ligand-to-metal charge transfer

Picture an electron that normally lives on a ligand — sitting on an oxygen or a chlorine wrapped around the metal — and then light arrives and hauls it inward onto the metal atom. That inward jump is a ligand-to-metal charge transfer, LMCT for short. For an instant the ligand is partly oxidized and the metal partly reduced, and the energy needed to make the jump shows up as an absorption band.

Mechanically, the electron starts in an orbital that is mostly ligand in character (a lone pair or a filled ligand orbital) and ends in an orbital that is mostly metal d in character. This is favored when the metal is in a high oxidation state and therefore hungry for electrons — easily reduced — and when the ligand has electrons that are easy to give up, such as a soft, polarizable ligand like sulfide or iodide. The more easily the metal is reduced and the ligand oxidized, the lower the energy of the LMCT band, so it slides from the ultraviolet down into the visible and the compound becomes deeply colored.

LMCT explains the most dramatic colors in main-group and high-oxidation-state chemistry, precisely where d-d transitions cannot help. Permanganate (Mn at +7, d0) and chromate (Cr at +6, d0) have no d electrons at all, so their intense purple and yellow are pure LMCT — oxygen lone pairs promoted onto the metal. The same logic colors many metal sulfides black or deeply hued (sulfide is very easy to oxidize) and lies behind the yellow of HgI2 and the orange of CrO3. A useful rule of thumb: a vivid, very strongly absorbing band on a metal in a high oxidation state is almost always LMCT.