The photochemical behavior of complex transition metal compounds is highly dependent on their structures and ligands. The nature of the ligands, their arrangement around the metal center, and the overall geometry of the complex can significantly influence the absorption of light, the excited states, and the subsequent photochemical reactions. Here, we will discuss some specific examples of coordination compounds and their photochemical behavior.1. Metal-to-Ligand Charge Transfer MLCT complexes: In these complexes, the photoexcitation results in the transfer of an electron from a metal-based orbital to a ligand-based orbital. A classic example is the tris bipyridine ruthenium II complex, [Ru bpy ], where bpy is 2,2'-bipyridine. Upon photoexcitation, an electron is transferred from a Ru II d-orbital to a * orbital of the bpy ligand. The MLCT excited state is relatively long-lived, making this complex an excellent candidate for applications in photocatalysis and solar energy conversion.2. Ligand-to-Metal Charge Transfer LMCT complexes: In these complexes, the photoexcitation results in the transfer of an electron from a ligand-based orbital to a metal-based orbital. An example is the hexacyanoferrate III complex, [Fe CN ]. Upon photoexcitation, an electron is transferred from a CN ligand to an Fe III d-orbital. LMCT complexes are often used as photosensitizers in photoredox reactions.3. Ligand-centered complexes: In these complexes, the photoexcitation results in the promotion of an electron within the ligand itself, without any significant involvement of the metal center. An example is the tetrakis 4-N-methylpyridyl porphyrinatozinc II complex, [Zn TMPyP ], where TMPyP is the porphyrin ligand. Upon photoexcitation, an electron is promoted within the porphyrin -system. Ligand-centered complexes are often used as photosensitizers in photodynamic therapy for cancer treatment.4. Metal-centered complexes: In these complexes, the photoexcitation results in the promotion of an electron within the metal d-orbitals. An example is the hexaaquachromium III complex, [Cr HO ]. Upon photoexcitation, an electron is promoted within the Cr III d-orbitals. Metal-centered complexes are often used as photochromic materials, which change color upon exposure to light.In conclusion, the photochemical behavior of complex transition metal compounds is highly dependent on their structures and ligands. The nature of the ligands, their arrangement around the metal center, and the overall geometry of the complex can significantly influence the absorption of light, the excited states, and the subsequent photochemical reactions. By carefully designing the coordination environment, it is possible to tailor the photochemical properties of these complexes for various applications, such as photocatalysis, solar energy conversion, photoredox reactions, photodynamic therapy, and photochromic materials.