Introduction
In spite of the obvious deterrence for experimentalists of the high
toxicity of beryllium compounds,[1] a great deal
is now known about the organometallic and coordination chemistry of
beryllium.[2] Beryllium compounds that have been
studied experimentally and especially computationally include various
systems that feature BeBe bonding and/or very short BeBe
distances.[3-19] For example, it has been shown
for various choices of X, such as a fluorine atom or an appropriate
N‑heterocyclic carbene ligand, that certain XBeBeX species feature Be−Be
bonds that are both significantly stronger and shorter than the weak and
rather long bond in Be2.[3-4] We
note that Liu et al. have interpreted the bonding in the octahedral
Be2(µ2-Li)4 cluster, and
others, in terms of a Be=Be double‑π bond[10] and
the bonding in Be2X4Y2
clusters (X = Li, Na and Y = Li, Na, K) in term of Be≡Be triple
bonds.[15] Calculations and analysis carried out
by Rohman et al. also support the notion of Be≡Be triple bonds in
various systems, including Be2X6
(X = Li, Na), but they found the BeBe bonding to be ultra-weak in spite
of the very short BeBe distances.[16-17] Indeed,
some systems have been studied both experimentally and computationally,
such as the rhombic Be2O2 cluster, which
feature very short BeBe distances in the absence of any direct BeBe
chemical bonding.[8-9, 11]
There has been significant recent computational interest in predicting
the existence of potentially stable beryllium complexes that feature
ultra-short BeBe distances,[6, 12-13, 18-19]
regardless of whether or not they actually involve any direct chemical
bonds between the beryllium atoms. The present work was motivated by one
such study in which three bridging hydrogen atoms were used potentially
to simulate the effect of a Be≡Be triple bond, with the outcome that
particularly short BeBe distances were predicted for various systems,
including the D 3h[BeH3Be]+
cation.[13] Our main goal here is to investigate
the nature of the bonding interactions in this type of system,
especially the D 3h[MH3M]+ cations (M = Be, Mg),
including those ‘capped’ by He or Ne atoms (as proxies for an inert gas
matrix). To this end, we follow all-electron CCSD(T)/cc‑pVQZ geometry
optimizations with spin-coupled generalized valence bond (SCGVB)
calculations and the analysis of localized natural orbitals and
domain-averaged Fermi holes.
It is our expectation that the various systems we study should all
correspond to local minima on their respective potential energy surfaces
and that their electronic structures are somewhat more likely to involve
highly polar three‑centre two‑electron (3c‑2e) M−H−M bonding character
rather than direct metal-metal chemical bonds. We note in this context
that Kalita et al.[14] detected in the
LiMH2MLi system (M =Be, Mg, Ca) with two bridging H
atoms the presence of two 3c‑2e M−H−M bonds that were said to be
reminiscent of the bonding situation in diborane.