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Electron-pair bonding is a central chemical paradigm. Here, we show that alongside the two classical covalent and ionic bond families, there exists a class of charge-shift (CS) bonds wherein the electron-pair fluctuation has the dominant role. Charge-shift bonding shows large covalent-ionic resonance interaction energy, and depleted charge densities, and features typical to repulsive interactions, albeit the bond itself may well be strong. This bonding type is rooted in a mechanism whereby the bond achieves equilibrium defined by the virial ratio. The CS bonding territory involves, for example, homopolar bonds of compact electronegative and/or lone-pair-rich elements, heteropolar bonds of these elements among themselves and with other atoms (for example, the metalloids, such as silicon and germanium), hypercoordinated molecules, and bonds whose covalent components are weakened by exchange-repulsion strain (as in [1.1.1]propellane). Here, we discuss experimental manifestations of CS bonding in chemistry, and outline new directions demonstrating the portability of the new concept.

This paper deals with a central paradigm of chemistry, the electron-pair bond. Valence bond (VB) theory and electron-localization function (ELF) calculations of 21 single bonds demonstrate that along the two classical bond families of covalent and ionic bonds, there exists a class of charge-shift bonds (CS bonds) in which the fluctuation of the electron pair density plays a dominant role. In VB theory, CS bonding manifests by way of a large covalent-ionic resonance energy, RE(CS), and in ELF by a depleted basin population with large variances (fluctuations). CS bonding is shown to be a fundamental mechanism that is necessary to satisfy the equilibrium condition, namely the virial ratio of the kinetic and potential energy contributions to the bond energy. The paper defines the atomic propensity and territory for CS bonding: Atoms (fragments) that are prone to CS bonding are compact electronegative and/or lone-pair-rich species. As such, the territory of CS bonding transcends considerations of static charge distribution, and involves: a) homopolar bonds of heteroatoms with zero static ionicity, b) heteropolar sigma and pi bonds of the electronegative and/or electron-pair-rich elements among themselves and to other atoms (e.g., the higher metalloids, Si, Ge, Sn, etc), c) all hypercoordinate molecules. Several experimental manifestations of charge-shift bonding are discussed, such as depleted bonding density, the rarity of ionic chemistry of silicon in condensed phases, and the high barriers of halogen-transfer reactions as compared to hydrogen-transfers.

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.

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Several density functional theories, chosen among the most popular, are shown to systematically overestimate three-electron bond dissociation energies in a series of model systems covering the full range of homonuclear three-electron X∴X bonds (X ) He, N, O, F, Ne, P, S, Cl, Ar) that can be found in organic cation radicals. The errors range from 15 to 54 kcal/mol with the SVWN local spin density method, from 6 to 45 kcal/mol with the gradient-corrected BLYP, BP86, and BPW91 functionals, and from 3 to 31 kcal/mol with the B3LYP, B3P86, and B3PW91 three-parameter hybrid functionals. The errors follow some regular tendencies according to the place of the X atom in the periodic table. The geometries and frequencies are also in error, the bond lengths being systematically too long and the frequencies too low. The errors are tentatively interpreted as consequences of electron self-interaction, leading to overstabilization of the Coulombic terms relative to the exchange-correlation terms in this type of bonds. At variance with these systematic errors, the BH&HLYP functional displays overall better results but still severely fails in some particular cases. Some lines of thought for devising modified DFT functionals are proposed.

National Natural Science Foundation of China[20873106]; Ministry of Science and Technology[2011CB808504]; Israel Science Foundation[ISF 53/09

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