The photochemical behavior of coordination compounds is primarily determined by their geometry, electronic structure, and the nature of the ligands. Octahedral and tetrahedral complexes exhibit different photochemical properties due to their distinct geometries and electronic configurations.1. Electronic transitions:In octahedral complexes, the d-orbitals split into two sets: the lower energy t2g set dxy, dxz, dyz and the higher energy eg set dx2-y2, dz2 . The energy difference between these two sets is called the crystal field splitting energy o . Upon absorption of light, electrons can be promoted from the t2g to the eg orbitals, leading to a variety of electronic transitions, such as d-d, charge transfer, and ligand-to-metal charge transfer LMCT transitions.In contrast, tetrahedral complexes have a different splitting pattern. The d-orbitals split into a lower energy e set dxy, dxz, dyz and a higher energy t2 set dx2-y2, dz2 . The energy difference between these two sets is called the crystal field splitting energy t . The splitting energy in tetrahedral complexes is generally smaller than that in octahedral complexes, leading to different electronic transitions and absorption spectra.2. Spin-forbidden transitions:In octahedral complexes, the Laporte selection rule states that d-d transitions are forbidden for centrosymmetric complexes. However, due to the presence of spin-orbit coupling and vibronic coupling, weak d-d transitions can still occur, leading to relatively low-intensity absorption bands.In tetrahedral complexes, the Laporte selection rule does not apply, and d-d transitions are allowed. As a result, tetrahedral complexes typically exhibit more intense absorption bands compared to octahedral complexes.3. Photochemical reactions:Octahedral and tetrahedral complexes can undergo different photochemical reactions upon light absorption. For example, octahedral complexes can undergo photoinduced ligand substitution reactions, where a ligand is replaced by another ligand or solvent molecule. This process is facilitated by the formation of a coordinatively unsaturated intermediate species.Tetrahedral complexes, on the other hand, are less likely to undergo ligand substitution reactions due to their smaller coordination number and higher steric constraints. Instead, they may undergo photochemical reactions involving changes in their electronic structure, such as electron transfer or bond cleavage processes.Experimental evidence:A classic example of the difference in photochemical behavior between octahedral and tetrahedral complexes is the study of metal carbonyl complexes. Octahedral carbonyl complexes, such as Cr CO 6, exhibit relatively weak absorption bands in the visible region due to the Laporte-forbidden d-d transitions. Upon irradiation with UV light, these complexes can undergo photoinduced CO substitution reactions.In contrast, tetrahedral carbonyl complexes, such as Ni CO 4, exhibit more intense absorption bands in the visible region due to the allowed d-d transitions. However, these complexes do not undergo photoinduced CO substitution reactions, as the tetrahedral geometry imposes steric constraints that prevent the formation of a coordinatively unsaturated intermediate species.In conclusion, the photochemical behavior of octahedral and tetrahedral coordination compounds differs significantly due to their distinct geometries, electronic structures, and ligand environments. These differences manifest in their electronic transitions, absorption spectra, and the types of photochemical reactions they can undergo.