1:1 Equimolar mixtures of Me3Al, Me3Ga, or Me3In and benzylamine, when heated to 90-120 °C in toluene solution, undergo methane elimination to yield the respective dimers, [Me2MN(H)CH2Ph]2, where M = Al, Ga, or In. These dimers exist as 1:1 mixtures of cis and trans isomers in toluene-d8 solution, over the temperature range 30-90 °C, consistent with the results of Beachley  for the aluminum derivative. However, single-crystal X-ray data indicate the dimers each crystallize in the trans form. The structure of each dimer exhibits a center of symmetry and each M2N2 core has a slightly distorted square planar geometry. The 1H- and 13C-NMR chemical shift assignments were made for the cis and trans isomers of the three compounds using a combination of 1D- and 2D-NMR techniques. Thermolysis of the 1:1 mixtures at higher temperatures (120-180 °C) generates the more condensed hexameric species, [MeAlNCH2Ph]6 and [MeGaNCH2Ph]6. [MeAlNCH2Ph]6 crystallizes in a rhombohedral structure where the A1-N cage consists of two six-membered (AlN)3 rings linked together by six transverse Al-N bonds. The results of these thermolyses involving benzylamine are compared with the results of analogous reactions employing dibenzylamine, where an orthometalated product, [MeMN(CH3Ph)-μ- (CH2C6H4)]2, (M = A1, Ga), is obtained. © 2002 Elsevier Science B.V. All rights reserved.