LEWIS BONDS

In the MOVB method, we use one Slater determinant with block-localized molecular orbitals to define individual VB configuration, called diabatic state. For example, the reactant state of the Sn2 reaction between HS- and CH3CI is defined as the Lewis bond structure of the substrate CH3CI ... 

As its name implies, AIM enables us to calculate such properties of atoms in a molecule as atomic charge, atomic volume, and atomic dipole. Indeed it shows us that the classical picture of a bond as an entity that is apparently independent of the atoms, like a Lewis bond line or a stick in a ball-and-stick model, is misleading. There are no bonds in molecules that are independent of the atoms. AIM identifies a bond as the line between two nuclei. 

Bond paths are observed between bonded atoms in a molecule and only between these atoms. They are usually consistent with the bonds as defined by the Lewis structure and by experiment. There are, however, differences. There is only a single bond path between atoms that are multiply bonded in a Lewis structure because the electron density is always a maximum along the internuclear axis even in a Lewis multiple bond. The value of pb does, however, increase with increasing Lewis bond order, as is shown by the values for ethane (0.249 au), ethene (0.356 au), and ethyne (0.427 au), which indicate, as expected, an increasing amount of electron density in the bonding region. 

As an example, in Figure 3.14 the spectra of pyridine molecularly adsorbed on P-Ca203 are shown. The spectra are due to molecular Lewis-bonded pyridine, without any absorption assignable to pyridinium ions. It is evident that the 8a and 19b pyridine bands (1580 and 1438 cm" in the liquid) are shifted significantly... 

The Lewis bonding model with its electron pairs can be used to define a more general kind of acid-base behavior of which the Arrhenius and Bronsted-Lowry definitions are special cases. A Lewis base is any species that donates lone-pair electrons, and a Lewis acid is any species that accepts such electron pairs. The Arrhenius acids and bases considered so far fit this description (with the Lewis acid, H, acting as an acceptor toward various Lewis bases such as NH3 and OH , the electron pair donors). Other reactions that do not involve hydrogen ions can still be considered Lewis acid-base reactions. An example is the reaction between electron-deficient BF3 and electron-rich NH3 ... 

The probe has been largely used because it gives well defined bands of pyridinium ion at 1540 cm l and of Lewis bonded pyridine at ca. 1450 cm-l for A1 Lewis acidity or at around 1430-1440 cm l for interaction with cations (10). The only disadvantage is that it can not enter very small pores. 

Fig. 12 The extension of the Lewis ideas for hypo-valent and h3rper-valent molecules. In the former, the electron deficiency is relieved by the formation of dative bonds either from lone pairs or B-H bonds. In the latter, the hyper-valency is brought into line with the EAN rule by using Lewis bond structures where the central atom bears positive charges which compensate the negative charges on the fluorines. The results in Table 1 suggest that x is associated with the number of three-centre four-electron Xe-F bonds in these structures...

These examples illustrate the way in which chemists have modified the formal two-centre two-electron Lewis bond representatimis in order to extend the descrip-ti(Mi to hypo- and hyper-valent compounds. It involves an extension of the ideas to encompass sharing of electron pairs between three rather than two atoms and the use of resonance in order to match the canonical forms to the symmetry of the molecule. For hypo- and hyper-valent compounds, the octet rule may be attained by forming multicentred or supplementary dative bonds. 

The Lewis bonding model stiU remains remarkably fruitful when it becomes connected with quantum chemical calculations even 100 years after its first presentation. 

Structural chemistry displayed by polynuclear carbonyl complexes, especially those have bridging carbonyls, requires the application of molecular orbital theory to understand the details of the electron delocalization and backbonding interactions, before plausible conclusions can be drawn about how the system should be described in Lewis pair theory. In this review, an expansion to Lewis bonding theory concerning system containing bridging carbonyls and its derivatives is introduced, largely based on the contemporary theoretical and computational work. 

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