Boron and Aluminum Compounds

The analogy between the trivalent boron compounds and car-bonium ions extends to the geometry. Although our arguments for a preferred planar structure in carbonium ions are indirect, there is electron diffraction evidence for the planar structure of boron trimethyl and the boron trihalides.298 Like carbonium ions, the boron and aluminum analogs readily form a fourth covalent bond to atoms having the requisite non-bonding electrons. Examples are the compounds with ammonia, ether, and fluoride ion.297 

The formation of the etherate from tri-/ -tolylaluminum is exothermic to the extent of about 20 kcal./mole.299 

Groups attached to aluminum or boron in this way have the expected increased electrophilic and decreased nucleophilic reactivities. For example, the complex of aluminum triphenyl and phenyl lithium lacks the usual Grignard-like reactivity of phenyl lithium towards-carbonyl compounds.800 

An instance of increased electrophilic reactivity is the behavior of complexed tetrahydrofuran.298-800 

A particularly interesting case is that of the bridgehead compounds of boron.801 Attempts to make the compound LI gave only 

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Stereoelectronic factors are also important in determining the stmcture and reactivity of complexes. Complexes of catbonyl groups with trivalent boron and aluminum compounds tend to adopt a geometry consistent with directional interaction with one of the oxygen lone pairs. Thus the C—O—M bond angle tends to be in the trigonal (120-140°) range, and the boron or aluminum is usually close to die carbonyl plane. ... 

Carbocations occupy a unique place in the world of electron-deficient cations. Electron-deficient nitrogen and oxygen compounds (nitrenium and oxenium ions, respectively) are extremely unstable and are rarely or never seen. Electron-deficient boron and aluminum compounds are quite common, but these species are not cations. Silenium ions (R3Si+) are extremely unstable kinetically still, it is useful to think of the R3Si+ group as a large, kinetically more stable H+. 

The number of species with incomplete octets is limited to some beryllium, boron, and aluminum compounds. Perhaps the best examples are the boron hydrides. Bonding in the boron hydrides will be discussed in Chapter 22. 

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