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Author(s): B. Rösch [sup.1] , T. X. Gentner [sup.1] , J. Eyselein [sup.1] , J. Langer [sup.1] , H. Elsen [sup.1] , S. Harder [sup.1] Author Affiliations: (1) Department of [...], A complex of a metal in its zero oxidation state can be considered a stabilized, but highly reactive, form of a single metal atom. Such complexes are common for the more noble transition metals. Although rare examples are known for electronegative late-main-group p-block metals or semimetals.sup.1-6, it is a challenge to isolate early-main-group s-block metals in their zero oxidation state.sup.7-11. This is directly related to their very low electronegativity and strong tendency to oxidize. Here we present examples of zero-oxidation-state magnesium (that is, magnesium(0)) complexes that are stabilized by superbulky, monoanionic, [beta]-diketiminate ligands. Whereas the reactivity of an organomagnesium compound is typically defined by the nucleophilicity of its organic groups and the electrophilicity of Mg.sup.2+ cations, the Mg.sup.0 complexes reported here feature electron-rich Mg centres that are nucleophilic and strongly reducing. The latter property is exemplified by the ability to reduce Na.sup.+ to Na.sup.0. We also present a complex with a linear Mg.sub.3 core that formally could be described as a Mg.sup.I-Mg.sup.0-Mg.sup.I unit. Such multinuclear mixed-valence Mg.sub.n clusters are discussed as fleeting intermediates during the early stages of Grignard reagent formation. Their remarkably strong reducing power implies a rich reactivity and application as specialized reducing agents. Strongly reducing [beta]-diketiminate complexes containing magnesium in its zero oxidation state are reported, among which is a compound with a linear triatomic Mg-Mg-Mg core.