Charge‐Shift Bonding: A New and Unique Form of Bonding

Abstract

Charge‐shift bonds (CSBs) constitute a new class of bonds different than covalent/polar‐covalent and ionic bonds. Bonding in CSBs does not arise from either the covalent or the ionic structures of the bond, but rather from the resonance interaction between the structures. This Essay describes the reasons why the CSB family was overlooked by valence‐bond pioneers and then demonstrates that the unique status of CSBs is not theory‐dependent. Thus, valence bond (VB), molecular orbital (MO), and energy decomposition analysis (EDA), as well as a variety of electron density theories all show the distinction of CSBs vis‐à‐vis covalent and ionic bonds. Furthermore, the covalent–ionic resonance energy can be quantified from experiment, and hence has the same essential status as resonance energies of organic molecules, e.g., benzene. The Essay ends by arguing that CSBs are a distinct family of bonding, with a potential to bring about a Renaissance in the mental map of the chemical bond, and to contribute to productive chemical diversity.

Comment on “The ‘Inverted Bonds’ Revisited. Analysis of ‘in Silico’ Models and of [1.1.1]Propellane Using Orbital Forces“

“…:F − . As such, the bond in F 2 − , like all (2c,3e) bonds, is a pure CSB. How does the DOF method account for the existence of this bond and its bonding energy?…”

“…At the same time, the covalent part alone C⋅‐⋅C, involving singlet‐coupling between the two electrons in the two HAOs, although being significantly stabilizing, cannot by itself overcome the large repulsions from the surrounding wing C−C bonds, and is thus overall only slightly repulsive. Such bonds that are dominated by the resonance energy of the constituent VB structures that differ by a shift of charge are qualified as “Charge‐shift bonds“ (CSBs) and constitute a bonding class on their own, quite different by many aspects from classical covalent bonds …”

History and Future of Dative Bonds

“…From am olecular orbital perspective,t he three-center,f ourelectron model of Rundlea nd Pimentel provides ag ood enough interpretation for the stability of hypervalent molecules. [71,76,80,84] Although this model was shown ad nauseam, we will present it here to help our cause. As before,w ew ill use Xe as ap rototype for an octet filled molecule, and therefore XeF 2 will do the job.…”

“…As before,w ew ill use Xe as ap rototype for an octet filled molecule, and therefore XeF 2 will do the job. [84] We can see in Scheme 10 that the bondingp art, the HOMO-1, is made by an ungerade s bond. The HOMO is symmetric, making it impossible for the xenon p z to participate in it.Ino ther words, it is at hree-center four-electron model, but it is at hree-center two-electron bond, or one electron per XeÀFb ond.…”

“…The HOMO is symmetric, making it impossible for the xenon p z to participate in it.Ino ther words, it is at hree-center four-electron model, but it is at hree-center two-electron bond, or one electron per XeÀFb ond. The HOMO centralized in the outer ligands creates ac harge accumulation in them, which must come from the central atom, explaining the stabilityo fh ypervalentm olecules with electronegative ligands (an effectt hat can be explained considering charge-shift bonding [84] ). In addition, the HOMO-1 forces the ligands to be at approximate-ly1808.T he pair of fluoridel igandsi nX eF 2 works very similarly in terms of electron count as the oxide in XeO.…”