Figure 4 . Molecular graphs of the compounds (first row for Be compounds and third row for Zn compounds) and 3D ELF plots of the compounds (second row for Be compounds and fourth row for Zn compounds). Population of the M-M disynaptic, V(M,M) (M = Be, Zn) basins are also given in |e|.
We then turned our attention to investigate the strength of M-M (M = Be, Zn) bonds. For, this we have used two isodemic equations (Scheme 2). Moreover, stability of the radical anions towards electron detachment is very important. To check the stability of the radical anions towards electron detachment, we have also calculated the adiabatic detachment energies (ADEs). All these are collected in Table 2. It is evident from Table 2 that the M-M BDEs based on both the isodesmic equations are significant suggesting strong 2-centre-1-bonds. It should also be noted that in Figure 1 the other minimum energy structures of Be compounds are higher in energy where there was no Be-Be interaction. Thus, the energy difference between the two isomers may also be taken as a direct measure of BDE of the Be-Be bonds. The BDE values obtained with this direct approach are very similar to those obtained using isodesmic equations. However, such direct measure of BDE for the Zn compounds cannot be made as we could not locate any high lying isomers for Zn compounds. In every case, the calculated BDEs are highest for X = CN. A positive value (or less negative value) of ADE suggests that the anion is less stable towards electron detachment. The calculated value of ADE for benzene is -19 kJ/mol whereas for all these radical anions studied here have significantly negative values of ADE (more negative than benzene) suggesting the stability of these anion radical towards electron detachment. The formation of M-M one electron bond is responsible to provide stability to these anion radicals.