Overall Bonding Scheme

These bonding schemes are examined in the following sections. 

An accurate determination of the electronic band structure and density of states is essential to obtain a precise representation of structure of these carbides and understand their bonding mechanisms and the relation between bonding characteristics and properties. The band structure is usually well characterized and experimental observations are feirly extensive for the simpler carbides such as the carbides of Groups IV (Ti, Zr, HQ and the monocarbides of Group V (V, Nb, Ta). However, the band structure for other compositions and non-stoichiometric compounds is not as thoroughly investigated and is not as well determined.  

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The geometry of the backbonding orbitals involved and the overall bonding scheme are depicted in Fig. 9. The backbonding phenomenon is general for other ligands, such as O2 and CO, as well. 

The overall bonding scheme for a carbon-carbon double bond includes both o and 7t bonds (and their empty antibonding counterparts) and is also shown in Figure 3.13. Carbon-carbon double bonds are quite short, with a typical bond distance being 1.33 A. 

We can use data from Table 10.2 to estimate the bond order for the two nitrogen-to-nitrogen bonds. From this information we can write plausible Lewis structures, and by applying VSEPR theory to the Lewis structures, we can predict a likely geometric shape of the molecule. Finally, with this information we can propose hybridization schemes for the central atoms and an overall bonding scheme for the molecule. 

Some systematic studies on the different reaction schemes and how they are realized in organic reactions were performed some time ago [18]. Reactions used in organic synthesis were analyzed thoroughly in order to identify which reaction schemes occur. The analysis was restricted to reactions that shift electrons in pairs, as either a bonding or a free electron pair. Thus, only polar or heteiolytic and concerted reactions were considered. However, it must be emphasized that the reaction schemes list only the overall change in the distribution of bonds and ftee electron pairs, and make no specific statements on a reaction mechanism. Thus, reactions that proceed mechanistically through homolysis might be included in the overall reaction scheme. 

With a reliable access to 15 at my disposal (see Scheme 5), the construction of 25 was first attempted. Thus, 15 was carefully hydrogenated over 5% Pd-C to afford the dihydro-product 51, which expectedly dehydrated to give 25 in 32% overall yield (Scheme 11 It was apparent that the least hindered double bond had been hydrogenated preferentially. 

The second category consists of starting from an appropriately mono- or di-substituted heterocycle and introducing the fifth atom, with the formation of two bonds (Scheme 49). An alternate method of construction of the second ring by forming two bonds is also shown in Scheme 49. It should be emphasized, however, that the classification involving the formation of two bonds is very arbitrary and is based on the overall synthesis. 

These reagents were introduced, respectively, by Meyers [305], Oshima [306], Trost [307] and Corey [308], and skillfully applied as shown in the syntheses of cyclobutanones [307] and a-unsaturated aldehydes [308], in which the initially formed carbon-carbon bond was the first step of the overall synthetic schemes elaborated. 

The term substitution in an unrestricted sense is rather too broad to be useful in classification of radical reactions, since most of them result in replacement of one group by another. We have already seen typical examples of bond homolysis, in which a molecule dissociates to yield two radicals which combine with each other or with another molecule. We are primarily concerned in this section with those elementary reaction steps in which a radical attacks directly an atom of another molecule (Equation 9.64), displacing from the site of attack another group, and with the overall reaction schemes in which these elementary reactions occur. 

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