Stabilizing a 20-Electron Metallaazulyne by Aromaticity

The 18-electron rule states that
metal complexes with 18 valence
electron metal centers are thermodynamically stable because nine valence
orbitals of transition metals including one s orbital, three p orbitals,
and five d orbitals can collectively accommodate 18 electrons, achieving
the same electron configuration as the noble gas in the period. Thus,
20-electron compounds are extremely rare due to a violation of such
a rule. Here, we demonstrate a 20-electron metallaazulyne via density
functional theory calculations stabilized by aromaticity, which was
supported by various aromaticity indices including nucleus-independent
chemical shift, anisotropy of the induced current density, the isochemical
shielding surface, and electron density of delocalized bonds. Interestingly,
when a transition metal fragment is first introduced into the aromatic
azulyne molecule, the resulting osmaazulyne becomes antiaromatic,
in sharp contrast to the previous transformation from pentalyne to
metallapentalyne. More interestingly, when osmaazulyne is reduced
by two electrons, the resulting 20e osmaazulyne becomes aromatic.
Our findings highlight an important application of aromaticity in
stabilizing 20e species, inviting experimental verification.