Charge- transfer complexes (abbreviated CT ) complexes are electron - donor-acceptor complexes that change by the absorption of light into a charge-separated state. By radiant and nonradiative transitions of the CT complex then returns back to the ground state. Charge-transfer complexes may be purely organic complexes or transition metal complexes. They often have an intense coloring.
In scientific publications CT complexes are also equated with electron donor-acceptor complexes. Strictly speaking, the definition of charge-transfer complexes is, however, more narrowly, electron donor- acceptor complex is considered as a generic term.
Charge transfer transitions
CT complexes are often complexes with metallic center. Depending on between which parts is taking place and in which direction the partial charge-transfer, different transitions can be distinguished.
Ligand to metal transition
These compounds and complexes, it is possible that an electron of the anion or ligand is transferred to the metal atom. If the orbitals are considered, the transition between the p- orbitals of the ligand and d- or s- orbitals of the metal. This transition is mainly found in compounds with highly charged cations. Typical examples are the permanganate and chromate ion, in which an electron is transferred to the oxygen of the manganese - or chromium atom. Another example is the thiocyanate complexes of trivalent iron, in which an electron from the thiocyanate ion is transferred to the iron ( III) ion.
Transition metal to ligand
The transition metal to ligand is the reverse of the transition metal ligand, then the metal of the donor and the acceptor ligand. The transition is now of occupied d orbitals of the metal into empty π * orbitals ( antibonding π orbitals ) of the ligand. Suitable ligands are, for example carbon monoxide, pyridine, or pyrazole. In bipyridine complexes of divalent iron, an electron from an occupied d- levels of the iron ( II ) ion is transferred to the low -lying unoccupied π * orbital of the bipyridine ligand.
Transition metal to metal
A charge transfer transition from a metal to the other is possible in compounds in which there is a metal in various oxidation states in a compound. A typical example is Prussian blue, can be moved in the electron between divalent and trivalent iron ions.
Transition ligand to ligand
This electron transfer can occur between different ligands, which, however, occur less frequently than in the charge-transfer transitions described previously.
Are the halogens, chlorine, bromine, iodine, or some organic compound in suitable solvents, often benzene, halogenated hydrocarbons or pyridine dissolved, there is characteristic colors. These are caused by unstable Lewis acid-base complexes in which electrons are transferred from the solvent to the solute molecules. Since different solvents may bind to different degrees, it comes in different solvents and at different colors. This phenomenon is also referred to as solvatochromism.
As the d-d transitions are also charge-transfer transitions usually in the visible or near ultraviolet spectral range. This requires that many charge-transfer complexes are colored. Here, the eye sees the complementary color of the absorbed light. So that at about 560 nm (green spectral region ) absorbing potassium permanganate appears characteristic violet.
In contrast to other transitions of charge-transfer transition quantum mechanically allowed. Therefore, a high absorption and thus intense colors are possible.
Solving charge-transfer complexes in different solvents, so there will be color shifts ( solvatochromism ). This play different interactions, such as dipole -dipole forces between the solvent molecules and the dissolved complex a role which cause a shift of the absorption maximum of light to longer or shorter wavelengths.
The color of many pigments based on charge-transfer transitions. Important examples are cadmium sulfide ( yellow), vermilion (red), lead chromate (yellow) or iron oxide pigments.