Figure 8: (a) Contour maps of the Laplacian distribution of electron density in the plane of 2Mo molecule. Dashed lines indicate regions of electronic charge concentration (\(\nabla^{2}(r)\)< 0), and solid lines denote regions of electronic charge depletion (\(\nabla^{2}(r)\) > 0). Small blue spheres represent bond critical points (BCPs) and small orange sphere represent ring critical point (RCP). Bond paths and interatomic surface paths are indicated by brown and blue lines. (b) Molecular electrostatic potential mapped on the molecular surface of 2Mo. Blue indicates N-atom and yellow indicates S-atom. Red color represents accumulation of positive charge and blue color indicates accumulation of negative charge. Surface local minima (Vmin) and maxima (Vmax) of ESP in kcal/mol are represented as cyan and orange spheres, respectively.
QTAIM analysis of 2Mo reveals the existence of four BCPs in between four S‒N bonds and a ring critical point at the center of the S2N2 ring (Figure 8a). The ρ(r)and \(\nabla^{2}(r)\) values for the BCPs and RCP of S2N2 ring in 2Mo are very similar to that in S2N2 ring in1Mo (Table 3). Two BCPs are observed between Mo1‒N1 and Mo2‒N2 bonds, which have similar electronic characteristics as observed for the Mo‒N1 bond in 1Mo (Table 3). Similar to 1Mo , charge depletion from N-atom to Mo as well as from Mo to N is also observed in the contour plot of Laplacian distribution of the electron density of2Mo indicating N→Mo donation and Mo→N back donation. Moreover, two BCPs are observed between S1···Cl3 and S2···Cl5, where the bond path passes through the σ-hole of S-atom. This interaction is similar to the chalcogen bonding between sulfur and chlorine atom in various other systems facilitated by σ-hole on S-atom.[71] In addition, two RCPs were also observed at the center of two Mo‒N‒S‒Cl rings. Note that, the AIM descriptors at these BCPs are similar to the BCPs of 1Mo . This indicates similarity in the chemical bonding of S2N2 with one and two transition metal fragments.
Scheme 5: Schematic representation of the possible bonding interaction between metal fragment group orbitals and S2N2 ligand group orbitals in2Mo chosen for EDA-NOCV analysis. Up and down arrows indicate electrons with opposite spin and the single headed arrow (\(\rightarrow\)) indicates donor acceptor interactions between fragments.
In order to understand the quantitative bonding interaction between two [Mo(NO)Cl4]¯ fragments and bridging S2N2 in the frozen geometry of 2Mo , EDA-NOCV analysis implemented in ADF 2018 program package were carried out (Table 5). The fragmentation scheme is given in scheme 5, which indicates two donor-acceptor N→Mo\(\sigma\)-interactions and one donor-acceptor Mo→N \(\pi\)-back donation. Table 5 shows the calculated EDA results for the interaction between [Mo2(NO)2Cl8]2¯ fragment and neutral η2-S2N2 bridging ligand. The major contribution to the total interaction energy, Eint comes from the electrostatic interaction, Eelstat (65.1%) rather than covalent interaction ΔEorb (34.9%). The breakdown of ΔEorbinto pairwise orbital interactions shows that the bonding interaction between the fragments comes mainly from the σ-interaction (ΔEσ, 72.5%) between the respective singlet fragments. The deformation density plots correspond to three\(\sigma\)-interactions viz. ρ1 ,ρ2 and ρ3 (Figure 9). Theρ1 and ρ3 correspond to the donation from anti -bonding and bonding combination of lone pair orbitals on N atoms in S2N2 ligand to the anti -bonding and bonding combination of the fragment orbitals of [Mo2(NO)2Cl8]2¯ . The ρ2 corresponds to donation from in-plane Cl ligand attached to the metal center to the S2N2 σ *-MO. This can be correlated with the Cl to S σ -hole interaction.[71] The deformation density plotρ4 corresponds to the \(\pi\)-back-donation from the transition metal fragment to the S2N2 \(\pi\)*-molecular orbital. This interaction contributes 7.8% of the total orbital interaction energy. Thus, S2N2 acts mainly as a strong σ-donor and weak π-acceptor both as a mono- and bi-dentate ligand. Note that, the percentage contribution of the σ- and π-interaction to the total Eorb is similar in mono- and bi-metallic complexes (Table 5). However, the absolute value of these interactions in 2Mo is roughly double than that in 1Mo . The major difference in the η2- coordination vs η1-coordination is observed in the higher dissociation energy of 2Mo (52.3 kcal/mol) as compared to 1Mo (6.1 kcal/mol), corroborating with the experimental isolation of a higher number of bi-metallic complexes (Scheme 2).[22-24]This nature of ligand is quite similar to heterocycles such as pyrazine in organometallic chemistry.[75] However interestingly, S2N2 can also acts as a σ-acceptor due to the presence of electro-positive S-atom, which can lead to additional stability to the transition metal complexes of S2N2 with appropriate donor ligands at the metal center. We have also studied the π-donation of S2N2 ligand to two 12 valence electron metal fragments [Mo(NO)Cl4]+ . The percentage of the π-interaction to the total orbital interaction energy in[S2N2(Mo(NO)Cl4)2]2+complex is found to be higher (10.1%) than that observed in2Mo (Table S2, Figure S5).[73] 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.