Bonding considerations Lewis structures

The foregoing considerations concerning CO bonds must clearly apply to all other bonds to oxygen, which in a Lewis structure are described as either single, double, or triple (including, e.g., BO, NO, PO, SO and CIO bonds). They also apply to bonds to nitrogen, which similarly are described as single, double, or triple. 

The nature of the bonding in this molecule has been the cause of considerable discussion. Its short length (112.8 pm) and its great strength (bond dissociation enthalpy 1072 kJ mol- ) are consistent with the usual triple-bond Lewis structure... 

Here, p(r) is the average density of atoms found in a thin shell at a radius r from an arbitrary atom in the material, and p is the average density of the entire material. For very small values of r, g(r) —> 0, since atoms cannot overlap one another. For large values of r, on the other hand, g(r) —> 1, because atoms that are separated from one another by large distances in a disordered material are not influenced by one another. The distribution functions calculated by Lewis et al. for liquid and amorphous InP are shown in Fig. 9.4. As might be expected, the amorphous material has considerably more structure than the liquid. One important feature in the amorphous material is the peak in the P-P distribution near r 2.2 A. This peak shows the existence of P-P bonds in the amorphous material, a kind of bond that does not exist in the crystalline solid. 

A 1,3-dipole is a compound of the type a—Het—b that may undergo 1,3-dipolar cycloadditions with multiply bonded systems and can best be described with a zwitterionic all-octet Lewis structure. An unsaturated system that undergoes 1,3-dipolar cycloadditions with 1,3-dipoles is called dipolarophile. Alkenes, alkynes, and their diverse hetero derivatives may react as dipolarophiles. Since there is a considerable variety of 1,3-dipoles—Table 15.2 shows... 

The pi MO s of the cyclopentadienide ion are instructive to consider because they not only illustrate the use of d and f GO s, but the bonding considerations in this ion are crucial to the treatment of ferrocene, our next example. The electron dash structure of C5Hf1 leads to five Lewis structures suggesting that there are six pi electrons which can circulate around the ring in the delocalized view. In Fig. 16 are shown tjie GO s and the pi SO s generated by them. Because there are no other atoms, the five SO s become the five MO s expected because there are five carbon AO s available for pi bonding. 

For many molecules or ions, the actual connectivity of the atoms may not be obvious and more information may be needed to choose the best among several possible Lewis structures. Compounds containing single, double, and triple bonds are described in the rest of this chapter, with consideration given to choosing among several possible Lewis structures. 

In Chapter 9, we discussed bonding in terms of the Lewis theory. Here we will stndy the shape, or geometry, of molecnles. Geometry has an important influence on the physical and chemical properties of molecules, such as density, melting point, boihng point, and reactivity. We will see that we can predict the shapes of molecules with considerable accuracy using a simple method based on Lewis structures. 

As we have seen, Lewis structures are a convenient way of showing the covalent bonds in many molecules or ions of the representative elements. In writing Lewis structures, the most important consideration for forming a stable compound is that the atoms attain a noble gas configuration. 

Many A reactions and oxidative additions lead to so-called hypervalent products. Sn2-S1 intermediates are also generally hypervalent that is, they contain main-group centers with more than eight valence electrons around them in a Lewis structure. As we saw from a case study (PF5), the bonding in such systems can be readily understood in terms of simple molecular orbital considerations. No special considerations are involved in applying arrow pushing to hypervalent compounds. 

These considerations imply that we may generate an "increased-valence" structure (V) from the standard Lewis structure (VI) by delocalizing one electron of the lone-pair of atom C into a vacant bonding BC orbital... 

Catalytically active sites apparently differ considerably in structure and nature between the reactions catalyzed. For example, dissociation of H2 and CH4 seems to be catalyzed by highly strained sites with high gradient of electrostatic field these are produced by severe dehydroxylation as described in the previous section. The carban-ionic nature of the intermediates for the dissociation of C — H bond of alkanes indicates that the active sites have basic character. On the other hand, strong Lewis acid plays an important role in skeletal isomerization of hydrocarbons, and impurity contained in alumina such as alkali strongly affects the activity. In some reactions both acidic and basic sites take part conceitedly. If one considers the high stability of C — H bonds of alkanes, it may be more reasonable to assume that the dissociation mentioned above is actually made possible by the concerted action of the basic and acidic sites of AhOa. 

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