HM⁺ and HM⁺‑He (M = Group 2 metal): chemical or physical interactions?

We investigate the HM+‑He complexes (M = Group 2 metal) using quantum chemistry. Equilibrium geometries are linear for M = Be and Mg, and bent for M = Ca–Ra; the explanation for this lies in the differing nature of the highest occupied molecular orbitals in the two sets of complexes. The difference primarily occurs as a result of the formation of the H–M+ bond, and so the HM+ diatomics are also studied as part of the present work. The position of the He atom in the complexes is largely determined by the form of the electron density. HM+…He binding energies are obtained and are surprisingly high for a helium complex. The HBe+…He value is almost 3000 cm−1, which is high enough to suspect contributions from chemical bonding. This is explored by examining the natural orbital density and by population analyses.