Compounds of Alkyl and Aryl Alanes

As compounds of coordinatively unsaturated trivalent aluminum, the organoaluminum compounds are Lewis acids and will combine with Lewis bases to form molecular compounds or complex anions. 

Cations with complexed aluminum of the type known among organoboron compounds, e.g., 

Suitable electron donators for organoalanes are amines (especially tertiary amines), ethers, or anions such as, for example, hydride, alkyl, alkoxy, or halogen. When organoaluminum compounds combine with these electron donors, compounds of tetravalent aluminum are formed. Stable anions with higher coordination numbers, i.e., analogs of [AlFe]3-, have not been observed so far among organoaluminum compounds. 

As most alkyl alanes are dimers or trimers (cf. Section III,B,1), complex formation with Lewis bases must be preceded by dissociation of the association complex, or the latter process must at least occur simultaneously. The tendency to form a complex anion according to 

Trialkyl and triaryl alanes give stable 1 1 adducts with neutral donor molecules (e.g., ethers, thioethers, tertiary amines, tertiary phosphines)  

---