The presence of a ligand can significantly affect the photochemical properties of a coordination compound. Ligands can influence the absorption and emission spectra, the excited-state lifetimes, and the photochemical reactivity of the complex. This is because ligands can alter the electronic structure of the metal center, leading to changes in the energy levels and transitions between them. Here are three examples of coordination compounds and their respective ligands that illustrate this phenomenon:1. Ruthenium II polypyridyl complexes: These complexes have the general formula [Ru bpy _3]^2+, where bpy is 2,2'-bipyridine, a bidentate ligand. The presence of bpy ligands results in strong absorption in the visible region, making these complexes excellent candidates for applications in solar energy conversion and photocatalysis. By replacing one or more bpy ligands with other polypyridyl ligands, such as 4,4'-dimethyl-2,2'-bipyridine or 1,10-phenanthroline, the absorption and emission properties of the complex can be fine-tuned for specific applications.2. Titanium IV citrate complexes: Titanium IV forms coordination compounds with citrate ligands, which are tridentate ligands derived from citric acid. The presence of citrate ligands in these complexes leads to strong absorption in the ultraviolet UV region, making them useful as UV absorbers in sunscreens and other applications. The photochemical properties of these complexes can be altered by varying the number of citrate ligands or by introducing other ligands, such as oxalate or tartrate.3. Copper I diimine complexes: Copper I forms coordination compounds with diimine ligands, such as 2,9-dimethyl-1,10-phenanthroline dmp or 2,2'-biquinoline biq . These complexes exhibit strong luminescence in the visible region, which can be tuned by varying the diimine ligand or by introducing additional ligands, such as phosphines or thiolates. The photochemical properties of these complexes have been exploited in applications such as luminescent sensors and photocatalytic systems.In summary, the presence of ligands can significantly influence the photochemical properties of coordination compounds, allowing for the fine-tuning of their absorption, emission, and reactivity for various applications. By understanding the relationship between ligand structure and photochemical properties, chemists can design coordination compounds with tailored properties for specific uses.