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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 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. Many such complexes can undergo an electronic transition into an excited electronic state. The excitation energy of this transition occurs very frequently in the visible region of the electro-magnetic spectrum, which produces the characteristic intense color 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 solids, liquids, and solutions. A well-known example is the complex formed by iodine when combined with starch, which exhibits an intense blue charge-transfer band. In inorganic chemistry, most charge-transfer complexes involve electron transfer between metal atoms and ligands. The charge-transfer bands of 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. ==Donor-acceptor association equilibrium== Charge-transfer complexes are formed by weak association of molecules or molecular subgroups, one acting as an electron donor and another as an electron acceptor. The association does not constitute a strong covalent bond and is subject to significant temperature, concentration, and host, e.g., solvent, dependencies. The charge-transfer association occurs in a chemical equilibrium with the independent donor (D) and acceptor (A) molecules: : In terms of quantum mechanics, this is described as a resonance between the non-bonded state |D, A> and the dative state |D+...A−>. The formation of the dative state is an electronic transition giving rise to the colorful absorption bands. The intensity of charge-transfer bands in the absorbance spectrum is strongly dependent upon the degree (equilibrium constant) of this association reaction. Methods have been developed to determine the equilibrium constant for these complexes in solution by measuring the intensity of absorption bands as a function of the concentration of donor and acceptor components in solution. The methods were first described for the association of iodine dissolved in aromatic hydrocarbons.〔H. Benesi, J. Hildebrand, ''A Spectrophotometric Investigation of the Interaction of Iodine with Aromatic Hydrocarbons'', J. Am. Chem. Soc. 71(8), 2703-07 (1949) doi:10.1021/ja01176a030.〕 The procedure is called the Benesi-Hildebrand method, named after the authors of the study. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Charge-transfer complex」の詳細全文を読む スポンサード リンク
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