CBSE Class 12-science Answered

Charge-transfer complex  or electron-donor-acceptor complex is an association of two or more molecules, or of different parts of one very large molecule, such that 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 molecule is called the electron acceptor. The nature of the attraction in a charge-transfer complex is not a stable chemical bond and is 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. As a result, the excitation energy of this resonance occurs very frequently in the visible region of the electro-magnetic spectrum. This produces the usually intense colours 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 bandsIn inorganic chemistry, most charge-transfer complexes involve electron transfer between metal atoms and ligands. The charge-transfer 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 complex 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 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.

The colour of charge-transfer bands, i.e., the charge-transfer transition energy, is characteristic of the specific type of donor and acceptor entities.

In CrO₄^2- , Cr⁶⁺ is orange in colour due to transfer of electrons from 2p orbitals of oxygen to 3d orbitals of Cr (Oxygen to metal charge transfer:  O^2-  → Cr⁶⁺). These transitions are responsible for strong absorption of violet, blue and green wavelengths resulting in orange to orange red colours.