The Nature of the Fourth Bond in the Ground State of C2: The Quadruple Bond Conundrum

Does, or doesn’t C2 break the glass ceiling of triple bonding? This work provides an overview on the bonding conundrum in C2 and on the recent discussions regarding our proposal that it possesses a quadruple bond. As such, we focus herein on the main point of contention, the 4th bond of C2, and discuss the main views. We present new data and an overview of the nature of the 4th bond—its proposed antiferromagnetically coupled nature, its strength, and a derivation of its bond energy from experimentally based thermochemical data. We address the bond‐order conundrum of C2 arising from generalized VB (GVB) calculations by comparing it to HCCH, and showing that the two molecules behave very similarly, and C2 is in no way an exception. We analyse the root cause of the deviation of C2 from the Badger Rule, and demonstrate that the reason for the smaller force constant (FC) of C2 relative to HCCH has nothing to do with the bond energies, or with the number of bonds in the two molecules. The FC is determined primarily by the bond length, which is set by the balance between the bond length preferences of the σ‐ versus π‐bonds in the two molecules. This interplay in the case of C2 clearly shows the fingerprints of the 4th bond. Our discussion resolves the points of contention and shows that the arguments used to dismiss the quadruple bond nature of C2 are not well founded. The glass ceiling of triple bonding! This work provides an overview on the bonding conundrum in C2 and on the recent discussions regarding the proposal that it possesses a quadruple bond. It is shown that C2 is quadruply bonded by all reasonable measures, which include analyses of the impact of the 4th bond on the force constant, bond length, and stretching frequency, and a thermochemical derivation of the strength of the 4th bond. Thus C2 and its isoelectronic first‐row diatomic species break the glass ceiling of triple bonding.