (for neutral metal atoms or neutral ligands). Complex formation is pictured as a sequence of hypothetical steps. Step 1: in an isolated gaseous metal ion, all five d orbitals are degenerate; as the ligands first form an imagined spherical field of negative charge around the metal, the energy of ALL five d orbitals rises together (electron-electron repulsion), while their average energy stays the same. Step 2: the ligands then approach along their real, directional bond axes rather than uniformly from every direction, so orbitals pointing toward the incoming ligands are repelled more than orbitals pointing between them -- splitting the five degenerate orbitals into two sets at different energies, called crystal field splitting. Step 3: as the ligands approach …