Hafnocene-based Bicyclo[2.1.1]hexene Germylenes – Formation, Reactivity, and Structural Flexibility

2,5-Disilylsubstituted
germole dianions 1 react with
hafnocene dichloride to give hafnocene-based bicyclo[2.1.1]­hexene
germylenes 3. Their formation proceeds via hafnocene-germylene
complexes 2 that were identified by NMR and UV spectroscopy.
Germylenes 3 are stabilized by homoconjugation between
the empty 4p­(Ge) orbital and the π-bond of the innercyclic C2C3 double bond. This interaction can be
understood as σ2, π-coordination of the butadiene
part to the dicoordinated germanium atom that leaves the 16e– hafnocene moiety electronically unsaturated. We demonstrate that
this new class of germylenes might serve as ligand to a variety of
low-valent transition-metal complexes. The structure of the germylene
ligand in complexes with Fe(0), Ni(0), and Au­(I) and in reaction products
with N-heterocyclic carbenes showed an intriguing structural flexibility
that allows to accommodate different electronic situations at the
ligating germanium atom. The origin of this structural adaptability
is the interplay between the topological flexible unsaturated germanium
ring and the hafnocene group.