Conclusions
The present work was motivated by a recent study that reported particularly short BeBe separations for various cations based on a central D 3h BeH3Be moiety,[13] with the presence of the three bridging hydrogen atoms being claimed to simulate the effect of a Be≡Be triple bond.[13] We have carried out all-electron CCSD(T)/cc‑pVQZ geometry optimization for theD 3h[MH3M]+ cations (M = Be, Mg), with and without ‘capping’ by He or Ne atoms (as proxies for an inert gas matrix). In order to investigate the nature of the chemical bonding we then used SCGVB and CASSCF calculations (amongst others), together with the analysis of valence localized natural orbitals and domain-averaged Fermi holes, as well as bond indices and QTAIM bond paths. We also briefly examined the ‘mixed’ system, i.e. theC 3v [BeH3Mg]+ cation.
In each case we found no evidence for any significant direct metal-metal bonding. Instead the short separations are the result of the positively-charged metal centres being held close together by the three negatively-charged hydrogen centres, with a stabilizing contribution from three equivalent sets of highly polar 3c‑2e M−H−M bonding character.
We concur with Zhao et al. [13] that theD 3h[BeH3Be]+ cation is a very interesting target for experimental work, whether it involves trapping the ‘bare’ cations in inert gas matrices or potentially isolating species based on [L→MH3M′←L]+ moieties. The present work shows that the same turns out to be true for the D 3h[MgH3Mg]+ cation and even for the ‘mixed’ C 3v [BeH3Mg]+ cation.
Keywords: Ultra-short metal-metal distances; Spin-coupled generalized valence bond (SCGVB) calculations; Localized natural orbitals; Domain-averaged Fermi hole analysis; Three-centre two-electron bonding.
Additional Supporting Information may be found in the online version of this article.