Magnesium systems
We carried out essentially the same modes of analysis for theD 3h[MgH3Mg]+ and [HeMgH3MgHe]+ cations as we have described for the beryllium systems. Again we found, without the imposition of any symmetry relations amongst the active orbitals, that the converged SCGVB(6) solutions for the ‘bare’D 3h[MgH3Mg]+ cation features two sets of three symmetry-related orbitals, with each ‘pair’ being primarily associated with a particular Mg−H−Mg linkage (see first two images in the top row of Figure 3). The orbital overlap\(\left\langle\phi_{1}\middle|\phi_{2}\right\rangle\) is 0.825 and the total active space spin function, \(\Theta_{0,0}^{6},\) is again dominated by the perfect-pairing mode: this system features three symmetry-equivalent highly polar 3c‑2e Mg−H−Mg bonding units, albeit SCGVB orbital \(\phi_{1}\) appears to have a larger relative contribution from H than was the case for the various beryllium species. The dominant valence LNOs for the D 3h[MgH3Mg]+ cation consist of three almost doubly-occupied symmetry-equivalent orbitals that are consistent with notions of 3c‑2e character (see the third image in the top row of Figure 3). Except again for a larger relative contribution from H, there are obvious similarities to the bonding situation that we found for the beryllium species.