Changing the ligands in metal complexes can significantly affect their photochemical properties, such as absorption spectra, emission spectra, and photochemical reactivity. This is because the ligands can influence the electronic structure of the metal center and the overall energy levels of the complex. The underlying mechanisms can be explained through the following factors:1. Ligand field strength: The strength of the ligand field can affect the energy gap between the metal-centered d-orbitals. Strong-field ligands, such as cyanide CN- and carbon monoxide CO , can lead to a larger energy gap, resulting in lower energy absorption and emission. In contrast, weak-field ligands, such as iodide I- and bromide Br- , can lead to a smaller energy gap, resulting in higher energy absorption and emission.Example: The color of transition metal complexes, such as [Co NH3 6]3+ violet and [Co CN 6]3- yellow , can be attributed to the difference in ligand field strength between NH3 and CN-.2. Charge transfer transitions: Changing the ligands can also affect the charge transfer transitions in metal complexes. These transitions involve the transfer of an electron from a ligand orbital to a metal orbital ligand-to-metal charge transfer, LMCT or vice versa metal-to-ligand charge transfer, MLCT . The energy of these transitions depends on the energy difference between the donor and acceptor orbitals, which can be influenced by the nature of the ligands.Example: In Ru II polypyridyl complexes, such as [Ru bpy 3]2+ bpy = 2,2'-bipyridine , the MLCT transitions are responsible for their strong absorption in the visible region and their luminescent properties. By changing the bpy ligands to other polypyridyl ligands, the absorption and emission properties can be fine-tuned.3. Ligand-centered transitions: Some ligands can also exhibit their own electronic transitions, which can be influenced by their coordination to the metal center. These ligand-centered transitions can interact with the metal-centered transitions, leading to changes in the overall photochemical properties of the complex.Example: In platinum II complexes with cyclometalated ligands, such as [Pt ppy 2] ppy = 2-phenylpyridine , the ligand-centered transitions can mix with the MLCT transitions, resulting in phosphorescent emission.4. Steric effects: The steric properties of the ligands can also affect the photochemical properties of metal complexes. Bulky ligands can protect the metal center from quenching by other molecules, leading to enhanced luminescence or longer-lived excited states.Example: In iridium III complexes, such as [Ir ppy 3], the use of bulky ppy ligands can help to shield the metal center from quenching, resulting in highly efficient phosphorescent emission.In summary, changing the ligands in metal complexes can have a significant impact on their photochemical properties through various mechanisms, including ligand field strength, charge transfer transitions, ligand-centered transitions, and steric effects. By carefully selecting the appropriate ligands, it is possible to design metal complexes with tailored photochemical properties for various applications, such as solar energy conversion, photocatalysis, and luminescent materials.