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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 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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Figure 1: Representations of bonding in C2.
Figure 2: VB wavefunctions and energy terms.
Figure 3: Schematic representation of the transformation of the TC wavefunction ( equation (3)) into a GVB wavefunction.
Figure 4: Semi-localized ɸLɸR orbitals, which form the fourth bond and their overlap S values.
Figure 5: ɸLɸR GVB orbital pairs and their overlap S values for the internal bonds in C2.

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Authors

Contributions

S.S. designed the project, analysed the FCI wavefunctions and wrote the paper. D.D. performed the VB, MRCI, FCI and bond order calculations. W.W. designed the initial VB calculations of C2. P.S. performed the initial set of VB calculations for C2. P.C.H. participated in the design of the VB determination of Din, in the analysis of the FCI wavefunctions, and contributed to writing the manuscript. H.R. initiated interest in the problem14, and explored probes for characterizing the bonding properties.

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Correspondence to Sason Shaik.

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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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