Reaction of MMe3 (M = Al, Ga) with one equivalent of NH2NPh2 affords a dimeric complex [Me2M-mu-N(H)NPh2](2) [M = Al (1), M = Ga (2)] as a mixture of trans and cis isomers. Purification of 1 and 2 by recrystallization gives only trans isomers 1a and 2a, respectively. Variable-temperature H-1-NMR studies reveal that both 1 and 2 in solution undergo trans --> cis isomerization with free energy of activation (Delta G(c)double dagger) values of 15.9 for 1 and 18.3 kcal mol(-1) for 2. The solid state structures of trans isomers 1a and 2a have been determined by single-crystal X-ray diffraction studies. The molecular geometries of la and 2a consist of a centrosymmetric and dimeric unit (M-N)(2) with two bridging hydrazido groups and two terminal methyl groups bound to each metal atom. The two N-NPh2 groups are trans to each other with respect to the (M-N)(2) core plane. The coordination geometry of both metal and nitrogen atoms are distorted tetrahedral. (C) 1999 Elsevier Science S.A. All rights reserved.