Reaction of GaMe3 with N,N-dimethylethylenediamine (DMEDA) produces adducts, Me3Ga:NH2C2H4NMe2 (1:1, 1) and Me3Ga:NH2C2H4NMe2:GaMe3 (2:1, 2). A fast concerted intermolecular: exchange of the two inequivalent GaMe3 moieties occurs in 2 at room temperature. Thermolysis of 2 at 210 degreesC in a sealed ampule produces a mixture of cis and trans isomers of tetrametallic imidogallane, [Me2Ga(mu -NC2H4NMe2GaMe2)](2) (3). Slow recrystallization of 3 affords only the cis isomer as colorless crystals, but in solution the cis isomer equilibrates with the trans isomer. The cis --> trans isomerization of 3 has been studied by H-1 NMR spectroscopy. The equilibrium has been observed to follow reversible first-order kinetics with DeltaH degrees = - 3.35 +/- 1.70 kJ mol(-1) and DeltaS degrees = - 10.4 +/- 1.7 J mol(-1) K-1. The activation parameters for the cis --> trans conversion are DeltaH(1)double dagger = 144.6 +/- 6.2 kJ mol(-1) and Delta S(1)double dagger = 105.6 +/- 1.5 J mol(-1) K-1, and those for the reverse process are DeltaH(-1)double dagger = 147.6 +/- 6.4 kJ mol(-1) and DeltaS(-1)double dagger = 115.4 +/- 1.5 J mol(-1) K-1. The isomerization process has been proposed to be assisted by the NMe2 end of DMEDA as an internal Lewis base. The molecular structures of 2 and cis-3 have been determined by single-crystal X-ray diffraction studies. The geometry about the gallium atom of 2 is distorted tetrahedral with a Ga-C (av) distance of 1.98(1) Angstrom and a Ga-N distance of 2.16(1) Angstrom. The molecular geometry of cis-3 involves one four-membered Ga2N2 ring and two five-membered GaN2C2 rings of cis conformation, and it has approximate C-2 symmetry with the C-2 axis going through the center of the Ga2N2 ring.