Quadruple bonding of carbon in terminal carbides

Despite its four valence electrons, carbon can at most form triple bond in ordinary organic complexes. Quadruple bonds for carbon had been considered as impossible for a long time. Recently we showed that quadruple bonding is viable in a triatomic uranium carbide oxide molecule CUO, where the terminal C is quadruply bonded with U via its nearly unhybridized 2s- and 2p-orbitals. Here we extend this new concept to a series of diatomic molecules consisting of tetravalent p-, d-, and f-elements and terminal carbide. Investigation has been focused on a series of CM-type molecules with possible quadruply-bonded carbon (QBC), CB-, CTi, CZr, CHf, CV+, CNb+, CTa+, and isoelectronic species of CUO. We have performed natural bond orbital (NBO), natural resonance theory (NRT), and atom-in-molecule (AIM) analyses at both density functional theory (DFT) and ab initio CASSCF levels to provide evidence for the feasibility of carbon quadruple bond in these systems. Our calculation results show that the C-M bond orders in these QBC species are comparable to that in CUO, indicating terminal carbides can have novel quadruple bonding when appropriate orbitals are available in the adjacent atoms.