Heteronuclear and Homonuclear Bimetallic Metallocenes of Alkaline Earths and Zinc

Hetero- and homonuclear dimetallocenes of group 2 (alkaline
earth)
and zinc metals, CpMM′Cp (M = Be, Mg, Ca, Sr, Ba, Zn), have
been systematically investigated using quantum chemical calculations
with a focus on novel metal–metal bonding, electronic structure,
and stability. The metal–metal bonding is generally low-valent
covalent bonding between M(I) centers. All BeM complexes along with
MgMg, CaCa, CaSr, CaBa, SrSr, and BaBa metallocenes are stable with
respect to dissociation, with loss of a neutral metal atom being the
most favorable dissociation pathway. As an indicator of electronic
stability, the largest highest occupied molecular orbital (HOMO)–lowest
unoccupied molecular orbital (LUMO) gaps are calculated for the Be-containing
(BeBe, BeZn, BeMg; 3.17–5.95 eV) and MgMg (4.84 eV) metallocenes.
Remarkably, CpBeBeCp with the first Be–Be bond has just been
isolated [Science 2023, 380, 1147−1149], with CpBeZnNacnac representing the first
Be–Zn bond. The BeZn and BeMg metallocenes are promising targets
for isolation, which would represent the first isolated Be–Mg
covalent bond. Quantum theory of atoms in molecule (QTAIM) calculations
highlight novel metal–metal bonding with BeBe, MgMg, CaCa,
and SrSr metallocenes, with a non-nuclear local maximum in their electron
density, termed a “non-nuclear attractor” (NNA) or “pseudoatom”.
CpBeBeCp is unique in potentially having two NNAs in the Be–Be
bond that contains 1.4 electrons that are delocalized and weakly bound.
The metal–metal bonds are not just a simple bond with one bond
critical point but rather by a bond exhibiting the more complex feature
of two (or more) bond critical points mediated by pseudoatoms. The
implications for bonding and reducing the reactivity of CpBeBeCp are
discussed.