Structural aspects of the metal alkyls

Some of the alkyls of the elements of the first three groups are covalently bound monomers, while others are associated electron-deficient molecules , and others may have ionic constitutions. 

No monomeric alkali metal alkyls or aryls are known, as those crystal structures which have been determined indicate electron-deficient, e.g. (MeLi), or ionic (K Me ) constitutions. The dialkyls of the lighter second group metals are mostly electron-deficient dimers or polymers, but those of zinc, cadmium and mercury are monomers with a linear structure as expected from participation of one (metal) s and one p orbital (with or without dji participation). In the third group the pattern is more complex. Whereas the trialkylboranes are monomeric, boron hydrides (and alkyl hydrides) and polyboron compounds form electron-deficient structures. Aluminium alkyls and alkyl hydrides are normally electron-deficient dimers or trimers gallium trialkyls are monomeric though the trivinyl is a dimer trimethylindium is a weakly associated tetramer in the solid state, otherwise all indium and thallium trialkyls appear to be monomers. 

Monomeric trimethylborane has a vacant 2p orbital on the boron, which may interact weakly with the methyl groups. It is believed that steric reasons are the main obstacle to dimerization forming Me B2. For example, the C—C distance across the four-membered ring in the dimer would be only 2-9 A, which is very short, and the B— B distance would be still less. 

Proton magnetic resonance spectra of Me Al2 should distinguish between the two different kinds of methyl group. However, at room temperature only a single resonance is observed owing to the rapidity of methyl exchange, but at —75° (in solution) there are two resonances in 2 1 area ratio. At about —20° the mean lifetime of a particular configuration is only 3 milliseconds. 

Since it appears that only the monomeric alkyls enter into the important reactions with olefins that are discussed below, p 93, the addition of MegM2 to other alkyls decreases their reactivity to olefins by converting them to dimers. 

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