Changing the ligands of a coordination compound can significantly affect its photochemical properties, such as absorption and emission spectra, photostability, and photochemical reactivity. This is because the ligands can influence the electronic structure, energy levels, and geometrical arrangement of the metal center, which in turn determine the compound's photochemical behavior. Here are three specific examples with explanations:1. Ligand field strength: The strength of the ligand field can affect the energy gap between the metal-centered d-orbitals. For example, consider the octahedral complexes [Fe CN 6]^3- and [Fe H2O 6]^3+. The cyanide ligand CN- is a strong-field ligand, while water H2O is a weak-field ligand. The strong-field ligand creates a larger energy gap between the t2g and eg orbitals, leading to a low-spin configuration and a lower energy absorption band. In contrast, the weak-field ligand results in a high-spin configuration and a higher energy absorption band. This difference in absorption spectra can be exploited in various photochemical applications, such as solar energy conversion and photocatalysis.2. Ligand-to-metal charge transfer LMCT transitions: The nature of the ligand can also affect the energy and intensity of LMCT transitions. For example, consider the complexes [Ru bpy 3]^2+ and [Ru phen 3]^2+, where bpy = 2,2'-bipyridine and phen = 1,10-phenanthroline. Both complexes exhibit strong MLCT transitions, but the energy and intensity of these transitions are different due to the different ligands. The phen ligand has a more extended -system, which results in a lower energy and more intense MLCT transition compared to the bpy ligand. This difference in photochemical properties can be utilized in designing efficient light-harvesting systems and photoredox catalysts.3. Steric effects and geometrical isomerism: The steric properties of the ligands can influence the geometrical arrangement of the coordination compound, which in turn affects its photochemical properties. For example, consider the cis and trans isomers of [Pt NH3 2Cl2]. The cis isomer has both chloride ligands on the same side of the platinum center, while the trans isomer has them on opposite sides. The cis isomer exhibits a more intense absorption band and a higher photoreactivity compared to the trans isomer. This is because the cis isomer can undergo photoinduced ligand exchange reactions more easily, as the chloride ligands are in close proximity to each other. This difference in photochemical properties has important implications in the design of photochemotherapy drugs, where the cis isomer cisplatin is a widely used anticancer agent, while the trans isomer is inactive.In summary, changing the ligands of a coordination compound can significantly affect its photochemical properties by influencing the electronic structure, energy levels, and geometrical arrangement of the metal center. This can be exploited in various photochemical applications, such as solar energy conversion, photocatalysis, and photochemotherapy.