On the stability of helium compounds: insights from charge displacement analysis

Helium, the most abundant element in the universe besides hydrogen, is unique in the noble gas (Ng) series, since no neutral compound involving it has been experimentally detected, except a very recently reported Na2He adduct, synthesized under high-pressure conditions.1 It is universally accepted that Ngs bind predominantly through dispersion and induction, with a small charge-transfer component in the case of the heavier Ngs. We recently rationalized the puzzling stability and short distances predicted by theory for helium adducts with some highly polar substrates, such as BeO or AuF.2 On the basis of high level quantum-chemical calculations, the charge transfer component is shown to play a key role in the helium adducts stabilization. In particular, we unambiguously ascertained that helium is able not only to donate electron density, but also to accept electron density from BeO or AuF. Accordingly, we look for suitable substrates, emphasizing the role of helium as electron acceptor. Inspired by the synthesis of a neutral (electronically excited) beryllium-carbene complex,3 we investigated the stability of the prototypical He-Be electronic states. Remarkably, for the Be*(1D)-He system we computed at a large full configuration interaction level of theory a 1 electronic state (correlating at large distance with Be*(1D) and He in its 1S ground state) stable by as much as 10 kcal/mol. The analysis of the electron charge displaced upon complex formation by employing the so-called charge displacement function3 confirms the key role of helium as acceptor of electron density. Our findings offer unprecedented important clues towards the design and synthesis of stable helium compounds.