Beryllium-beryllium double-π bonds in the octahedral cluster of Be(μ-X) (X = Li, Cu, BeF).

Metal-metal double-π bonding in s-block metals has not been reported until now. The octahedral clusters of Be(μ-X) only possessing degenerate Be-Be double-π bonds were first achieved by an innovative concerted electron-donating strategy from the vertical plane of the Be-Be midpoint using four s-type electron-donating ligands. Moreover, this novel strategy is universal and independent of electronegativity. Three typical clusters were discussed herein including Be(BeF), BeLi and BeCu. Especially, WBI in BeLi reached up to 1.96. The significant π-bonding character was confirmed by various bond analysis strategies.