The coordination number of the central metal ion in metalloporphyrins and metallophthalocyanines significantly affects their electronic structure and spectroscopic properties. Metalloporphyrins and metallophthalocyanines are macrocyclic complexes with a central metal ion coordinated to nitrogen atoms from the porphyrin or phthalocyanine ligand. The electronic structure and spectroscopic properties of these complexes are influenced by the nature of the central metal ion, its oxidation state, and its coordination number.1. Electronic structure:The coordination number determines the geometry of the complex, which in turn affects the electronic structure. For example, a metalloporphyrin with a coordination number of 4 has a square planar geometry, while a coordination number of 6 results in an octahedral geometry. This difference in geometry affects the splitting of d-orbitals and the energy levels of the complex, leading to different electronic structures.In metalloporphyrins, when the central metal ion has a coordination number of 4, the d-orbitals split into two sets: dxz and dyz non-bonding and dxy, dx2-y2, and dz2 bonding . In contrast, when the coordination number is 6, the d-orbitals split into three sets: t2g non-bonding and eg bonding . This difference in electronic structure affects the stability, reactivity, and redox properties of the complexes.2. Spectroscopic properties:The coordination number also affects the spectroscopic properties of metalloporphyrins and metallophthalocyanines, such as their UV-Vis and magnetic circular dichroism MCD spectra. The differences in electronic structure due to coordination number lead to different absorption bands and intensities in the UV-Vis spectra.For example, in the case of metalloporphyrins, the Soret band a strong absorption band in the UV-Vis spectrum is sensitive to the coordination number of the central metal ion. When the coordination number increases, the energy of the Soret band decreases, resulting in a redshift in the spectrum. This phenomenon has been observed in various metalloporphyrins, such as iron III porphyrins, where the Soret band shifts from 400 nm for a coordination number of 4 to 450 nm for a coordination number of 6 .Experimental evidence:A study by Ghosh et al. Inorg. Chem., 1994, 33, 2077-2083 investigated the effect of coordination number on the electronic structure and spectroscopic properties of iron III porphyrins. They synthesized a series of iron III porphyrin complexes with different axial ligands, resulting in different coordination numbers. They found that the coordination number significantly affected the electronic structure, as evidenced by the changes in the MCD spectra and redox potentials of the complexes.Another study by Kobayashi et al. J. Am. Chem. Soc., 2000, 122, 10716-10717 examined the effect of coordination number on the electronic structure and spectroscopic properties of metallophthalocyanines. They synthesized a series of metallophthalocyanines with different central metal ions and coordination numbers and found that the coordination number had a significant impact on the UV-Vis and MCD spectra of the complexes.In conclusion, the coordination number of the central metal ion in metalloporphyrins and metallophthalocyanines plays a crucial role in determining their electronic structure and spectroscopic properties. Changes in coordination number lead to differences in geometry, electronic structure, and absorption spectra, which can be experimentally observed and correlated to the coordination number.