S2N2
The singlet S2N2 is more stable than the triplet state by 58.8 kcal/mol (Figure S1). Hence, we have limited our discussion on singletS2N2 .S2N2 is a near-perfect square with an equal S‒N bond length of 1.665 Å and near right-angular bond angles of ⦟SNS = 88.8° and ⦟NSN = 91.2°. Therefore,S2N2 shows the characteristic geometric criteria for traditionally aromatic molecules.[57] The natural population analysis ofS2N2 is given in table 1. The partial atomic charge on more electronegative N-atoms (\(\chi_{N}\) = 3.04)[58] and less electronegative S-atoms (\(\chi_{S}\) = 2.58)[58] inS2N2 are -0.85 e and 0.85 e. In spite of the difference in the electronegativity of N- and S-atoms, the population in the perpendicular pz -orbital on N-atoms (1.45 e) and S-atoms (1.50 e) are comparable. This indicates a nearly uniform π-electron distribution in electron rich 6πS2N2 , similar to cyclic S3N3systems.[18,59] The HOMO-2 (σ7) and HOMO (π3) of S2N2are σ- and π-molecular orbitals (MO) having major coefficient on N-atoms (Figure 2). The LUMO (σ8) and LUMO+1 (π4) are σ- and π-MOs having major coefficient on S-atoms. Thus, in principle, N-atom can act as σ- and π-donor while S-atom can act as σ- and π-acceptor. A S‒N bond order of 1.22 inS2N2 indicates a reduction in the S‒N double bond character and corroborates well with the description of electron rich π-systems (Table 1).[18,59]
The Nucleus-Independent Chemical Shift (NICS) value at the ring center, NICS(0), and 1Å above, NICS(1), and the axial components of the shielding tensor NICS(0)zz and NICS(1)zzindicates weakly aromatic character (Table 2). However, the low negative values of NICS(1)zz indicate a relatively poor delocalization of π-electrons as compared to standard 6π aromatic systems like benzene and its analogues.[38] Our NICS values show a parallel with the reported NICS values ofS2N2 at a higher level of theory.[60]Note that, Head-Gordon, Schleyer, and co-workers assigned S2N2 as essentially a weakly aromatic 2π electrons system based on structural, energetic and magnetic criteria.[61]
Table 1 : Atomic partial charge distribution, atomic orbital population and bond order given by the natural population analysis forS2N2 , S2N2[Mo(NO)Cl4]¯(1Mo ) and S2N2[Mo(NO)Cl4]2(2Mo ) at the M06/def2-TZVPP level of theory by Gaussian G09 program package.