Motivated by the particularly short metal-metal distance that has been predicted for the D3h [BeH3Be]+ cation, comparable to those anticipated for triple bonds, we investigate the nature of the bonding interactions in the D3h [MH3M]+ cations (M = Be, Mg). CCSD(T)/cc pVQZ calculations are used to determine optimized geometries for all of the various species, including those ‘capped’ by He or Ne atoms (as proxies for an inert gas matrix). The primary tools that are then used to investigate the nature of the chemical bonding are spin-coupled generalized valence bond calculations and the analysis of localized natural orbitals and of domain-averaged Fermi holes. The various results for all of the systems considered indicate the presence of highly polar three-centre two-electron M−H−M bonding character instead of any significant direct metal-metal bonding.