Gallium alkyl hydrides

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. 

Donor adducts of aluminum and gallium trihydride were the subject of considerable interest in the late 1960s and early 1970s.1 Thin-film deposition and microelectronic device fabrication has been the driving force for the recent resurgence of synthetic and theoretical interest in these adducts of alane and gallane.24 This is directly attributable to their utility as low-temperature, relatively stable precursors for both conventional and laser-assisted CVD,59 and has resulted in the commercial availability of at least one adduct of alane. The absence of direct metal-carbon bonds in adducts of metal hydrides can minimize the formation of deleterious carbonaceous material during applications of CVD techniques, in contrast to some metal alkyl species.10, 11... 

Lithium gallium hydride reduces effectively both primary and secondary alkyl bromides and iodides to the corresponding hydrocarbons (Table 41).435... 

Table 41 Reduction of alkyl bromides with lithium gallium hydride. Reproduced with permission from the Korean Chemical Society...

Tris[ (l S,2i )-6,6-dimethylbicylo[3.1.1]heptan-2-yl methyl]gallium reacts with ketones above room temperature, and optically active alcohols are obtained as main products (Scheme 145).438 LiGaH4, in combination with an S,0-chelate ligand, 2-hydroxy-2 -mercapto-l,T-binaphthyl (MTBH2), forms an active hydride catalyst for an asymmetric reduction of prochiral ketones with catecholborane. Enantiofacial differentiation is based on the steric requirement of the ketone substituents. Aryl//z-alkyl ketones are reduced in 90-93% ee and branched ketones RC(0)Me (e.g., R = Pr , oC6H11 Bu ) in 60-72% ee (Table 43).439 440... 

Aluminum alkyls, alkylaluminum halides, and alkylalu-minum hydrides strongly associate in solution and in the solid state.The association of trialkylaluminum compounds is in stark contrast to the monomeric nature of trialkylboron see Boron Organoboranei) and trialkylgallium see Gallium Organometallic Chemistrj compounds. 

Aluminum alkyls are stronger Lewis acids than the trialkyls of boron and gallium, and complex formation in the presence of Lewis bases is an important aspect of the reaction chemistry of organoaluminum compounds. The Lewis acidity of organoaluminum compounds accounts for the strong association of aluminum alkyls and hydrides by three-center two-electron bonds, as well as association of organoaluminum... 

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