Computational design of species with ultrashort Be-Be distances using planar hexacoordinate carbon structures as the templates

A current project in metal-metal bonding chemistry is to achieve ultrashort metal-metal distances (USMMDs, denoted by d_M-M < 1.900 Å) between main group metal beryllium atoms. A valid way for achieving such USMMDs is the substitution of a carbon atom in a planar pentacoordinate environment with the isoelectronic Be₂ moiety. In the present work, we report our recent findings that a similar substitution can be applied to the carbon atom in a planar hexacoordinate environment. Using species CN₃Be₃⁺ and CO₃Li₃⁺ and related analogues as the templates, the Be₂N₃M₃⁺ (M = Be, Mg, Ca) and Be₂O₃M₃⁺ (M = Li, Na, K) species with axial ultrashort Be-Be distances of 1.627-1.870 Å were designed computationally. The ultrashort Be-Be distances in these species represent a balance between the lengthening effect of axial Be-Be electrostatic interactions and the shortening effects of the strong X-Be bonding and repulsive X-X-X electrostatic interactions. In addition, the shorter axial Be-Be distances were determined firstly by the smaller size of the bridging electronegative X atoms and secondly by the lower electronegativity of the peripheral M atoms, while the stabilities of the newly designed species were closely related to the types of valence electron pairs, whereby the localized two-center two-electron bonds were better for stabilization than the non-bonding valence lone pairs. Among the newly designed species, Be₂N₃Be₃⁺ and Be₂N₃Mg₃⁺ were characterized to be the kinetically stable global minima, thereby providing promising targets for the experimental realization of species with USMMDs between main group metals.