Pivoting to the question of metal-carbon quadruple bonding in the terminal carbides examined, four canonical MOs indeed exhibit significant M-Ccarbido bonding. These are the two M(dp)-C(pp) p-bonding MOs, a predominantly dz2-pz-based s-bonding MO, and a predominantly dz2-2s-based s-bonding MO (Figure ). NBO analysis, however, indicates only three MOs with predominant M-Ccarbido bonding character, namely two p and one s NBO, along with a 2s-like lone pair on the carbide carbon. The second s-bond found in diatomics such as RhB 12-15 does not appear to survive in the presence of the strongly s-donating triscarbene supporting ligand. The Mayer bond order for the M-Ccarbido bond hovers around 2.8 for all the terminal carbides examined, suggesting in essence an M-C triple bond.