Theories of bonding in complexes

Metal-ligand bonding in coordination compounds has historically been described by two separate theories, developed to answer slightly different questions. Valence Bond Theory (VBT, sections 9.9.1-9.9.5) explains bonding in terms of hybridisation of the metal ion's atomic orbitals, and is good at predicting a complex's GEOMETRY and magnetic behaviour (paramagnetic/diamagnetic) from first principles, once the spin state is already known. Crystal Field Theory (CFT, sections 9.9.6-9.9.9) instead treats the ligands as simple point negative charges producing a purely electrostatic field around the metal ion, and is specifically able to explain things VBT cannot -- most importantly, WHY a given ligand causes high spin or low spin behaviour in the first place, and WHY coordination compounds are so often coloured. The two theories are complementary rather than competing: CFT's limitations …