Substantial Energies of Stabilization for Hg···S Spodium Bonding Interactions Beyond Standard van der Waals Separations

The nature of long Hg···S spodium bonding, i.e., separations beyond standard van der Waals radii, is investigated for a series crystalline, quasi-dimeric aggregates of [Hg(S2CNRR’)2]2 is investigated. Through density functional theory (DFT) calculations, molecular electrostatic potential (MEP) surfaces, quantum theory of atoms in molecules (QTAIM), noncovalent interaction plots (NCIplot), and natural bond orbital (NBO) analysis, the strength and nature of the long Hg···S interactions are analyzed. This study differentiates two distinct quasi-dimer motifs, namely Type I and Type II, based on the orientation of the Hg···S contacts within the quasi-dimers. The Type I dimers exhibit stronger Hg···S interactions (∼6 kcal/mol per contact), while Type II motifs are characterized by weaker interactions (∼3 kcal/mol per contact), in agreement with their longer Hg···S distances. NBO analysis reveals that these contacts involve LP(S) → σ*(Hg–S) charge-transfer contributions, with significant stabilization energies supporting their role in dimer formation. The findings contribute to a deeper understanding of secondary interactions in mercury-based supramolecular assemblies and highlight the subtle interplay of electronic and steric factors in crystal engineering.