Electron-pair bonding in real space. Is the charge-shift family supported?

Abstract

Charge-shift bonding (CSB) has been introduced as a distinct third family of electron-pair links that adds to the covalent and ionic tradition.

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

“…There is nothing in these profiles that suggests that CSBs are simply stretched covalent bonds,a sa rgued recently. [64] Thus,a tl ong distances,t he Laplacian for HÀHi sv irtually zero,a nd in the range near the equilibrium distance, R = 2-0.75 ,itdescends rather fast and becomes strongly negative, as expected for ac ovalent bond. In contrast, the respective Laplacian curve for FÀFc ontinuously rises from zero at the interval 4.5-3.0 ,r eaching am aximum positive value near the equilibrium distance of the bond, as expected for aC SB dominated by Pauli repulsion.…”

“…Of course,o pposite points of view have appeared and suggested that:1 )CSBs are merely covalent bonds,w hich exhibit the same orbital shrinkage and kinetic-energy-lowering in the bonding region as in H À H, [84] and 2) positive Laplacians simply typify long covalent bonds;hence,the long F-F bond is merely astretched covalent bond with apositive Laplacian, and the same would be found if we were to stretch HÀH. [64] However,w es howed that these objections are not firmly founded.…”

“…Similarly,w ere FÀFs imply like the stretched HÀH covalent bond, [64] the weights of its ionic structures would have been quite low; but quite to the contrary,one finds that the ionic structures of F 2 have larger weights than those of H 2 (see Table S3). Further, the Laplacians of H À Ha nd F À Fare very different across the bond distances ( Figure 12), and the behavior of the Laplacian of the latter bond is entirely dominated by the charge-shift resonance energy of this bond.…”

“…Besides,t he density at the bond critical point is quite significant for F 2 ,while it is totally insignificant for stretched covalent bonds such as the stretched H-H bond. What characterizes aCSB in QTAIM analysis is the combination of ap ositive Laplacian along with as ignificant density at the bond critical points.A ss uch, CSBs and covalent bonds are different bond types.A nd neither objection [64,84] forms ap articularly useful basis for understanding the features of CSBs that are discussed in this Essay.…”

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

“…Of course, opposite points of view have appeared and suggested that: 1) CSBs are merely covalent bonds, which exhibit the same orbital shrinkage and kinetic‐energy‐lowering in the bonding region as in H−H, and 2) positive Laplacians simply typify long covalent bonds; hence, the long F–F bond is merely a stretched covalent bond with a positive Laplacian, and the same would be found if we were to stretch H−H . However, we showed that these objections are not firmly founded.…”