= Solution
Trialkylaluminium compounds commonly form <dimers> with two bridging alkyl groups and four terminal alkyl groups. Each electron-deficient bridge is a <three-centre two-electron bond> spanning Al–C–Al; the carbon–metal bond <electrons> are shared with the second aluminium center. A localized ordinary two-center bond to each Al would overcount the <electrons>.
For $\mathrm{[R_2AlH]_2}$, \b[hydride bridges are expected to be especially favorable], giving a four-membered Al–H–Al–H core with two terminal alkyl groups on each Al. The Al–H–Al links are again <three-centre two-electron bonds>. Hydride is small and can bridge without the steric cost of a bulky alkyl bridge. Aggregation still depends on substituent size, donor solvent and concentration; this is the natural dimeric model, not a statement that all dialkylaluminium hydrides are dimers under every condition.
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