Figure 7 : Important molecular orbitals of theS2N2[Mo(NO)Cl4]2(2Mo) . Energy eigenvalues are in eV. The isosurface values for molecular orbitals are 0.01 e/bohr3.
NBO charge and population analysis (Table 1) were performed to gain a more detailed insight into the electronic structure of 2Mo as well as on the nature of S2N2 as a bridging ligand. The positive group charge on S2N2 ring (0.24 e) indicates a higher extent of the net donation from S2N2bridging ligand to the two metal fragments as compared to 1Mo . The Wiberg bond indices of all the S‒N bonds reduced significantly as compared to S2N2, which manifests donation from filled d-orbitals of metal fragments to the anti-bonding π*-molecular orbitals of S2N2 ligand. It is interesting to note that, the overall atomic population in the perpendicular pz -orbitals on S and N is similar to that observed in S2N2. However the atomic population in the perpendicular pz -orbitals on N atoms (1.54 e) has increased and that in the perpendicular pz -orbitals on the S-atoms has decreased (1.43 e). This clearly denotes a significant\(\pi\)-back donation from the metal fragments to the \(\pi\)*-MO of S2N2 ring (LUMO, π4 in Figure 2). Thus, even though the total population in the π-orbitals of S2N2 in 2Mo remains similar to free S2N2, we can observe a reversal of the direction of polarization of the π-electron density in 2Mo . The second order perturbation theory analysis by NBO on 2Moshows a significant donation from lone pair of chloride ligands (Cl3/Cl7) to S1‒N2/S2‒N1 \(\sigma\)*-orbitals (3.7 kcal/mol), which is well complemented with the Cl3···S1/Cl5···S2 distances in the geometrical analysis. This interaction is similar to the chalcogen bonding described by Cremer and co-workers.[71]
The higher negative NICS(1)zz value observed for the S2N2 ring in 2Mo corroborates with the increase in the π-electron density due to back donation from themetal fragment (Table 2). The more negative ESP on the metal fragments as compared to the S2N2 ring, however, indicates a net transfer of electrons to the metal fragments validating the positive charge on S2N2(Figure 8b). In order to understand the π bonding strength of S2N2 in 2Mo , the elongated S–N bonds (S1–N2 and S2–N1) are cleaved leading to two quartet SN[Mo(NO)(Cl)4]¯fragments (Scheme S1). The major contributions to the total orbital interaction energy (ΔEorb), between the fragments is higher than for the S2N2molecule implying greater covalent character (Table 3). Also, the π-contribution to the orbital interactions are –81.0 kcal/mol in2Mo and -73.2 kcal/mol inS2N2 molecule. Thus, the π bonding strength of S2N2 in the2Mo in more than the π bonding strength of S2N2 molecule. The inspection of the deformation density plot (Δρ2 in Figure S8) exhibits depletion of electron density from the two metal fragments to the S2N2 ring. Thus, the double donation makes an uniform increase of the π-electron density in the S2N2 ring in 2Mo as compared to S2N2 ring in 1Mo . Therefore, an uniform increase in the population of pz-orbitals (Table 1) and higher aromatic character (Table 2) of 2Mo is validated.