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]