The photochemical behavior of coordination compounds is significantly influenced by the nature of their ligands. Ligands are ions or molecules that bind to a central metal atom to form a coordination complex. The type of ligand, its electronic properties, and the strength of its bond to the metal center can all affect the photochemical properties of the complex. This can be observed in the absorption and emission spectra, photo-induced electron transfer, and photochemical reactions of the coordination compounds.Experimental evidence and observed trends:1. Ligand-to-metal charge transfer LMCT and metal-to-ligand charge transfer MLCT transitions:The electronic transitions in coordination compounds can involve the transfer of an electron from a ligand to the metal center LMCT or from the metal center to a ligand MLCT . The energy of these transitions depends on the nature of the ligands and their energy levels. For example, strong-field ligands like cyanide CN- can lead to low-energy LMCT transitions, while weak-field ligands like water H2O can result in high-energy LMCT transitions. Similarly, the energy of MLCT transitions can be tuned by changing the ligands. This can be observed in the absorption spectra of coordination compounds, where the position and intensity of the bands depend on the ligand type.2. Ligand field strength and photostability:The strength of the ligand field can affect the photostability of coordination compounds. Strong-field ligands can lead to a large ligand field splitting, which stabilizes the low-energy d-electron configurations and makes the complexes less susceptible to photo-induced decomposition. On the other hand, weak-field ligands can result in small ligand field splitting, making the complexes more prone to photodegradation. This can be observed in the photostability of transition metal complexes with different ligands, such as the increased photostability of ruthenium II polypyridyl complexes compared to their analogs with weaker ligands.3. Ligand substitution and photochemical reactions:The nature of the ligands can also affect the photochemical reactions of coordination compounds. For example, photo-induced ligand substitution reactions can occur when a coordination compound absorbs light and undergoes a change in its electronic configuration, which weakens the bond between the metal center and one of the ligands. The ease of ligand substitution depends on the strength of the metal-ligand bond, which can be influenced by the ligand type. This can be observed in the photochemical reactions of metal carbonyl complexes, where the substitution of CO ligands by other ligands can be induced by light irradiation.In conclusion, the photochemical behavior of coordination compounds is highly dependent on the nature of their ligands. The electronic properties of the ligands, their field strength, and their bonding to the metal center can all influence the absorption and emission spectra, photo-induced electron transfer, and photochemical reactions of the complexes. By carefully selecting the ligands, it is possible to tune the photochemical properties of coordination compounds for various applications, such as photocatalysis, solar energy conversion, and luminescent materials.