Quadruple bonding in C ₂ and analogous eight-valence electron species

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 C ₂ 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 invertedg 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 C 2 that contain higher-row elements, such as Si 2 and Ge 2, exhibit only double bonding.