a The percentage values in parentheses give the contribution to the total attractive interactions ΔEelstat + ΔEorb.b The percentage values in parentheses give the contribution to the total orbital interactions ΔEorb.
The bonding interaction between the metal fragment and S2N2 ligand has 64.1% electrostatic and 35.9% covalent character, which is in good agreement with the QTAIM and NBO analysis (Tables 1 and 3). The Eorb has contributions from three major donor-acceptor interactions, that is, two\(\sigma\)-type interactions and one \(\pi\)-type interaction (Figure 6). The two \(\sigma\) interactions contribute 69.3% of the total orbital interaction energy. The deformation density plot, ρ1 and ρ2 mainly correspond to the donation from \(\sigma\)-MO of S2N2mainly localized on N-atom (lone pair) to the a 1fragment orbital of the metal fragment (Figure 6). A careful visual inspection of ρ1 as well as ρ2 suggests accumulation of electron density on S1 and depletion of electron density from Cl3, which represents a donation from lone pair of electrons from Cl3 (HOMO–1 of [Mo(NO)Cl4], Figure S2) to the S2N2 σ*-MO (LUMO in Figure 2). This can be corroborated with σ-hole interaction between an electrophilic region of a S atom with a Lewis base like Cl-atom in the same molecular entity.[71] Thus, the longer S1‒N2 bond length and the less positive charge on S1 can be recognized from this interaction. On the other hand, the deformation density plot ρ3corresponds to the \(\pi\)-back-donation from the transition metal fragment to the S2N2 \(\pi\)*-molecular orbital. This interaction contributes only 8.2% of the total orbital interaction energy. Hence, S2N2 in1Mo can act as a strong σ-donor and weak π-acceptor ligand, which is similar to N-heterocyclic aromatics like pyridines.[6,9] Furthermore, it can also act as σ-acceptor at the S-atom by the charge transfer of N lone pair to S2N2 σ*-MO. In order to investigate the π-donation property of S2N2, we have carried out the EDA-NOCV analysis on the theoretically modeled electron-deficient 12 valence electron fragment [Mo(NO)Cl4]+ with the S2N2 ligand (Table S2, Figure S4).[73] The EDA analysis indicates a substantial contribution (25%) of the π-donation from S2N2 π-MO (HOMO in Figure 2) to the e-type metal fragment orbital. The σ-donation from S2N2 σ-MO (HOMO-1 in Figure 2) to thea1 -type metal fragment orbital contributes 45.9% to the orbital interaction. Thus, in the presence of suitable metal fragment S2N2 can also act as π-donor ligand (Table S2).[73]