This study details the formal hydrogenation of two magnesium(I) dimers {(Nacnac)Mg}(2) (Nacnac = [{(C6H3R2-2,6)NCMe}(2)CH](-); R = Pri ((Dip)Nacnac), Et ((Dep)Nacnac)) using 1,3-cyclohexadiene. These reactions afford the magnesium(II) hydride complexes, {(Nacnac)Mg(mu-H)}(2). Their reactions with excess CO are sterically controlled and lead cleanly to different C-C coupled products, viz. the ethenediolate complex, ((Dip)Nacnac)Mg{kappa(1)-O-[((Dip)Nacnac)Mg(kappa(2)-O,O-O2C2H2)]}, and the first cyclopropanetriolate complex of any metal, cis-{((Dep)Nacnac)Mg}(3){mu-C-3(H-3)O-3}. Computational studies imply the CO activation processes proceed via very similar mechanisms to those previously reported for related reactions involving f-block metal hydride compounds. This work highlights the potential magnesium compounds hold for use in the Fischer-Tropsch-like transformation of CO/H-2 mixtures to value added oxygenate products.