A Theoretical Search for Terminal Carbides and d-Block–p-Block Quadruple Bonds

Exploratory scalar-relativistic DFT calculations (B3LYP*-D3/ZORA-STO-TZ2P) have been used to examine the stability and bonding of pseudotetrahedral Group 8 (Fe, Ru, Os) and Group 9 (Co, Rh, Ir) terminal carbide complexes with tripodal tris(N-heterocyclic-carbene) supporting ligands. The complexes examined are all charge-neutral, with d4 ground states. The complexes exhibit relatively low adiabatic ionization potentials in the 4.3-5.9 eV range and sizable adiabatic singlet-triplet gaps in the 0.9-1.6 eV range. Furthermore, all the complexes exhibit near-zero or strongly negative electron affinities, indicating high reductive stability. These calculated results suggest that, once successfully synthesized, the majority of the complexes examined should be isolable and moderately stable. As far as bonding in the metal-carbido moiety is concerned, NBO analyses suggest a triple bond, with a 2s-like lone pair on the carbido carbon.