Helium Bonding Possibilities Explored

A hypervalent carbon, boron, or beryllium atom bound on one side by a cyclic organic ligand could, on the other side, bind to helium and open up a new kind of noble gas chemistry, proposes Henry S. Rzepa of Imperial College London. Rzepa computationally modeled a series of structures incorporating carbon, boron, or beryllium complexed with various cyclopropene, cyclobutadiene, and cyclopentadiene rings to gauge their ability to form bonds with helium (Nat. Chem., DOI: 10.1038/nchem.596). The shortest (1.19 Å) carbon-helium bond was formed when a carbon-cyclobutadiene compound was modeled. Rzepa suggests that the helium bonds are so-called charge-shift bonds, which are stabilized by resonance between covalent and ionic interactions. Rzepa notes that helium can dissociate quickly from the compounds, even when large groups such as 2,6-diisopropylphenyl are used as ring substituents, but he holds out hope that other sterically bulky ring substituents could serve to stabilize the compounds.