Reactions of two magnesium(I) compounds, [{(ArNacnac)Mg}2] (ArNacnac=[HC(MeCNAr)2]−; Ar=mesityl (Mes) or o-xylyl (Xyl)), with CO in the presence of [Mo(CO)6] lead to the reductive hexamerization of CO and formation of magnesium benzenehexolate complexes, [{(ArNacnac)Mg}6(C6O6)]. [Mo(CO)6] is not consumed in these reactions, but is apparently required to initiate (or catalyze) the CO hexamerizations. A range of studies were used to probe the mechanism of formation of the benzenehexolate complexes. The magnesium(I) reductive hexamerizations of CO are closely related to Liebig's reduction of CO with molten potassium (to give K6C6O6, amongst other products), originally reported in 1834. As the mechanism of that reaction is still unknown, it seems reasonable that magnesium(I) reductions of CO could prove useful homogeneous models for its elucidation and for the study of other C−C bond forming reactions that use CO as a C1 feedstock (e.g. the Fischer–Tropsch process). © 2020 Wiley-VCH GmbH