charge-transfer band

Why is permanganate such a screaming, intense purple, and a chromate so saturated a yellow, when copper sulfate is merely a soft blue? The difference is the kind of electronic jump behind the color. Copper's color comes from a faint, forbidden d-d transition; permanganate's comes from a charge-transfer band — a transition in which an electron actually moves from one part of the molecule to another, between the ligands and the metal.

In a charge-transfer transition the electron is not just shuffled between two d orbitals on the same metal; it leaps across the metal-ligand bond, from an orbital that is mostly on the ligand to one that is mostly on the metal (or the reverse). Because this is a genuine relocation of charge over a real distance, and because it is usually fully allowed by both the Laporte and the spin rules, it absorbs light enormously more strongly than a d-d band — molar absorptivities of tens of thousands rather than tens. That is the whole reason the color is so vivid: a tiny amount of the absorbing ion gives a deep, intense hue.

Charge-transfer bands come in two flavors named by which way the electron travels: ligand-to-metal (LMCT) and metal-to-ligand (MLCT). They usually sit in the ultraviolet, but when the metal is easily reduced or oxidized, or the ligand is easily oxidized or reduced, the band slides down into the visible and produces the most spectacular colors in inorganic chemistry. A practical caveat: because CT bands are so strong, even a trace of an impurity that allows charge transfer can dominate the visible spectrum and mask the much weaker, more informative d-d bands underneath.