Charge-transfer Complex

A charge-transfer complex (CT complex) or electron-donor-acceptor complex is an association of two or more molecules, or of different parts of one very large molecule, in which a fraction of electronic charge is transferred between the molecular entities. The resulting electrostatic attraction provides a stabilizing force for the molecular complex. The source molecule from which the charge is transferred is called the electron donor and the receiving species is called the electron acceptor.

The nature of the attraction in a charge-transfer complex is not a stable chemical bond, and is thus much weaker than covalent forces. The attraction is created by an electronic transition into an excited electronic state, and is best characterized as a weak electron resonance. The excitation energy of this resonance occurs very frequently in the visible region of the electro-magnetic spectrum, which produces the usually intense color characteristic for these complexes. These optical absorption bands are often referred to as charge-transfer bands (CT bands). Optical spectroscopy is a powerful technique to characterize charge-transfer bands.

Charge-transfer complexes exist in many types of molecules, inorganic as well as organic, and in all phases of matter, i.e. in solids, liquids, and even gases. A well-known example is the blue charge-transfer band exhibited by iodine when combined with starch.

In inorganic chemistry, most charge-transfer complexes involve electron transfer between metal atoms and ligands. The charge-transfer bands in transition metal complexes result from shift of charge density between molecular orbitals (MO) that are predominantly metal in character and those that are predominantly ligand in character. If the transfer occurs from the MO with ligand-like character to the metal-like one, the complex is called a ligand-to-metal charge-transfer (LMCT) complex. If the electronic charge shifts from the MO with metal-like character to the ligand-like one, the complex is called a metal-to-ligand charge-transfer (MLCT) complex. Thus, a MLCT results in oxidation of the metal center, whereas a LMCT results in the reduction of the metal center. Resonance Raman Spectroscopy is also a powerful technique to assign and characterize charge-transfer bands in these complexes.