Figure 1 . Optimized geometries of the radical anions featuring 2 centre 1 electron bonds. Bond lengths are in Å and Wiberg bond indices (WBI) are given within parenthesis. Relative energies are in kJ/mol. T1 diagnostic values of the CCSD(T)/def2-TZVP calculations are given within parenthesis.
The electronic structure of these radical anions were analyzed with adaptive natural density partitioning (AdNDP) scheme.16 AdNDP is a powerful tool which helps in understanding the distribution of electron pairs involving single, double and multiple centres. It provides unique identification of n-centre-n-electron bonds where the highest value of n is the total number of atoms in the molecule. Figure 2 shows the Be-Be and Zn-Zn bonds recovered using AdNDP scheme. AdNDP analyses identifies the presence of 2 centre 1-electron Be-Be and Zn-Zn bonds in all these molecules with occupation number close to 1.0 |e|. Analyses of the spin density also reveals that the location of the spin density is between two Be and Zn atoms in their lowest energy structures indicating the formation of one electron bond between them. In contrast, the spin density is mostly localized in the C-Be bonds in the bond stretch isomers of 1,2-diBeX anion radicals (Figure 3).