The shape and size of metal complexes play a crucial role in determining their photochemical properties, which include absorption and emission spectra, excited-state lifetimes, and photochemical reactivity. The coordination geometry, ligand type, and the overall size of the complex can significantly influence these properties. Here are some specific examples and experimental evidence to support this claim:1. Coordination geometry: The geometry of a metal complex can affect the energy levels of its orbitals and, consequently, its electronic transitions. For example, octahedral and tetrahedral complexes of transition metals exhibit different absorption spectra due to the different energy gaps between their d orbitals. In octahedral complexes, the d orbitals split into two sets t2g and eg , while in tetrahedral complexes, they split into two different sets e and t2 . This difference in energy levels results in distinct absorption spectra for the two geometries.Experimental evidence: A classic example is the comparison between the absorption spectra of [Co NH3 6]3+ octahedral and [CoCl4]2- tetrahedral . The former exhibits a strong absorption band at around 500 nm, while the latter has a band at around 700 nm, demonstrating the influence of coordination geometry on the absorption properties of metal complexes.2. Ligand type: The nature of the ligands coordinated to the metal center can also affect the photochemical properties of metal complexes. Strong-field ligands, such as CO and CN-, can increase the energy gap between the metal's d orbitals, leading to a blue shift in the absorption spectrum. In contrast, weak-field ligands, such as Cl- and H2O, result in a smaller energy gap and a red shift in the absorption spectrum.Experimental evidence: The absorption spectra of [Fe CN 6]3- and [Fe H2O 6]3+ provide a clear example of this effect. The former, with strong-field CN- ligands, has an absorption band at around 420 nm, while the latter, with weak-field H2O ligands, has a band at around 800 nm.3. Size of the complex: The overall size of a metal complex can influence its photochemical properties by affecting the spatial distribution of its orbitals and the extent of orbital overlap between the metal center and the ligands. Larger complexes may exhibit more extensive delocalization of their orbitals, leading to lower energy gaps and red-shifted absorption spectra.Experimental evidence: A study by Zhang et al. Inorg. Chem., 2011, 50, 12446-12453 investigated the effect of size on the photochemical properties of a series of ruthenium polypyridyl complexes. They found that as the size of the complex increased, the absorption and emission maxima shifted to lower energies longer wavelengths , indicating a relationship between the size of the complex and its photochemical properties.In conclusion, the shape, size, and ligand type of metal complexes can significantly impact their photochemical properties. Understanding these relationships is essential for the design and optimization of metal complexes for various applications, such as photocatalysis, solar energy conversion, and luminescent materials.