crystal field theory
/ CFT /
Why is a solution of copper salt blue, a ruby red, and a chromium compound green or violet — yet a zinc salt is colorless? Why is one iron complex strongly magnetic and another barely so? Crystal field theory is the first simple model that answers these questions for the transition metals. It looks at what happens to the d electrons of a metal ion when other groups, called ligands, cluster around it.
The trick is to treat each ligand as a tiny point of negative charge — a lone pair pointing at the metal, modeled as a naked negative dot. A free metal ion has five d orbitals all at the same energy (they are degenerate). When the ligand point charges crowd in around the metal, their negative fields push on the d electrons. But the five d orbitals point in different directions, so some get pushed harder than others. The orbitals aimed straight at the ligands climb to higher energy; the orbitals aimed between the ligands stay lower. The once-equal d orbitals split into groups separated by an energy gap, and that gap, sitting in the visible-light range, is what colors and magnetizes these compounds.
Be honest about what this model is. It is purely electrostatic: it pretends the metal-ligand interaction is just the repulsion between point charges, with no electron sharing at all. That is a fiction — real metal-ligand bonds are partly covalent, and treating ammonia or carbon monoxide as a bare negative point is crude. Yet the model is astonishingly useful for predicting splitting patterns, colors, magnetism, and stability trends, which is why it remains the everyday workhorse. When the electrostatic picture breaks down, chemists upgrade to ligand field theory, which folds covalency back in.
Solid Ti(III) in water gives [Ti(H2O)6]3+, which is violet. It has a single d electron sitting in the lower split set. That electron absorbs green-yellow light to jump the crystal field gap, and the leftover transmitted light looks violet — a textbook case of color produced entirely by d-orbital splitting.
One d electron and a single absorption band already give a colored ion.
Crystal field theory is a deliberate simplification: it ignores all covalency and treats ligands as point charges, so it cannot by itself explain why neutral CO splits d orbitals more strongly than negative fluoride — that needs ligand field theory.