Boron bond type

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. 

Hydroboration is a reaction m which a boron hydride a compound of the type R2BH adds to a carbon-carbon bond A carbon-hydrogen bond and a carbon-boron bond result... 

Boron resonances (Table III) appear in the range expected for sandwich-type complexes with metal-boron bonding (for reference data see Ref. 50). In cases where pairs of B-methyl and B-phenyl derivatives (as, e.g., 9 and 17) are known, the nB resonance of the methyl compound is at lower field by 2.0-3.7 ppm (average, 2.6 ppm). 

It is readily apparent that the three a bonds are capable of holding the six bonding electrons in the a t and e molecular orbitals. The possibility of some 7r-bonding is seen in the molecular orbital diagram as a result of the availability of the a2" orbital, and in fact there is some experimental evidence for this type of interaction. The sum of the covalent radii of boron and fluorine atoms is about 152 pm (1.52 A), but the experimental B-F bond distance in BF3 is about 129.5 pm (1.295 A). Part of this "bond shortening" may be due to partial double bonds resulting from the 7r-bonding. A way to show this is by means of the three resonance structures of the valence bond type that can be shown as... 

The results provide a clear indication of the importance of the change in hybridization between cyclopropane ( sp2 for the external bonds) and propane ( sp3). Electropositive groups prefer to be bonded to the more electron-withdrawing cyclopropane ring, whereas electronegative substituents prefer to be bonded to C2 of propane. The one anomaly in Table 3 is found with the BH2 substituent which leads to the most exothermic group transfer reaction. This is presumably a result of the interaction of the cyclopropane ring with the empty p orbital at boron. This type of interaction will be discussed in the next section. 

Metalated ferrocenes have served as valuable intermediates for the synthesis of a number of other derivatives. Treatment of lithiated ferrocenes with tributyl borate followed by hydrolysis leads to ferroceneboronic acid (XXXIII) as well as the diboronic acid (73). Ferroceneboronic acid, like benzeneboronic acid, is readily cleaved by cupric bromide or cupric chloride to form the corresponding halo derivatives (XXXIV). Ferrocene-l,l -diboronic acid reacts in the same manner, and either one or two carbon-boron bonds can be cleaved. Further reactions of this type have led to a variety of mixed dihaloferrocenes (73, 75). 

At low temperatures in a variety of solvents, hexaborane(lO) undergoes exchange of a maximum of five hydrogens with deuterodiborane(6) 209>. The product is shown by boron-11 and proton nmr to be substituted only at basal terminal sites. There is no precedent for an exchange of this type at low temperature for a boron hydride without BH2 groups and it has been proposed 209> that the unique short boron-boron bond in the base of the pentagonal pyramidal framework of hexaborane(lO) is involved in the reaction intermediate. 

As for hydrides, borides, and carbides, different types of nitrides are possible depending on the type of metallic element. The classifications of nitrides are similarly referred to as ionic (salt-like), covalent, and interstitial. However, it should be noted that there is a transition of bond types. Within the covalent classification, nitrides are known that have a diamond or graphite structure. Principally, these are the boron nitrides that were discussed in Chapter 8. 

The conjugate base of a cluster generated by deprotonation (see Section 5.1) is a Lewis base. Any metal-metal, metal boron, or boron boron bond is a potential Lewis base. The former have considerable nucleophilic character if anionic and effectively interact with electrophiles more complex than the proton. As the site of basicity is associated with framework bonding, this reaction type results in cluster building, for... 

Reactions of this type are somewhat less satisfactory for preparation of mixed diboron derivatives containing boron-carbon bonds. Thus, while tetrakis(dimethylamino)diborane(4) is readily hydrolyzed to tetrahydroxy-diborane(4) in aqueous acid, l,2-bis(dimethylamino)-l,2-diethylborane(4) reacts with formation of hydrogen and cleavage of the boron-boron bond 14). Reaction of the dibutyl analog with a twofold excess of water gave the dimethylamine adduct of the boroxole... 

Intermolecular reactions of this type can be slow if either of the reaction partners has a bulky substituent. Reactions with chloroboranes R2BCI or RBCI2 go under much milder conditions. The stereochemistry of the carbon-boron bond is retained in the product for example, the borane (11) gave the amine (12 90%) on reaction with azidocyclohexane. 

The classification of structural types can often be done more conveniently on the basis of valence electron counts. Various schemes for relating electron counts to structures have been proposed, with most proposals based on the set of rules formulated by Wade. " The classification scheme based on these rules is summarized in Table 15-7. In this table, the number of pairs of framework bonding electrons is determined by subtracting one B—H bonding pair per boron the n + 1 remaining framework electron pairs may be used in boron-boron bonding or in bonds between boron and other hydrogen atoms. 

The reactions of boron-substituted heteroaromatic compounds may be formally classified into two types (1) reactions relating to the carbon-boron bond, and (2) all other reactions. Very few investigations have been reported concerning the latter. 

Ethylene provides us with a second bond type. Instead of making use of sp3 hybrid orbitals as in the methane molecule, the carbon atoms in ethylene use sp2 hybrid orbitals, leaving a 2p orbital, say the 2pz orbital unused for the moment. The sp2 hybrid orbitals on each carbon atom are used to overlap with a) two hydrogen Is orbitals and (b) each other. The sp2 hybrid orbitals are arranged at 120° to one another as in the boron trichloride molecule. This is illustrated in Figure 17a. All the bonds in this framework are of the a type. 

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