Ethan Saunders ‘26 asks: “We learned about carbon-carbon single, double, and triple bonds…but do carbon-carbon quadruple bonds exist?” The short answer is: “maybe”….

Although quadruple bonding and beyond…(perhaps even up to sextuple bonding in Cr2, W2, and U2 has been observed for metal-metal bonds, triple bonding is typically considered to be the limit for main-group bonding. However, Sason Shaik and coworkers argued (on the basis of theoretical calculations that) C2 (and its isoelectronic molecules CN+, BN, CB-) in a 2012 Nature Chemistry paper that these compounds all have quadruple bonding. In a 2016 Chem: Eur. J paper they claim that this also helps rationalize the structure and reactivity of C2.

However, Oliveira de Sousa and Nascimento argue on the basis of energy partitioning calculations that the speculated “fourth” bond is destabilizing J. Chem. Theory. Comput. 2016. Highly accurate quantum Monte Carlo calculations by Genovese and Sorella suggest a partial agreement with the quadruple bond picture, with the main bonding effect resulting from spin fluctuations. *J. Chem. Phys. 2020

What do experiments say? Diatomic C2 was first discovered by Swan in 1857 in high temperature flames, and subsequent work was done using electric arcs. This complicates the ability to try to assign the nature of the C2 ground state. Reently, Miyamoto et al. (Nature Commun 2020) described a room-temperature chemical synthesis of C2 from hypervalent alkynl-\lambda^3-iodane. If C2 has a quadruple bond, it should be a singlet biradical. Based on its chemical reactivity, Miyamoto support this claim, and in turn, support the prediction of a quadruple bond. However, Rzepa has commented that Miyamoto’s reaction sysnthesis of C2 would imply a highly endoenergic reaction (based on DFT and CCSD(T)), which seems inconssistent with their results. (Nature Commun 2021) This calls into question what exactly was made in the Miyamoto et al. experiment, and in turn, the implications for quadruple bonding.

What should we make of this? It is important to realize that the chemical bond is a convenient fiction. As Charles Coulson wrote, “A chemical bond is not a real thing: it does not exist: no one has ever seen it, no one ever can. It is a figment of our own imagination.”(The Spirit of Applied Mathematics, 1952) For unusual molecules like C2, we are reaching the limit of the simple localized picture of bonding that is enforced by Lewis theory and simple formulations of valence bond theory.