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. 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(1-6), it is a challenge to isolate early-main-group s-block metals in their zero oxidation state(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 Mg2+ cations, the Mg-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+ to Na-0. We also present a complex with a linear Mg-3 core that formally could be described as a Mg-I-Mg-0-Mg-I unit. Such multinuclear mixed-valence Mg-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.