Figure 5 : (a) Contour maps of the Laplacian distribution of electron density in the plane of S2N2 in 1Mo 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 of1Mo . 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.
Four BCPs and one RCP are observed in the coordinated S2N2 ring in 1Mo . Theρ(r) and \(\nabla^{2}(\rho)\) at the BCP between S and N atoms and at the RCP in S2N2 ring remains close to that observed in S2N2 (Table 3). The electron density at the BCP (ρ(r) ) between Mo and N1 is significantly higher (0.1058 a. u.) and the Laplacian of electron density (\(\nabla^{2}(\rho)\)) is positive (0.1683 a. u.), which indicates a strong electrostatic interaction between Mo and N1. However, negative H(r) and –G(r)/V(r) values indicate important contribution from covalent interaction as well. The inspection of the contour plot indicates depletion of charge density from N-atom as well as from Mo along the Mo-N1 bond, thus indicating donation and back donation interaction. QTAIM analysis also shows the existence of a BCP between Cl3 and S1 in 1Mo complex (ρ(r)and \(\nabla^{2}(\rho)\) are 0.0222 a. u. and 0.0603 a. u. respectively), where the bond path passes through the possible σ-hole near the S-atom along the extension of S‒N bond. In addition, an RCP at the center of Mo‒N1‒S1‒Cl3 is also observed. Small positive H(r)and higher –G(r)/V(r) at the Cl3···S1 BCP identifies Cl‒S interaction as majorly non-covalent. The role of σ-hole in stabilizing such interaction are also reported.
Scheme 4: Schematic representation of the different possible bonding interactions between metal fragment group orbitals and S2N2 ligand group orbitals in1Mo 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.
EDA-NOCV analysis using ADF 2018 program package has been carried out to understand the quantitative nature of bonding between the transition metal fragment and S2N2 (Table 5). The bonding possibility in scheme 4 represents the interaction between neutral S2N2 ligand and 14 electron metal fragment, [Mo(NO)Cl4]¯ in1Mo complex. The bonding possibility in scheme 4 represents two donor-acceptor interactions viz. from N lone pair to transition metal fragment (\(\sigma\)1) and from metal fragment to the π*-MO of S2N21).
Table 5: EDA results of the possible bonding representation for the Mo‒N bond in S2N2[Mo(NO)Cl4]¯(1Mo ) and S2N2[Mo(NO)Cl4]2(2Mo ) at the BP86/TZ2P/ZORA level of theory according to the fragmentation described in scheme 3 and 4. Energies are in kcal/mol.