Covalent bonds classical description

In this chapter the electrochemical properties of materials with covalent bonds and stoichiometric composition will be described. Many of these materials are semiconductors. In the literature there is a classical description of their electrochemistry by Morrison. Memming has given a comprehensive review on semiconductor electrochemistry, Sato has described the electrochemistry for oxides and semiconductor electrodes, and Trasatti has edited a book on catalytic aspects of oxides. 

It remained for G.N. Lewis (2), M.L. Huggins (3), and their contemporaries to interpret the primary and secondary valences of Werner in terms of the newly emerging electron patterns which were being used to explain "valence." Primary valences were normal covalent bonds (one electron from each bonded atom) and secondary valences were coordinate covalent bonds (two electrons for bond formation supplied by the ligand). The terms Lewis base and Lewis acid entered the literature. The Werner and Lewis-Huggins descriptions of stmcture and bonding were excellent for the metal-ammines, halo complexes of metals, and related species. Classical Werner coordination compounds fit the Wemer-Lewis description, and there were many of such compounds. 

It might appear that d orbitals have to be included in the case of hypervalent compounds, such as SFe, PCI5 or SiFg, because the 35, and 3p AOs can be used to form no more than four covalent bonds. Thus, the textbook description of SFe goes back to Pauling s classical paper [2] and postulates six equivalent SF bonds formed after prior sp d hybridization of the sulphur AOs. Doubts... 

Let us restrict ourselves momentarily to a purely classical description of a covalent bond between two atoms, in which an electron from each atom is moved from a point close to its own nucleus to a point between the two nuclei. Initially, each electron is close to one nucleus, and the two electrons are rather far apart. In other words, each electron is rather strongly attracted to its own nucleus, and because the electrons are rather far apart from each other, the electron-electron repulsion is relatively low. The movement of the electrons to a point between the two nuclei reduces the electron-nucleus attractions and increases the electron-electron repulsions. As a result, the movement of electrons from near their respective nuclei to the intemudear region involves an increase in (potential) energy. Thus, by focusing only on the electrostatic interactions, it is difficult to rationalize how the sharing of electrons has a stabilizing effect. 

The classical VB wave function, on the other hand, is build from the atomic fragments by coupling the unpaired electrons to form a bond. In the H2 case, the two electrons are coupled into a singlet pair, properly antisymmetrized. The simplest VB description, known as a Heitler-London (HL) function, includes only the two covalent terms in the HF wave function. 

The flexibility of the valence bond self-consistent field (VBSCF) method can be exploited to calculate VB wave functions based on orbitals that are purely localized on a single atom or fragment. In such a case, the VBSCF wave function takes a classical VB form, which has the advantage of giving a very detailed description of an electronic system, as, for example, the interplay between the various covalent and ionic structures in a reaction. On the other hand, since covalent and ionic structures have to be explicitly considered for... 

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