(1Mo)
The bonding interaction of S2N2 with the metal fragment can be better understood from the fragment orbital analysis of [Mo(NO)Cl4]¯. The fragment [Mo(NO)Cl4]¯ is square pyramidal in structure and a valence electron count ascertains that it has 14 valence electrons. The metal centric fragment orbitals available for bonding in the 14 VE transition metal square pyramidal ML5 fragment are shown in scheme 3. The eight d metal electrons can occupy b2 , e andb1 set of orbitals, leaving thea1 orbitals that can accept lone pair from N in S2N2 to form Mo‒N σ-bond. Thus, HOMO-2 of S2N2 majorly concentrated on N has the right symmetry to donate to a1 fragment d-orbital and the perpendicular π-type e fragment d-orbital is likely to donate back to the LUMO+1 (S‒N π*-MO) of the S2N2 to form 1Mo (Figure 2). Thus, the bonding in 1Mo can be qualitatively regarded as similar to Dewar-Chatt-Duncanson model of σ-donation from ligand to metal and π-back donation from metal to ligand.[68,69] However, a detailed bonding analysis will be helpful in examining the relative strength and effect of these interactions in a more quantitative way.
Scheme 3: Schematic representation of the d-orbitals in a square pyramidal ML5 fragment.
The optimized geometry ofS2N2[Mo(NO)Cl4]¯(1Mo ) is shown in figure 1. The four membered S2N2 ring is planar in 1Mo , having unequal S‒N bond lengths. The S1‒N1 and S1‒N2 bonds are longer, while S2‒N1 and S2‒N2 bond length is comparable to S‒N bond length inS2N2 molecule. The S1‒N1 (1.670 Å) and S1‒N2 (1.685 Å) bond elongation can be correlated with the back-donation of electrons from the metal fragment toS2N2 π*-MO. The bond angle at N-atoms increases from 88.7° inS2N2 to 91.9° in 1Moto facilitate metal coordination. Furthermore, the S1···Cl3 distance (2.937 Å) is within the sum of the van der Waals radius of Cl and S-atoms (3.55 Å),[70] indicating a non-covalent type intramolecular interaction between Cl3 and S1. This rather short S‒Cl distance is already noted by Dehnicke and coworkers in many bi-metallic complexes of N2S2.[23]Hence, the σ-hole at the chalcogen atom receives the electron pair from Cl and thus plays a crucial role in the chalcogen bond formation between Cl and S.[71,72] The molecular orbital analysis identifies HOMO-12, HOMO and HOMO-1 as N→Mo σ-donation, Mo→N π-back donation, and S1···Cl3 interaction, respectively (Figure 4).