This study details syntheses of unsymmetrical magnesium(I)-adduct complexes, [(ArNacnac)(D)Mg-Mg(ArNacnac)] (ArNacnac = [(ArNCMe)2CH]-), 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 [{(DipNacnac)(D)Mg(μ-C3O3)Mg(DipNacnac)}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. © 2019 American Chemical Society.