Pair-bonding model

In applying an electron-pair bond model for the hypervalent molecules studied, the (apical, equatorial) and (equatorial, equatorial) isomers, namely 23-eq and 23b, respectively, are both local minima. This means that the tetracoordinated species are less strained than the pentacoordinated ones, due to less antibonding properties of the electron delocalization between the geminal ring bonds on the hypervalent sulfur atom. Yet, the more strained (eq, eq) isomers are, surprisingly, more stable than the (ap, eq) isomers in most of the hypervalent three-membered rings studied <2001PCA10711>. 

There is a vast range of neutral compounds and anions. Except in the ion, the compounds show complex structures, which cannot be interpreted using simple electron pair bonding models. 

The motivation for this work has been the need to develop a systematic description of structural chemistry that is applicable to inorganic materials. The standard electron-pair bond model of organic chemistry does not work for these compounds, and physical theories tend to be either too simplistic or too complex to be widely useful. 

In the following text we compare and contrast the recoupled pair bond model with three models that have also been proposed to rationaHze the existence and stability... 

The democracy principle of Cooper and coworkers [7] states that any valence electron can participate in bonding if it is energetically favorable to do so. This implies a similar conditionaUty as found with the recoupled pair bonding model. The recoupled pair bonding model provides insights into the conditions under which it is energetically favorable to form the type of bonds - recoupled pair bonds - that lead to hypervalence. 

The description of bonding in boranes [54,71] turned out to be a straightforward application of QCT. This, in spite of boranes being notorious for their difficult rationalisation in terms of the two-centre electron-pair bond model, an issue that has attracted much discussion [72-74]. 

Until about 20 years ago, the valence bond model discussed in Chapter 7 was widely used to explain electronic structure and bonding in complex ions. It assumed that lone pairs of electrons were contributed by ligands to form covalent bonds with metal atoms. This model had two major deficiencies. It could not easily explain the magnetic properties of complex ions. 

The Lewis structures encountered in Chapter 2 are two-dimensional representations of the links between atoms—their connectivity—and except in the simplest cases do not depict the arrangement of atoms in space. The valence-shell electron-pair repulsion model (VSEPR model) extends Lewis s theory of bonding to account for molecular shapes by adding rules that account for bond angles. The model starts from the idea that because electrons repel one another, the shapes of simple molecules correspond to arrangements in which pairs of bonding electrons lie as far apart as possible. Specifically ... 

In molecular orbital theory, electrons occupy orbitals called molecular orbitals that spread throughout the entire molecule. In other words, whereas in the Lewis and valence-bond models of molecular structure the electrons are localized on atoms or between pairs of atoms, in molecular orbital theory all valence electrons are delocalized over the whole molecule, not confined to individual bonds. 

The lone pairs on the oxygen atom in disiloxane, disilaoxirane, and 1,3 -cyclodis-iloxane have been shown [131] by the bond model analysis [132-134] to delocalize significantly to the silicon atoms throngh the interaction of the n-orbital... 

In the 7r-bonding model, diamides and triamides are expected to show longer M—N distances than their monoamido counterparts, as there are two or three nitrogen lone pairs competing for just one empty p-orbital... 

Once computed on a 3D grid from a given ab initio wave function, the ELF function can be partitioned into an intuitive chemical scheme [30], Indeed, core regions, denoted C(X), can be determined for any atom, as well as valence regions associated to lone pairs, denoted V(X), and to chemical bonds (V(X,Y)). These ELF regions, the so-called basins (denoted 2), match closely the domains of Gillespie s VSEPR (Valence Shell Electron Pair Repulsion) model. Details about the ELF function and its applications can be found in a recent review paper [31],... 

Wang CS, Yang ZZ (1999) Atom-bond electronegativity equalization method. II. Lone-pair electron model. J Chem Phys 110(13) 6189-6197... 

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