Figure 8: (a) Contour maps of the Laplacian distribution of
electron density in the plane of 2Mo 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 of 2Mo. 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.
QTAIM analysis of 2Mo reveals the existence of four BCPs in
between four S‒N bonds and a ring critical point at the center of the
S2N2 ring (Figure 8a). The ρ(r)and \(\nabla^{2}(r)\) values for the BCPs and RCP of
S2N2 ring in 2Mo are very
similar to that in S2N2 ring in1Mo (Table 3). Two BCPs are observed between Mo1‒N1 and Mo2‒N2
bonds, which have similar electronic characteristics as observed for the
Mo‒N1 bond in 1Mo (Table 3). Similar to 1Mo , charge
depletion from N-atom to Mo as well as from Mo to N is also observed in
the contour plot of Laplacian distribution of the electron density of2Mo indicating N→Mo donation and Mo→N back donation. Moreover,
two BCPs are observed between S1···Cl3 and S2···Cl5, where the bond path
passes through the σ-hole of S-atom.
This interaction is similar to the
chalcogen bonding between sulfur and chlorine atom in various other
systems facilitated by σ-hole on S-atom.[71] In
addition, two RCPs were also observed at the center of two Mo‒N‒S‒Cl
rings. Note that, the AIM descriptors at these BCPs are similar to the
BCPs of 1Mo . This indicates similarity in the chemical bonding
of S2N2 with one and two transition
metal fragments.
Scheme 5: Schematic representation of the possible bonding
interaction between metal fragment group orbitals and
S2N2 ligand group orbitals in2Mo 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.
In order to understand the quantitative bonding interaction between two
[Mo(NO)Cl4]¯ fragments
and bridging S2N2 in the frozen geometry
of 2Mo , EDA-NOCV analysis implemented in ADF 2018 program
package were carried out (Table 5). The fragmentation scheme is given in
scheme 5, which indicates two donor-acceptor N→Mo\(\sigma\)-interactions and one donor-acceptor Mo→N \(\pi\)-back
donation. Table 5 shows the calculated EDA results for the interaction
between
[Mo2(NO)2Cl8]2¯ fragment and neutral
η2-S2N2 bridging
ligand. The major contribution to the total interaction energy,
Eint comes from the electrostatic interaction,
Eelstat (65.1%) rather than covalent interaction
ΔEorb (34.9%). The breakdown of ΔEorbinto pairwise orbital interactions shows that the bonding interaction
between the fragments comes mainly from the σ-interaction
(ΔEσ, 72.5%) between the respective singlet fragments.
The deformation density plots correspond to three\(\sigma\)-interactions viz. ρ1 ,ρ2 and ρ3 (Figure 9). Theρ1 and ρ3 correspond to
the donation from anti -bonding and bonding combination of lone
pair orbitals on N atoms in S2N2 ligand
to the anti -bonding and bonding combination of the fragment
orbitals of
[Mo2(NO)2Cl8]2¯ .
The ρ2 corresponds to donation from in-plane Cl
ligand attached to the metal center to the
S2N2 σ *-MO. This can be
correlated with the Cl to S σ -hole
interaction.[71] The deformation density plotρ4 corresponds to the \(\pi\)-back-donation from
the transition metal fragment to the
S2N2 \(\pi\)*-molecular orbital. This
interaction contributes 7.8% of the total orbital interaction energy.
Thus, S2N2 acts mainly as a strong
σ-donor and weak π-acceptor both as a mono- and bi-dentate ligand. Note
that, the percentage contribution of the σ- and π-interaction to the
total Eorb is similar in mono- and bi-metallic
complexes (Table 5). However, the absolute value of these interactions
in 2Mo is roughly double than that in 1Mo . The major
difference in the η2- coordination vs
η1-coordination is observed in the higher dissociation
energy of 2Mo (52.3 kcal/mol) as compared to 1Mo (6.1
kcal/mol), corroborating with the experimental isolation of a higher
number of bi-metallic complexes (Scheme 2).[22-24]This nature of ligand is quite similar to heterocycles such as pyrazine
in organometallic chemistry.[75] However
interestingly, S2N2 can also acts as a
σ-acceptor due to the presence of electro-positive S-atom, which can
lead to additional stability to the transition metal complexes of
S2N2 with appropriate donor ligands at
the metal center. We have also studied the π-donation of
S2N2 ligand to two 12 valence electron
metal fragments
[Mo(NO)Cl4]+ . The
percentage of the π-interaction to the total orbital interaction energy
in[S2N2(Mo(NO)Cl4)2]2+complex is found to be higher (10.1%) than that observed in2Mo (Table S2, Figure S5).[73] Thus, if
exploited wisely, the versatile ligand property of the π-electron rich
SN inorganic aromatic compounds might lead to hitherto unprecedented
reactivities of transition metal complexes.