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

Abstract

Electron-deficient covalent bonds are a type of covalent bonds without electron accumulation at their bonding regions. Compared with normal covalent bonds, they are quite sensitive to chemical environments. Electron-deficient and normal covalent bonds are not isolated from each other. An electron-deficient bond may change to a normal one upon the change of substituting groups. Neither bond elongation nor atom electronegativity is directly related to the electron deficiency in an electron-deficient bond. Atoms in molecules (AIM) analyses suggest that electron-deficient bonds are characterized by positive Laplacians and small ρ(BCP) values. The positive Laplacian is caused by insignificant electron accumulation perpendicular to the bond path. On the basis of electron localization function (ELF) descriptors, electron-deficient bonds have small basin populations, low η values and high relative fluctuations. There may be one or two bond basins for an electron-deficient bond. In addition, such a bond may correlate with two more valence basins close to the two participating atoms. Electron-deficient bonds are usually weak and long. This is consistent with the low s characters in their natural bond orbitals (NBOs).

Depicting electronic distributions from accurate computational first principles: On the relationship between the complex patterns of bonding interaction and the back‐donation phenomenon

“…[]). Therefore, from this analysis it is right to state that the NiC interaction is of CSB type because as shown above, the electronic descriptors clearly indicate that the interactions are neither covalent nor ionic ones but intermediate with a marked depletion of the electron density …”

“…[4] It seems to occur for typical ionic interactions and for an intermediate interaction between the pure covalent and ionic interactions of the often called charge-shift bond (CSB) or deficient covalent bond type. [42][43][44] These two types of atomic interactions can be differentiated within the density view, according to the electron population that each atom houses. Let us compare for that goal, the present NiC case with that of the ionic NaCl.…”

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

“…Thus,the three methods lead to the same classification of the homonuclear bonds into two families.E LF and QTAIM diagnose the attractive/repulsive nature of the covalent "shared densities", while VB brings about energetic components that highlight the dominant role of the RE CS energy in the CSB group.N ote that the quoted QTAIM properties for [1.1.1]propellane are derived from experimental densities, [52] and as such, lend experimental support to the characterization of the inverted C À Cb ond as CSB.Asimilar consensus was recently reported for the O À Ob ond of Rubrene endoperoxide, [54] for Mn À Mn and Mn À CO bonds in Mn 10 (CO) 10 , [55] as well as other bonds. [57][58][59][60][61][62][63]…”

The Mysticism of Pericyclic Reactions: A Contemporary Rationalisation of Organic Reactivity Based on Electron Density Analysis

“…These QC topological tools allowed, for the first time, analysis of the electron density obtained from QC calculations. Although some authors have drawn attention to some of the discrepancies between ELF and QTAIM in the analysis of some bonding properties, ELF has shown to be fully consistent with other QC topological tools, including QTAIM, in the characterisation of chemical bonds …”

“…Although some authors have drawn attention to some of the discrepancies between ELF and QTAIM in the analysis of some bonding properties, [24] ELF has shown to be fully consistent with other QC topological tools, including QTAIM, in the characterisation of chemical bonds. [25][26][27] The most attractive QC procedure for describing a reaction mechanism consists in the characterisation of the reorganisation of electron density to evidence bonding changes during a reaction path. [28,29] To perform these analyses quantitatively, bonding evolution theory (BET), consisting of the joint use of the ELF topology and Thom's catastrophe theory (CT), [30][31][32] was proposed by Krokidis et al [33] as a new tool to analyse the electronic changes in chemical processes, being widely applied in the study of different elementary reactions.…”