C2 is a diatomic molecule that can be seen in the blue color of hot hydrocarbon flames but cannot be held in a vial. However, its bonding is the source of much debate. Sason Shaik, Hebrew University of Jerusalem, Israel, Henry S. Rzepa, Imperial College London, UK, and Roald Hoffmann, Cornell University, New York, USA, have recently discussed their opinions on this topic in a trialogue that covers structure, stability, and reactivity of this omnipresent yet elusive molecule.
The lively exchange includes an overview of the molecular orbitals of C2 and an analysis of valence bond calculations. They also critically consider that some states of C2 also contain quadruple bonds, as originally postulated by Robert Mulliken in 1939. They also point out that atomization energies, which predict C2 to have the second strongest homonuclear bond, are irrelevant in this case. Although the nature of the bonding in C2 remains unresolved, the authors do emphasize how powerful valence bond and molecular orbital theories are in quantum chemistry.