Covalent-ionic superposition

The success of the HL model and its relation to Lewis model, posed a wonderful opportunity for the young Pauling and Slater to construct a general quantum chemical theory for polyatomic molecules. They both published, in the same year, 1931, several seminal papers in which they each developed the notion of hybridization, the covalent-ionic superposition, and the resonating benzene picture.Especially effective were Pauling s papers that linked the new theory to the chemical theory of Lewis, and that rested on an encyclopedic command of chemical facts. In the first paper, Pauling presented the electron-pair bond as a superposition of the covalent HL form and the two... 

The Pauling Covalent-Ionic Superposition Scheme of the Two-Electron Bond. . 172... 

Scheme 1 The traditional covalent and ionic bond families based on Pauling s covalent-ionic superposition scheme. Reproduced from [9] with permission of Wiley-VCH...

As such, in practice, the Pauling covalent-ionic superposition scheme has traditionally become associated with two bond families, based on a criterion of static charge distribution these are the covalent (polar-covalent) bond and ionic bond families in Scheme 1. In the first family, the major contribution to bonding comes from spin pairing. Importantly, in homopolar bonds, the REqs contribution was assumed - in Pauling s original scheme and in subsequent treatments based on... 

Finally, Fig. 2e, f shows the NaF and NaCl braids. It is clear that for both bonds, the dominant VB stmcture is ionic and it is very close to the exact covalent-ionic superposition curve, with a negligible REqs contribution. These two bonds are classically ionic. 

The resonance between the covalent and ionic bond structures of a molecule produces, by the superposition of the electron clouds of the ionic bond and of the covalent bond, a transitional electron cloud. This is discussed below in terms of wave mechanics. The electron cloud of the bond, however, will of course be continuous and the splitting into component parts, which this method of treatment has incurred, is the direct result of the attempt to describe a complex chemical bond in terms of two simpler types of bonds which may be represented by classical structural symbols. 

The structure of zinc sulphide on the basis of an ionic bond, would consist of Zn + and S ions arranged tetrahedrally. The covalent structure, however, requires the sulphur to be doubly charged, positive and tetravalent and the zinc to be doubly charged negative and tetravalent, both the Zn - and the thereby being in an i/ valency state. The actual molecule will be a superposition of these two extreme states and although the contribution of the covalent form may be small, it does nevertheless determine both the configuration of the atoms and their coordination numbers. 

In bonds between different atoms the dipole moment may have intermediate values owing to the superposition of covalent and ionic states. In such cases the bond is described by the function... 

Let us take the example of the NaCl molecule (/rj may describe the ionic Na Cl distribution, while 2 rnay correspond to the covalent bond Na-Cl. The adiabatic wave function (r R) of the NaCl molecule may be taken as a superposition of and 2- The valence bond (VB) wave functions (VB structures) described in Chapter 10 may be viewed as diabatic states. 

We first describe the noninteracting solutions. These are the molecular orbital eigenstates listed in Table 5.1. The goundstate is a linear superposition of the covalent and ionic basis states, 1) and 2). The first excited singlet state is the odd-parity ionic state, 2), whereas the tripiet excitation is the covalent state 3). The second singlet excitation is an antisymmetric linear combination of 1) and 2). 

Thus, it is not possible to establish the general dependence of B on V, since the energy of the real chemical bond is a superposition of ionic, covalent and polarizing components which depend differently on the distance. In our work [278, 279] such characteristics of polar compounds as the bond ionicity, the atomic valence, the effective nuclear charge, the Born s factor of repulsion and the interatomic distance are used to derive an equation suitable for approximate calculations of Bo in crystal substances with Nc from 4 to 8 with a 10 % accuracy. 

What makes the introduction of CF3O into pharmaceutically relevant compounds particularly intriguing is their unique electron distribution. The geminal combination of an alkoxy or aryloxy group with a fluorine atom offers the possibility of bonding/ non-bonding resonance which can be formally expressed by the superposition of a covalent and an ionic limiting stracture (Fig. 6). 

The Heitler-London papers mark the birth of VB theory [2], which was developed by Pauling as a quantum mechanical version of the Lewis model. This quantum mechanical articulation of Lewis shared-pair model has culminated in a generalizing intellectual constmct [25], which described the electron-pair bond A-X as a superposition of covalent (cov) and ionic forms, a+x— and 4>a—x+ [Eq. (2a) and (2b)]. 

We have seen that the closed-shell bonding and antibonding configurations each provide an uncorrelated description of the electronic system but that a superposition of these configurations introduces correlation. In the ground state, the electrons tend to be located around opposite nuclei, whereas, in the excited state, there is a tendency for the electrons to be located around the same nucleus. Further insight into the correlation problem may be obtained by isolating the pure covalent and ionic states, where the electrons are either always located around opposite nuclei or always located around the same nucleus. 

---