The photochemical behavior of transition metal complexes is significantly influenced by the type of ligands attached to them. Ligands can alter the electronic structure, stability, and reactivity of the metal center, which in turn affects the absorption and emission properties, as well as the photochemical reactions that the complex can undergo. Here, we will discuss some specific examples and the underlying mechanisms behind these changes.1. Ligand field strength: The strength of the ligand field can affect the energy levels of the metal d-orbitals, which in turn influences the absorption and emission properties of the complex. For example, in octahedral complexes, strong-field ligands e.g., CO, CN- lead to a larger splitting of the d-orbitals, resulting in low-spin complexes with low-energy absorption bands. In contrast, weak-field ligands e.g., H2O, OH- lead to high-spin complexes with high-energy absorption bands. This can be explained by the Ligand Field Theory, which describes the interaction between the metal center and the ligands in terms of electrostatic and covalent bonding.2. Charge transfer transitions: The nature of the ligands can also affect the charge transfer transitions in transition metal complexes. For example, metal-to-ligand charge transfer MLCT transitions are common in complexes with -acceptor ligands e.g., bpy, phen , where the electron density is transferred from the metal to the ligand upon photoexcitation. This can lead to long-lived excited states and interesting photophysical properties, such as luminescence. On the other hand, ligand-to-metal charge transfer LMCT transitions can occur in complexes with -donor ligands e.g., NO2-, Cl- , where the electron density is transferred from the ligand to the metal upon photoexcitation. This can result in short-lived excited states and different reactivity patterns.3. Steric effects: The steric properties of the ligands can also influence the photochemical behavior of transition metal complexes. For example, bulky ligands can protect the metal center from unwanted side reactions, such as solvent or substrate coordination, which can be important for selective photochemical reactions. Additionally, the steric environment can affect the conformation and rigidity of the complex, which can influence the non-radiative decay pathways and the luminescence properties.4. Redox properties: The redox properties of the ligands can also play a crucial role in the photochemical behavior of transition metal complexes. For example, redox-active ligands e.g., ferrocene, bipyridine can undergo photoinduced electron transfer reactions with the metal center, leading to the formation of reactive intermediates and interesting photochemical applications, such as photocatalysis and photovoltaics.In summary, the photochemical behavior of transition metal complexes can be significantly affected by the type of ligands attached to them. The ligands can influence the electronic structure, stability, and reactivity of the metal center, which in turn affects the absorption and emission properties, as well as the photochemical reactions that the complex can undergo. Understanding these effects is crucial for the rational design of new transition metal complexes with tailored photochemical properties and applications.