How many electrons form chemical bonds in the NgBeS (Ng = Ar, Kr, Xe) molecules? Topological study using the electron localisation function (ELF) and electron localisability indicator (ELI-D)

Nature of bonding in the NgBeS (Ng = Ar, Kr, Xe) molecules has been studied using topological analysis of ELF, ELI-D functions with the wave function approximated at the DFT (M062X, B3LYP + ZORA), MP2, CCSD and CASSCF level of calculations. Both Xe–Be and Be=S bonds display topological features typical for the covalent-dative bonding. The V2(Xe) attractor characterising electron density, involved in interaction with the beryllium atom, is closer to the C(Be) core than to C(Xe). The population of the respective basin ranges between 1.59e (B3LYP + ZORA) and 1.83e (CCSD). The beryllium–sulphur bond is described by the bonding disynaptic basin V(Be,S) with the population between 3.22e (CASSCF) and 3.48e (M062X). The approximate weights for the Be–S and Be=S resonance forms are 0.3 and 0.7, respectively, in all molecules. Both the NgBe and BeS bonds are highly polarised with the values of the p SBe and p NgBe polarity indices (CCSD) of 0.8 and 0.9–1.0 for all studied molecules.