Figure 4: Important molecular orbitals of theS2N2[Mo(NO)Cl4]¯(1Mo) . Energy eigen values are in eV. The isosurface values for molecular orbitals are 0.01 e/bohr3.
Group charge analysis in 1Mo (Table 1) by the natural population indicates a slight positive group charge of S2N2 (0.10 e), suggesting net electron transfer from S2N2 ligand to the metal fragment. The Wiberg bond indices reveal significant reduction for S1‒N1 and S1‒N2 bond order (1.10) as compared to that of S‒N bond order in S2N2 molecule (1.22), while the S2‒N1 and S2‒N2 bond order remains almost the same as that in S2N2. This supports the variable S‒N bond distances in 1Mo (Figure 1). The atomic orbital population in the pz -orbital of N1 is slightly higher (1.57) as compared to the population in the pz -orbitals of N2, S1 and S2. This increase in the population could be correlated to the back donation from metal fragment to S‒N π*-MO. Thus, the bond order and population analysis suggest metal fragment to S‒N π*-back donation, which in turn gives rise to polarization in the S2N2 π-electron density. In addition, the second-order perturbation analysis by NBO on 1Mo complex shows donation of chloride (Cl3) lone pair to S1‒N2 \(\sigma\)*-MO (5.2 kcal/mol), which is well complemented with the shorter Cl3···S1 distance. Here, the S-atom acts as acceptor in the Cl‒S chalcogen bond.[71,72] The uneven distribution of π-electrons in the pz-orbitals of N- and S-atoms is further supported by positive NICS(1)zz value (Table 2), which indicates non-aromatic character of S2N2 ligand in1Mo . The ESP plot on the molecular van der Waals surface of1Mo shows negative potential at the metal fragment and relatively slight positive potential on S2N2, supporting the positive group charge of S2N2 (Figure 5b).
In order to understand the π-bonding strength of S2N2 ring in the1Mo EDA-NOCV analysis were carried out using quartet SN and SN[Mo(NO)(Cl)4]¯fragments (Scheme S1). The bonding interaction between the SN[Mo(NO)(Cl)4]¯ and SN fragments has 36.1% electrostatic and 63.9% covalent character. The Eorb has contributions from 85.8% \(\sigma\)-type interactions and 11.1% \(\pi\)-type interactions. The value of ΔEπ suggests that π bonding strength of S2N2 ring in 1Mo is higher than the S2N2 molecule. The inspection of the deformation density plot (Δρ2 in Figure S7) indicates that the increase in ΔEπ in 1Mo as compared toS2N2 might arise due to the donation of π-electron density from metal fragment to the S-N π*-MO, which has more coefficient on N1. Thus the donation of electron from the metal fragment to the S-N π*-MO would not lead to uniform π-electron distribution in the S2N2 ring and can not be taken as an increase in the aromaticity of 1Mo . This is also supported by higher population of the pz-orbital on N1 (Table 1) and lower aromaticity (Table 2) of 1Mo .