Dichlorogallane [HGaCl₂]₂1 can be prepared in quantitative yield from equimolar quantities of GaCl₃ and Et₃SiH at −13 °C in the absence of a solvent. Treatment of 1 with dilithium N,N′-di-tert-butyl-cis-ethenediamide at −78 °C leads to high yields of [(^tBu₂N₂(CH)₂)GaH]₂2. According to IR and NMR observations, compound 2 exists in the form of two isomers in benzene solution. Crystallization from pentane affords only one isomer which was identified as a centrosymmetrical dimer with a four-membered ring (Ga–N)₂ as the core unit, two hydrogen atoms in trans position and two five-membered rings fused to opposite edges also in trans position (type B). The analogous reaction of [^tBu₂N₂(CH)₂]Li₂ with anhydrous [GaCl₃]₂ gives the corresponding chloride 3 which exists as only one isomer in solution. Crystals obtained from hexane contain four independent dimeric molecules, all of which have the same structure with inequivalent gallium atoms owing to a redistribution of the chloride ligands. The central four-membered ring (Ga–N)₂ has the two five-membered rings fused to neighbouring edges in trans position, and the two chlorine atoms are associated with the same gallium atom. This isomer [^tBu₂N₂(CH)₂]Ga(GaCl₂) 3 (type D) is unprecedented in the structural chemistry of the “diazabutadiene” complexes of Group 13 elements. The analogous cyclohexyl compound was prepared in high yield following the same route. It is formed as only one isomer which has the centrosymmetrical structure of type B, [(^cHex₂N₂(CH)₂)₂GaCl]₂ (^cHex = cyclohexyl). The isomerism is probably governed by steric effects.