The insertion of 1,1-diphenylethylene into the Mg-Mg bond of two magnesium(I) dimers, [(Nacnac)Mg-] (Ar = CHMe-2,4,6 (Mes); CHEt-2,6 (Dep)), yielding 1,2-dimagnesioethane products, [{(Nacnac)Mg}(μ-CHCPh)], is described. These reactions are readily reversible at room temperature and thus represent the first examples of room-temperature reversible redox processes for s-block metal complexes. The 1,2-dimagnesioethane products are highly activated magnesium alkyls and show unprecedented, uncatalyzed reactivity toward H, CO, and ethylene. Computational studies have investigated the mechanisms of all presented reaction types.