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 S2N2 (π1).
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]22¯(2Mo ) at the BP86/TZ2P/ZORA level of theory according to the
fragmentation described in scheme 3 and 4. Energies are in kcal/mol.