The photochemical behavior of transition metal complexes is significantly influenced by their coordination geometry. Coordination geometry refers to the arrangement of ligands around the central metal ion in a complex. Different coordination geometries can lead to different electronic structures, which in turn affect the photochemical properties of the complexes. Some common coordination geometries include octahedral, tetrahedral, square planar, and square pyramidal.1. Octahedral geometry: In this geometry, six ligands are arranged around the central metal ion, forming an octahedron. Octahedral complexes often exhibit d-d transitions, which are typically spin-forbidden and have low molar absorptivities. However, some octahedral complexes can undergo metal-to-ligand charge transfer MLCT or ligand-to-metal charge transfer LMCT transitions, which are spin-allowed and have higher molar absorptivities. An example of an octahedral complex with interesting photochemical properties is [Ru bpy ] bpy = 2,2'-bipyridine , which is a well-known photosensitizer used in photoredox catalysis.2. Tetrahedral geometry: In this geometry, four ligands are arranged around the central metal ion, forming a tetrahedron. Tetrahedral complexes usually have higher energy d-d transitions compared to octahedral complexes due to the lower ligand field splitting. This can lead to more intense absorption bands in the visible region, making them potentially useful as chromophores. An example of a tetrahedral complex is [CuCl], which exhibits a strong absorption band in the visible region due to a d-d transition.3. Square planar geometry: In this geometry, four ligands are arranged around the central metal ion in a square plane. Square planar complexes are often found with d metal ions, such as Pt II and Pd II . These complexes can exhibit MLCT or LMCT transitions, which can lead to interesting photochemical properties. For example, cisplatin cis-[Pt NH Cl] is a square planar complex used as an anticancer drug. Upon photoactivation, it can undergo ligand exchange reactions, which are crucial for its biological activity.4. Square pyramidal geometry: In this geometry, five ligands are arranged around the central metal ion, with four in a square plane and one axial ligand. Square pyramidal complexes can exhibit a combination of d-d, MLCT, and LMCT transitions, depending on the nature of the ligands and the metal ion. An example of a square pyramidal complex is [VO acac ] acac = acetylacetonate , which exhibits a strong absorption band in the visible region due to a d-d transition.In summary, the photochemical behavior of transition metal complexes is strongly influenced by their coordination geometry. Different geometries can lead to different electronic structures and transitions, which in turn affect the photochemical properties of the complexes. Understanding these relationships is crucial for the design of new photoactive materials and catalysts.