What condition in a complex makes MLCT likely to occur?

Metal-to-ligand charge transfer (MLCT) involves the promotion of an electron from a metal d orbital to an empty or partially filled π* orbital of the ligand. For MLCT to be likely, the ligands must possess low-lying π* orbitals that can effectively accept electron density from the metal when excitation occurs.

The presence of ligands with low-lying π* orbitals creates a favorable environment for MLCT since the energy required for an electron to transition from the metal to these vacant orbitals is not prohibitively high. This energetic alignment enhances the likelihood of MLCT transitions occurring during electronic excitation, making such complexes particularly interesting in the context of photochemistry and photophysical properties.

In summary, ligands with low-lying π* orbitals are crucial for facilitating MLCT, as they provide accessible empty orbitals for the metal to transfer its electronic density into during the transition process. This leads to significant optical and electronic properties that can be exploited in various applications, including sensors and light-harvesting materials.