A New Area in Main-Group Chemistry: Zerovalent Monoatomic Silicon Compounds and Their Analogues

ConspectusMonoatomic zerovalent main-group element complexes emerged very
recently and attracted increasing attention of both theoretical and
experimental chemists. In particular, zerovalent silicon complexes
and their congeners (metallylones) stabilized by neutral Lewis donors
are of significant importance not only because of their intriguing
electronic structure but also because they can serve as useful building
blocks for novel chemical species. Featuring four valence electrons
as two lone pairs at the central atoms, such complexes may form donor–acceptor
adducts with Lewis acids. More interestingly, with the central atoms
in the oxidation state of zero, they could pave a way to new classes
of compounds and functional groups that are otherwise difficult to
realize.In this Account, we mainly describe our contributions
in the chemistry
of monatomic zerovalent silicon (silylone) and germanium (germylone)
supported by a chelate bis-N-heterocyclic carbene
(bis-NHC) ligand in the context of related species developed by other
groups in the meantime. Utilizing the bis-NHC stabilized chlorosilyliumylidene
[:SiCl]+ and chlorogermyliumylidene [:GeCl]+ as suitable starting materials, we successfully isolated silylone
(bis-NHC)Si and germylone (bis-NHC)­Ge, respectively. The electronic
structures of the latter complexes established by theoretical calculations
and spectroscopic data revealed that they are genuine metallylone
species with electron-rich silicon(0) and germanium(0) centers. Accordingly,
they can react with 1 molar equiv of GaCl3 to form Lewis
adducts (bis-NHC)­E­(GaCl3) (E = Si, Ge) and with 2 molar
equiv of ZnCl2 to furnish (bis-NHC)­Si­(ZnCl2)2. Conversion of the metallylones with elemental chalcogens
affords isolable monomeric silicon­(II) and germanium­(II) monochalcogenides
(bis-NHC)­EX­(GaCl3) (X = Se, Te), representing molecular
heavier congeners of CO. Moreover, their reaction with elemental chalcogens
can also yield monomeric silicon­(IV) and germanium­(IV) dichalcogenides
(bis-NHC)­EX2 (X = S, Se, Te) as the first isolable complexes
of the molecular congeners of CO2. Moreover, (bis-NHC)­Si
could even activate CO2 to afford the monomolecular silicon
dicarbonate complex (bis-NHC)­Si­(CO3)2 via the
formation of SiO and SiO2 complexes as intermediates. Furthermore,
starting with a chelate bis-N-heterocyclic silylene
supported [:GeCl]+, we developed two bis-N-heterocyclic silylene stabilized germylone→Fe­(CO)4 complexes. Our achievements in the chemistry of metallylones demonstrate
that the characteristic of monatomic zerovalent silicon and its analogues
can provide novel reaction patterns for access to unprecedented species
and even extends the series of functional groups of these elements.
With this, we can envision that more interesting zerovalent complexes
of the main-group elements with unprecedented reactivity will follow
in the near future.