Conclusion
Electronic structure and ligand property of S2N2 compound towards the 14 valence electron metal fragments have been studied using quantum mechanical calculations. Our detailed theoretical calculation on S2N2 shows an area of charge concentration close to the direction of lone pair of N atoms of theS2N2 and an area of charge depletion in the extended direction of the S‒N bonds of theS2N2 ring. In order to explore the ligand property of S2N2 , we have studied the complex formation of S2N2 with one and two square pyramidal 14 valence electron [Mo(NO)Cl4]¯metal fragment(s). The molecular orbital and QTAIM analysis suggest that N→Mo σ-donation, Mo→N π-back donation, and Cl···S interaction through σ-hole at S-atom are present in both 1Mo and 2Mo . EDA-NOCV analysis has been carried out to understand the quantitative nature of bonding between [Mo(NO)Cl4]¯ metal fragments and S2N2 ligand. Our results indicate that the interaction between S2N2 and metal fragments has a higher electrostatic character than a covalent character. The nature of S2N2 as a ligand is similar in the bi-metallic complex S2N2[Mo2(NO)2(Cl)8]2¯ as well. On the contrary, the σ-lone pair on N-atom in S2N2 is donated to the electron-deficient 12 valence electron [Mo(NO)Cl4]+ fragment in S2N2[Mo(NO)Cl4]+and the electrons from the S2N2 π-MO are donated to the vacant d-orbitals of the metal fragment. Similar bonding nature is also observed in the bi-metallic S2N2[Mo2(NO)2(Cl)8]2+ complex. In addition, all these complexes show donation of lone pair on Cl attached to transition metal fragment to the S‒N σ*-MO, which is majorly located on the S-atom. Here, the S-atom in S2N2 can be considered as a σ-hole, which is involved in the chalcogen bond with Cl-atom. Hence, our theoretical results suggest that the S2N2 is a versatile ligand which can be tuned as σ-donor, σ- acceptor, π-donor and π-acceptor. Thus, if exploited wisely, the versatile ligand property of the π-electron rich SN inorganic aromatic compounds might lead to hitherto unprecedented reactivities of transition metal complexes. We also analyzed the strength of the π bonding and aromaticity of S2N2 ring has been investigated using the EDA–NOCV analysis. EDA results indicate that S2N2 ligand in S2N2[Mo2(NO)2(Cl)8]2¯ is more aromatic than S2N2 molecule.