What is metal to metal charge transfer (MMCT)?

 Metal-to-metal charge transfer (MMCT) refers to a type of electronic transition that occurs between two metal centers in a complex or compound. In MMCT, an electron is transferred from a metal ion or atom (the electron donor) to another metal ion or atom (the electron acceptor).

MMCT transitions typically involve metal ions or atoms with different oxidation states or electronic configurations. The electron transfer leads to a change in the oxidation state or electronic configuration of the metal centers involved.

The energy of the absorbed or emitted light in MMCT transitions corresponds to the energy difference between the initial and final states of the electron donor and acceptor metals. This energy difference is influenced by factors such as the electronic structure of the metal centers, the ligands coordinating to the metals, and the environment surrounding the complex.

MMCT transitions are often observed in transition metal complexes and are responsible for their characteristic colors and spectroscopic properties. These transitions can have implications in various fields, including coordination chemistry, photochemistry, and materials science.

Examples

Here are a few examples of metal-to-metal charge transfer (MMCT) complexes:

1. Prussian Blue: Prussian Blue is a well-known MMCT complex consisting of Fe(II) and Fe(III) ions. The electron transfer occurs between the Fe(II) and Fe(III) centers, resulting in a deep blue color.

2. Ruthenium(II)-Osmium(II) Complexes: These complexes feature a MMCT transition between the ruthenium(II) and osmium(II) metal centers. The absorption of light in the visible region leads to distinctive colors ranging from red to purple.

3. Copper(I)-Copper(II) Complexes: Certain copper complexes exhibit MMCT transitions between Cu(I) and Cu(II) ions. These transitions result in intense absorption bands in the visible region, giving rise to characteristic colors such as blue and green.

4. Nickel(II)-Copper(II) Complexes: Complexes containing both nickel(II) and copper(II) ions can exhibit MMCT transitions. The electron transfer occurs from the nickel(II) center to the copper(II) center, leading to distinct colors and spectroscopic features.

5. Manganese(III)-Ruthenium(III) Complexes: In certain manganese(III)-ruthenium(III) complexes, MMCT transitions occur between the two metal centers. These transitions are responsible for the absorption of light in the visible region, resulting in various colors.

These examples illustrate the variety of MMCT complexes involving different combinations of transition metal ions, oxidation states, and ligands. The specific colors and properties observed in each complex depend on the nature of the metal centers and their coordination environment.