On the basis of density functional theoretical approach, we have assessed the ground state geometries and absorption spectra of recently synthesized monometallic ruthenium (II) complex of composition [(bpy)2Ru(H3Imbzim)](ClO4)2·2H2O where bpy = 2,2′-bypyridine and H3Imbzim = 4,5-bis(benzimidazol-2-yl)imidazole. The all different kinds of charge transfers such as ligand-ligand, and metal-ligand have been quantified, compared, and contrasted with the experimental results. In addition, the effect of solvent on excitation energies has been evaluated. In spite of some digital discrepancies in calculated and observed geometries, as well as in absorption spectra, the density functional theory (DFT) seems to explain the main features of this complex. 1. Introduction In recent times, the importance of inorganic complexes has been reported extensively keeping in mind the wide range of applicability these complexes possess in different domains of life. Thanks to the synthetic chemists for synthesizing, characterizing, and demonstrating the wide range of applicability of these complexes. Among the entire applicability domain, the recognition and sensing of anions is one of the recently emerged and challenging areas in the field of research. This is due to the important role played by anions in the field of biological, industrial, agricultural, and environmental processes [1–8]. This importance can be visualized from the facts that majority of enzymes bind anions as either substrate or cofactor and many act as ubiquitous nucleophiles, bases, redox agents, and phase transfer catalysts [9, 10]. Diseases like cystic fibrosis and Alzheimer’s are induced by the malfunction of natural anion regulation processes [11, 12]. Even, from environmental point of view, the eutrophication of water is an important issue which is caused by phosphate and nitrate ions and used in agriculture fertilizers [13–16]. The advantage of the transition metal complex of composition [(bpy)2Ru(H3Imbzim)]2+ where bpy = 2,2′-bypyridine and H3Imbzim = 4,5-bis(benzimidazol-2-yl)imidazole to act as a sensor is that it contains three potent NH bonds that can be donated for the hydrogen bonding to the anions. Given this importance, it is not surprising that extensive experimental studies have been dedicated to design of simple artificial anion receptors and sensors [17–20]. However, there is not very clear understanding about the guiding principles about the selection of right kind of moiety as a chromophore and ligand as an anion sensor. In such situation, quantum chemical based electronic structure
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