Lewis electron-pair model

The physical basis of the Lewis electron pair model... 

G.A. GALLUP, The Lewis electron-pair model, spectroscopy and the role of the orbital picture describing the electronic structure of molecules. 

The fact that the dihalides form stable molecules, that the monohalides are much less stable than the dihalides, and that tri- and higher halides are unknown, is, of course, in perfect agreement with the Lewis electron pair model. Each of the two atoms forming the bond contributes one electron to a bond pair that moves in the space between and around the two kernels X M X. 

We shall refer to a Group 13 atom that forms three single bonds as predicted by the Lewis electron pair model as Lewis-valent and to a Group 13 atom that forms just one single bond as subvalenl. M-Cl bond distances and bond dissociation energies of the gaseous... 

The fact that the Group 14 elements form four single bonds is in agreement with the Lewis electron pair model. In these compounds the central atom is surrounded by four bonding electron pairs and the structure is tetrahedral in agreement with the VSEPR model. The observation that the E-Cl bonds in the tetrachlorides are shorter than in the dichlorides is, however, difficult to rationalize in terms of the VSEPR model. 

Even though the nature of the bonding in the dimetallenes is well understood, the properties of digermenes and distannenes leave us in a terminological quandary should we describe the E-E bonds in these molecules as double and accept that a double bond may be weaker than a single bond between the same atoms, or should we describe the bonds as single and accept that Lewis electron pair model has failed ... 

He, somewhat mischievously, made the following comment on the relationship between his molecular orbital analysis and the Lewis electron-pair model "Now I have a favourite argument that Lewis electron pair bonding is better described by a pair of electrons in a molecular orbital than by the Heitler-London bond. If the chemical bond has any polarity, it is necessary to add an ionic term, that is a Heitler-London plus an ionic term, to represent the bond. That is rather a messy description whereas the molecular orbital- this is not the spectroscopic but the chemical molecular orbital, the delocalized molecular orbital fits very nicely to the... 

However, a descriptive outline of the theory is presented at the beginning of Section 3. Section 4 introduces the properties of the laplacian of the electron density distribution and its relation to the Lewis electron pair model. 

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