An Excursion from Normal to Inverted CC Bonds Shows a Clear Demarcation between Covalent and Charge‐Shift CC Bonds

Abstract

What is the nature of the C-C bond? Valence bond and electron density computations of 16 C-C bonds show two families of bonds that flesh out as a phase diagram. One family, involving ethane, cyclopropane and so forth, is typified by covalent C-C bonding wherein covalent spin-pairing accounts for most of the bond energy. The second family includes the inverted bridgehead bonds of small propellanes, where the bond is neither covalent nor ionic, but owes its existence to the resonance stabilization between the respective structures; hence a charge-shift (CS) bond. The dual family also emerges from calculated and experimental electron density properties. Covalent C-C bonds are characterized by negative Laplacians of the density, whereas CS-bonds display small or positive Laplacians. The positive Laplacian defines a region suffering from neighbouring repulsive interactions, which is precisely the case in the inverted bonding region. Such regions are rich in kinetic energy, and indeed the energy-density analysis reveals that CS-bonds are richer in kinetic energy than the covalent C-C bonds. The large covalent-ionic resonance energy is precisely the mechanism that lowers the kinetic energy in the bonding region and restores equilibrium bonding. Thus, different degrees of repulsive strain create two bonding families of the same chemical bond made from a single atomic constituent. It is further shown that the idea of repulsive strain is portable and can predict the properties of propellanes of various sizes and different wing substituents. Experimentally (M. Messerschmidt, S. Scheins, L. Bruberth, M. Patzel, G. Szeimies, C. Paulman, P. Luger, Angew. Chem. 2005, 117, 3993-3997; Angew. Chem. Int. Ed. 2005, 44, 3925-3928), the C-C bond families are beautifully represented in [1.1.1]propellane, where the inverted C-C is a CS-bond, while the wings are made from covalent C-C bonds. What other manifestations can we expect from CS-bonds? Answers from experiment have the potential of recharting the mental map of chemical bonding.

Two-Center Two-Electron Covalent Bonds with Deficient Bonding Densities

“…The electron density of a molecule could be determined accurately and reliably. [16][17][18][19] In another trial, the F-F bond in F 2 and O-O bond in H 2 O 2 were described to be protocovalent rather than covalent. The improvement was to replace spherical atoms with oriented atoms and the new model resulted in the expected density buildup at the bonding region.…”

Ein Molekül, zwei Atome, drei Ansichten, vier Bindungen?

“…C 2 -Chemie, und ein Kosmos kreiert allein aus C 2 -Einheiten! % 29 % in der C-C-Bindung von Cyclopropan, [38] und es kçnnte der Beginn einer Konvergenz hin zu der prozentualen Verteilung in Ladungstransferbindungen sein, z. HOMO von gestaffeltem Ethan, nach Jørgensen und Salem.…”

The mechanics of charge-shift bonds: A perspective from the electronic stress tensor

“…In all other cases there are two compressive modes and one tensile mode, so the stress-tensor eigenvalues occur in the sequence k 1 < k 2 < 0 < k 3 . The compressive modes seem to become stronger as charge-shift bonding character increases, perhaps due to Paul repulsions from the electron-rich surroundings of the charge-shift bonds [41]. However, the tensile modes also become stronger with increasing charge-shift bonding character.…”

“…Using the language of atoms in molecules, one can analyze the electron density and related properties at the bond critical point associated with the candidate for a charge-shift bond. At the bond critical point of chargeshift bonds, the electron-density Laplacian is either positive or nearly zero [38], the ratio of the positive-definite kinetic-energy density to the electron density at the bond critical point is greater than one [39], and the energy-density is nearly zero [40,41]. The bond-classifying indicator of Bohorquez, Matta, and Boyd can also be used to identify charge-shift bonds [42,43].…”