Boron, bonding in compounds

The nature of the chemieal bonding in boron compounds is governed by the well-known two-electron-three-center bond, i.e., three boron atoms share two common... 

This diagonal relationship is not readily understood and cannot be interpreted in terms of charge density since the bonding in boron compounds and in silicon compounds is exclusively covalent. The elements are, however, both metalloids, have similar electronegativities, and have similar sizes leading to similar chemical behavior. 

Boranes are typical species with electron-deficient bonds, where a chemical bond has more centers than electrons. The smallest molecule showing this property is diborane. Each of the two B-H-B bonds (shown in Figure 2-60a) contains only two electrons, while the molecular orbital extends over three atoms. A correct representation has to represent the delocalization of the two electrons over three atom centers as shown in Figure 2-60b. Figure 2-60c shows another type of electron-deficient bond. In boron cage compounds, boron-boron bonds share their electron pair with the unoccupied atom orbital of a third boron atom [86]. These types of bonds cannot be accommodated in a single VB model of two-electron/ two-centered bonds. 

Other boron modifications have been described and also compounds such as B50C2 or B50N2, previously believed to be pure boron forms. For a detailed discussion on the bonding in boron modifications and boron-rich compounds see King (1993). 

Boron-sulfur bonds, addition, to alkynes, 10, 778 Boron trihalides, in boron compound synthesis, 9, 146 Boron-zinc exchange and copper-catalyzed substitutions, 9, 518 for organozinc halide preparation, 9, 89 Borostannylation, enynes, 10, 334... 

Covalent chemistry is the characteristic property of non-metals. Hydrogen can form only one covalent bond and in boron compounds we find an incomplete octet. 

However, since the elements boron and nitrogen are only two groups apart, we are justifiably reluctant in representing the bonds in the compound boron nitride as B3+N3. Finally it would seem to show very bad chemical intuition to represent the bonds in graphite or diamond as C+4C-4. 

Borane is an electron-deficient compound. It has only six valence electrons, so the boron atom in BH3 cannot have an octet. Acquiring an octet is the driving force for the unusual bonding structures ( banana bonds, for example) found in boron compounds. As an electron-deficient compound, BH3 is a strong electrophile, capable of adding to a double bond. This hydroboration of the double bond is thought to occur in one step, with the boron atom adding to the less substituted end of the double bond, as shown in Mechanism 8-6. 

With k being the dissociation constant (n-log k j ) of the substrate-enzyme complex for a metalloprotein with direct coordination of substrate S to the catalytic center (or in cases where nucleophilic substrates are retained by non-metal electrophiles such as aldehydes (sugars), H-bond donors, boron compounds for which c and x parameters are known) this is identical with Fq. 2.4. Thus k can be represented using x and c values, and the largest possible turnover at a given substrate... 

In nonoxide ceramics, nitrogen (N) or carbon (C) takes the place of oxygen in combination with silicon or boron. Specific substances are boron nitride (BN), boron carbide (B4C), the silicon borides (SiB4 and SiBg), silicon nitride (SisN4), and silicon carbide (SiC). All of these compounds possess strong, short covalent bonds. They are hard and strong, but brittle. Table 22.5 lists the enthalpies of the chemical bonds in these compounds. 

The end result is replacement of boron by an OH group. Because replacing boron by an OH group is an oxidation reaction, the overall reaction is called hydroboration-oxidation. An oxidation reaction increases the number of C — O, C — N, or C—X bonds in a compound (where X denotes a halogen), or it decreases the number of C — H bonds. 

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