Sesquicarbene Complexes: Bonding at the Interface Between M–C Single Bonds and MC Double Bonds

Allylic
dimetalated complexes [M2C­(vinyl)]+ (M = Au­(IPr)
and Cr­(CO)5–) incorporate
a new coordination mode of carbon. Digold complexes of this type have
recently been detected experimentally. The intrinsic bond orbitals,
partial charges, and structural parameters of the gold complexes and
of chromium analogs were studied computationally and compared to those
of the respective monometalated species and hydrocarbons. This showed
that such digold complexes have a carbene character at both Au–C
bonds comparable to typical carbene complexes of gold. Dichromium
complexes with their stronger π-backdonation compete for interaction
with carbon’s π-orbital; each of the chromium atoms partakes
in double bonding that is significant but weaker than that in the
carbene analogs. Containing two M–C bonds on the interface
between single and double bonds, these bridged complexes can be conceived
as “sesquicarbene complexes”. The π-system acted
in a very adaptive manner and employed additional stabilization of
the vinyl system only where needed. Significant carbene character
is found simultaneously in both M–C bonds at the same carbon
center. The discovery of these complexes with relatively strong double
bond character between one carbon and two metal atoms could bring
unusual single-carbon-centered organometallic cascade reactions to
the horizon.