Conclusion
Electronic structure and ligand property of
S2N2 compound towards the 14 valence
electron metal fragments have been studied using quantum mechanical
calculations. Our detailed theoretical calculation on
S2N2 shows an area of charge
concentration close to the direction of lone pair of N atoms of theS2N2 and an area of charge
depletion in the extended direction of the S‒N bonds of theS2N2 ring. In order to explore
the ligand property of S2N2 , we
have studied the complex formation of
S2N2 with one and two square pyramidal
14 valence electron
[Mo(NO)Cl4]¯metal fragment(s). The
molecular orbital and QTAIM analysis suggest that N→Mo σ-donation, Mo→N
π-back donation, and Cl···S interaction through σ-hole at S-atom are
present in both 1Mo and 2Mo . EDA-NOCV analysis has
been carried out to understand the quantitative nature of bonding
between [Mo(NO)Cl4]¯ metal
fragments and S2N2 ligand. Our results
indicate that the interaction between
S2N2 and metal fragments has a higher
electrostatic character than a covalent character. The nature of
S2N2 as a ligand is similar in the
bi-metallic complex
S2N2[Mo2(NO)2(Cl)8]2¯ as well. On the contrary, the σ-lone pair on N-atom in
S2N2 is donated to the
electron-deficient 12 valence electron
[Mo(NO)Cl4]+ fragment in
S2N2[Mo(NO)Cl4]+and the electrons from the S2N2 π-MO are
donated to the vacant d-orbitals of the metal fragment. Similar bonding
nature is also observed in the bi-metallic
S2N2[Mo2(NO)2(Cl)8]2+ complex. In addition, all these complexes show donation of lone pair on
Cl attached to transition metal fragment to the S‒N σ*-MO, which is
majorly located on the S-atom.
Here, the S-atom in
S2N2 can be considered as a σ-hole,
which is involved in the chalcogen bond with Cl-atom. Hence,
our theoretical results suggest
that the S2N2 is a versatile ligand
which can be tuned as σ-donor, σ- acceptor, π-donor and π-acceptor.
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. We also
analyzed the strength of the π bonding and aromaticity of
S2N2 ring has been investigated using
the EDA–NOCV analysis. EDA results indicate that
S2N2 ligand in
S2N2[Mo2(NO)2(Cl)8]2¯ is more aromatic than S2N2 molecule.