Triple bonding is conventionally considered to be the limit for multiply bonded main group elements, despite higher metal–metal bond orders being frequently observed for transition metals and lanthanides/actinides. Here, using high-level theoretical methods, we show that C2 and its isoelectronic molecules CN+, BN and CB− (each having eight valence electrons) are bound by a quadruple bond. The bonding comprises not only one σ- and two π-bonds, but also one weak ‘inverted’ bond, which can be characterized by the interaction of electrons in two outwardly pointing sp hybrid orbitals. A simple way of assessing the energy of the fourth bond is proposed and is found to be ~12–17 kcal mol−1 for the isoelectronic species studied, and thus stronger than a hydrogen bond. In contrast, the analogues of C2 that contain higher-row elements, such as Si2 and Ge2, exhibit only double bonding.
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Shaik, S., Danovich, D., Wu, W. et al. Quadruple bonding in C2 and analogous eight-valence electron species. Nature Chem 4, 195–200 (2012). https://doi.org/10.1038/nchem.1263
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