Results and discussion
Key geometrical parameters for D 3h[MH3M]+ and [NgMH3MNg]+ cations, optimized at the all-electron CCSD(T)/cc‑pVQZ level of theory, are presented in Table1 . Our values for the beryllium species are very similar to those reported by Zhao et al.,[13] with the small differences being mostly due to the use here of a different basis set. Whether M is Be or Mg, we observe from Table 1 that the capping ofD 3h[MH3M]+ cations with Ng atoms leads only to very small increases in the MHM angle and thus in the MM distances, but it has practically no effect on the MH distances. As such we can anticipate that the nature of the bonding interactions in the central MH3M moieties should change relatively little when the D 3h[MH3M]+ cations are capped with He or Ne atoms.