metal-to-ligand charge transfer

Now run the charge transfer the other way. Instead of light pulling an electron from a ligand onto the metal, it pushes an electron from the metal outward onto a ligand. That outward jump is a metal-to-ligand charge transfer, MLCT. For a moment the metal is partly oxidized and the ligand partly reduced, and the energy of that jump appears as an intense absorption band — the source of color in many of the brightly colored complexes used in solar cells and light-emitting devices.

The electron starts in a filled metal d orbital and lands in an empty orbital on the ligand — specifically an empty pi-antibonding orbital (a pi-star orbital) of a ligand that can accept electrons, such as carbon monoxide, cyanide, or an aromatic nitrogen ligand like 2,2'-bipyridine. MLCT is favored when the metal is in a low oxidation state and electron-rich (easily oxidized) and the ligand is a good pi-acceptor with low-lying empty orbitals to receive the electron. The better the metal pushes and the ligand pulls, the lower the band energy and the more it moves into the visible.

MLCT is the workhorse behind a whole class of functional complexes. The deep orange of tris(bipyridine)ruthenium(II), the photoactive heart of many dye-sensitized solar cells and photocatalysts, is an MLCT band: light promotes a ruthenium d electron onto the bipyridine ligand, creating a charge-separated excited state that can then do useful chemistry. Like all charge-transfer bands MLCT is fully allowed and therefore intense. The honest subtlety: telling MLCT from LMCT is not done by color alone — you reason from the oxidation state (low favors MLCT, high favors LMCT) and the nature of the ligand (pi-acceptors favor MLCT, easily oxidized donors favor LMCT).