On the nature of the boron-sulfur multiple bonds from the quantum chemical topology perspective – The theoretical investigation using electron localisation function (ELF)

The electron localization function (ELF) is a powerful quantum chemical tool for analysing bonding in molecules. This study applies ELF topological analysis to boron-sulfur (BS) bonds, distinguishing between single, double, and formally triple bonds. The results reveal that BS bonds exhibit strong polarity due to sulfur’s high electron-withdrawing capability. ELF analysis demonstrates that the concept of a triple BS bond is not supported, as the electron populations in bonding basins do not reach expected values for true triple bonds. Instead, these interactions are better described as resonance hybrids with covalent and dative contributions.

This study investigates 16 molecules containing BS bonds, selected to cover a range of bonding scenarios. The analysis of V(B,S) basin populations and BS bond lengths confirms distinct bonding characteristics. Formally single BS bonds exhibit the longest bond lengths (up to 1.922 Å) and the lowest basin populations (∼1.69e). Formally double bonds present intermediate bond lengths (1.799–1.850 Å) and basin populations around 1.97–1.98e. Molecules proposed to contain triple BS bonds display significantly lower basin populations (3.14–3.23e) than expected for true triple bonds, challenging their classification.

These findings refine our understanding of multiple bonding in boron chemistry and provide insights into electronic structure and reactivity. ELF analysis proves essential for characterizing bonding interactions beyond traditional valence models. The results have implications for designing BS-containing materials, particularly in catalysis and advanced applications. Further studies should examine external factors such as pressure, temperature, and substituent effects on BS bonding.