Reversible addition of ethene to gallium(<scp>i</scp>) monomers and dimers

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[double bond, length as m-dash]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 <i>via</i> 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.