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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 ??- and two ??-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.

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Contributed by Steven Bachrach.Reposted from Computational Organic Chemistry with permissionA short note here mainly to call to the readers attention a fascinating trialogue on the C2 molecule.1Shaik, Danovich, Wu, Su, Rzepa, and Hiberty2 recently...
A short note here mainly to call to the readers attention a fascinating trialogue on the C2 molecule.1 Shaik, Danovich, Wu, Su, Rzepa, and Hiberty2 recently presented a full CI study of C2 and concluded that the molecule contains a quadruple bond (see my previous...
Quadruple and higher order metalmetal bonds are known for transition metals, lanthanoids and actinoids. But for main group elements? Using four different computational methods, Shaik et al. [1] show that C2 and its isoelectronic molecules CN+, BN and CB (each...
Sason Shaik, David Danovich, Wei Wu, Peifeng Su, Henry S. Rzepa, Philippe C. Hiberty, Nature Chemistry 2012, 4, 195 (Paywall)Contributed by Steven Bachrach.Reposted from Computational Organic Chemistry with permissionInspired by a blog post...
Inspired by a blog post of Henry Rzepa (see here) Shaik and co-workers examined the C2 species with an eye towards the nature of the bond between the two carbon atoms.1 Using both a valence bond approach and a full CI approach, they end up at the same place:...