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 .