title: Quadruple bonding in C2 and analogous eight-valence electron species.
abstract: 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 '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 C(2) that contain higher-row elements, such as Si(2) and Ge(2), exhibit only double bonding.
authors.affiliation: Institute of Chemistry and The Lise Meitner-Minerva Center for Computational Quantum Chemistry, Hebrew University of Jerusalem, 91904, Jerusalem, Israel. sason@yfaat.ch.huji.ac.il
authors.forename: Sason
authors.initials: S
authors.lastname: Shaik
authors.forename: David
authors.initials: D
authors.lastname: Danovich
authors.forename: Wei
authors.initials: W
authors.lastname: Wu
authors.forename: Peifeng
authors.initials: P
authors.lastname: Su
authors.forename: Henry S
authors.initials: HS
authors.lastname: Rzepa
authors.forename: Philippe C
authors.initials: PC
authors.lastname: Hiberty