Charge transfer complexes

The first highly-conductive organic compounds were the Charge transfer complexes. In 1954, researchers at Bell Labs and elsewhere reported Charge transfer complexes with resistivities as low as 8 ohms-cm . In the early 1970's, salts of tetrathiafulvalene were shown to exhibit almost metallic conductivity, while superconductivity was demonstrated in 1980. Broad research on charge transfer salts continues today.
Charge Transfer (CT) bands in transition metal complexes result from movement of electrons between molecular orbitals (MO) that are predominantly metal in character and those that are predominantly ligand in character. If the electron moves from the MO with ligand like character to the metal like one, the complexes is called Ligand to Metal Charge Transfer (LMCT) complex. If the electron moves from the MO with metal like character to the ligand like one, the complexes is called 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 a powerful technique to assign and characterize charge transfer bands.