Reversible interactions of organic substrates with transition metal compounds are a hallmark of their chemistry and its catalytic applications, but remain uncommon for low-valent p-block compounds. We report here the preparation of amidophosphine-supported gallium(I) compounds that exhibit equilibria between monomeric gallylene and dimeric digallene (Ga=Ga) states. The monomer-dimer equilibrium is controlled by the steric and electronic properties of the phosphine donor in the ligand employed. Regardless of their preference for monomeric or dimeric state, reactions of the Ga(I) systems with B(C6F5)3 affords monomeric gallylene adducts, whilst reactions with ethene produce 1,2-digallacyclobutanes via formal [2+2] cycloadditions. These ethene additions are reversible, with the digallacyclobutanes releasing ethene upon heating or treatment with Lewis acids to regenerate the gallium(I) species
