Direct combination of the Lewis acids, Me3M (where M = B, Al, Ga, or In), and the diamines, Me2N(CH2)nNMe2 (where n = 1, 2, or 3) has given two series of complexes. A range of 1:1 complexes can be isolated with Me2NCH2NMe2 as ligand. With the other diamines, use of a 1:1 molar ratio of reactants yields crystalline 2:1 complexes with two moles of Me3M per mole of ligand. Excluding the reaction between Me3B and Me2NCH2NMe2, 2:1 complexes can be isolated by reacting the various Lewis acids with the appropriate molar quantity of the diamines. The complexes have been characterized by i.r. and proton n.m.r. spectroscopy and molecular weight measurements.A number of complexes of Me3Ga with methyl substituted ethylenediamines have been prepared and the effects of substitution on the nitrogen ligand atoms studied. Pyrolysis of these complexes has demonstrated the ready elimination of methane and has resulted in the formation of condensed Ga—N species.