Designer Mg−Mg and Zn−Zn single bonds facilitated by double aromaticity in the M₂B₇⁻ (M=Mg, Zn) clusters^†

The simple homodinuclear M-M single bonds for group II and XII elements are difficult to obtain as a result of the fulfilled s² electronic configurations, consequently, a dicationic prototype is often utilized to design the M⁺ −M⁺ single bond. Existing studies generally use sterically bulky organic ligands L⁻ to synthesize the compounds in the L⁻ −M⁺ −M⁺ −L⁻ manner. However, here we report the design of Mg-Mg and Zn-Zn single bonds in two ligandless clusters, Mg₂B₇⁻ and Zn₂B₇⁻, using density functional theory methods. The global minima of both of the clusters are in the form of M₂²⁺(B₇³⁻), where the M-M single bonds are positioned above a quasi-planar hexagonal B₇ moiety. Chemical bonding analyses further confirm the existence of Mg-Mg and Zn-Zn single bonds in these clusters, which are driven by the unusually stable B₇³⁻ moiety that is both σ and π aromatic. Vertical detachment energies of Mg₂B₇⁻ and Zn₂B₇⁻ are calculated to be 2.79 eV and 2.94 eV, respectively, for the future comparisons with experimental data.