The photochemical behavior of metal complexes can be significantly influenced by the nature of the ligands attached to the metal center. Ligands can affect the absorption properties, excited-state lifetimes, and photoreactivity of the metal complex. Here, we will discuss two specific examples to illustrate how different ligands can impact the photochemical behavior of metal complexes.Example 1: Ruthenium II polypyridyl complexesRuthenium II polypyridyl complexes are well-known for their rich photochemistry. The general formula for these complexes is [Ru L N-N 2]^2+, where L is a diimine ligand e.g., 2,2'-bipyridine or 1,10-phenanthroline and N-N is a bidentate nitrogen donor ligand.Diagram 1: [Ru bpy 3]^2+ complex https://www.researchgate.net/profile/Ana-Petronilho/publication/281449612/figure/fig1/AS:614153674076162@1523414049456/Chemical-structure-of-Ru-bpy-3-2.png When the ligands are changed, the absorption properties and excited-state lifetimes of the complexes can be significantly altered. For example, replacing one or more bipyridine bpy ligands with phenanthroline phen ligands in [Ru bpy 3]^2+ can red-shift the absorption maxima and increase the excited-state lifetimes.Example 2: Platinum II complexes with different ancillary ligandsPlatinum II complexes with different ancillary ligands can exhibit different photochemical behavior. One example is the difference between cisplatin and transplatin, which are both platinum II complexes with two ammine NH3 ligands and two chloride Cl- ligands.Diagram 2: Cisplatin and transplatin structures https://upload.wikimedia.org/wikipedia/commons/thumb/4/4d/Cisplatin_and_transplatin.png/300px-Cisplatin_and_transplatin.png Cisplatin cis-diamminedichloroplatinum II has a square planar geometry with the two ammine ligands and two chloride ligands in cis positions. In contrast, transplatin trans-diamminedichloroplatinum II has the two chloride ligands in trans positions. The photochemical behavior of these two complexes is quite different. Cisplatin is known to be photoactive, undergoing photodissociation of one chloride ligand upon UV irradiation, which is an important step in its mode of action as an anticancer drug. On the other hand, transplatin is not photoactive and does not undergo photodissociation of its chloride ligands.In summary, the photochemical behavior of metal complexes can be significantly influenced by the nature of the ligands attached to the metal center. Different ligands can affect the absorption properties, excited-state lifetimes, and photoreactivity of the metal complex, as illustrated by the examples of ruthenium II polypyridyl complexes and platinum II complexes with different ancillary ligands.