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.