A Theoretical Search for Stable Terminal Carbides

Exploratory scalar-relativistic
density functional theory (B3LYP*-D3/ZORA-STO-TZ2P)
calculations have been used to examine the stability and bonding of
pseudotetrahedral Group 8 (Fe, Ru, and Os) and Group 9 (Co, Rh, and
Ir) terminal carbide complexes with dδ4 ground states and tripodal tris(N-heterocyclic-carbene)
supporting ligands. The calculations predict relatively low adiabatic
ionization potentials of 4.3–5.9 eV for the charge-neutral
species and sizable adiabatic singlet–triplet gaps in the 0.9–1.6
eV range. Furthermore, the complexes exhibit near-zero or negative
electron affinities, indicating at least moderate reductive stability.
The calculated results suggest that, once successfully synthesized,
the majority of the complexes examined should be isolable and moderately
stable. As far as the bonding in the metal–carbido moiety is
concerned, natural bond orbital (NBO) analyses suggest a triple bond,
with a 2s-like lone pair on the carbido carbon.