The coordination number of a complex refers to the number of ligands directly bonded to the central metal ion. In the complex [Eu en NO ], europium Eu is the central metal ion, ethylenediamine en is a bidentate ligand, and nitrate NO is a monodentate ligand.Since ethylenediamine is a bidentate ligand, it can form two coordinate bonds with the central metal ion. There are two en ligands in the complex, so they contribute a total of 2 2 = 4 coordinate bonds. Additionally, there are three nitrate ions, each forming one coordinate bond with the europium ion, contributing another 3 coordinate bonds. Therefore, the coordination number of the complex is 4 + 3 = 7.The coordination geometry of a complex with a coordination number of 7 is typically a capped octahedron or a capped trigonal prism. In the case of [Eu en NO ], the coordination geometry is more likely to be a capped octahedron due to the presence of bidentate ligands en and the size of the europium ion.Regarding the spectroscopic properties of europium in the complex, europium belongs to the lanthanide series, which is known for its unique electronic transitions and spectroscopic properties. Europium III has a 4f electronic configuration, and its absorption and emission spectra are dominated by f-f transitions within the 4f shell. These transitions are parity forbidden, which means they have low molar absorptivity and low emission intensity. However, the presence of ligands like ethylenediamine and nitrate can enhance the absorption and emission properties of europium by inducing a ligand-to-metal charge transfer LMCT or an antenna effect. This can lead to an increase in the intensity of the f-f transitions and make the complex suitable for applications in luminescence and fluorescence spectroscopy.