This study details syntheses of unsymmetrical magnesium(I) adduct complexes, [((Ar)Nacnac)-(D)Mg-Mg ((Ar)Nacnac)] ((Ar)Nacnac = [(ArNC-Me)(2)CH](-)), Ar = xylyl (Xyl), mesityl (Mes), 2,6-diethylphenyl (Dep), or 2,6-diisopropylphenyl (Dip); D = N-heterocyclic carbene or 4-dimethylaminopyridine, DMAP), which X-ray crystallographic studies show to have markedly elongated Mg-Mg bonds. Two of these highly reactive species are shown to reductively trimerize CO to yield rare crystallographically characterized examples of the planar, aromatic deltate dianion, incorporated in the complexes [{((DiP)Nacnac)(D)Mg(mu-C3O3)Mg((DiP)Nacnac)}(2)] (D = DMAP or :C{N(Me)C-(Me)}(2)). DFT calculations suggest that these complexes form via stepwise two-electron reductions of three CO molecules, resulting in the formation of three C C bonds within the cyclic deltate unit. This work highlights the utility of activated magnesium(I) adduct complexes as soluble organometallic models for the study of reductive C C bond-forming events in, for example, the heterogeneously catalyzed Fischer-Tropsch process.