Electron Deficient Compounds. VII. The Structure of the Trimethylaluminum Dimer

Abstract
The trimethylaluminum dimer has a bridge structure with a skeletal symmetry of D2h within the accuracy of a structure determination by x‐ray diffraction of single crystals. Bridge bonds, Al–C, equal 2.24A, vs Al–C bonds of 1.99A for the exterior, ``normal'' methyls. The bridge angle Al–C–Al=70°, the exterior angle C–Al–C=124°. It is shown that a simple model provides the right order of magnitude for the heat of dimerization and indicates the relative importance of Al–C and Al–Al bonds. It is suggested that the sharp bridge angle required for good bridge bonding leads to metal‐metal repulsions which should increase as the size of the metal increases. This probably plays a role in the instability and nature of polymers of the heavier trialkyl‐metal compounds.