Hexamethyldialane bridging methyl groups

The dissociation energy of (CH3)4Al2(ii - CH3)2, defined as the energy of the reaction 

Triethylaluminum and other trialkylaluminum compounds also form alkyl-bridged dimers, but the dissociation energies of the dimers decrease with increasing bulk of the alkyl groups the dissociation energy of dimeric triethylaluminum is 69 kJ mol and the dissociation energy of dimeric tri-iso-butylaluminum 32 kJ moP. The latter is 60% dissociated as a pure liquid at 10°C. Tri-tert-butylaluminium appears to be 100% monomeric under all conditions, there is probably not room for four so voluminous alkyl groups around an A1 atom. 

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