Strongly reducing magnesium(0) complexes.

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, β-diketiminate ligands. Whereas the reactivity of an organomagnesium compound is typically defined by the nucleophilicity of its organic groups and the electrophilicity of Mg ²⁺ 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.