Studies on several isomers of the C2On2- and Si2On2- series of dianionic systems are performed at the Hartree-Fock, configuration interaction, Moller-Plesset perturbation theory and outer-valence Green’s function levels of theory. All examined C2On2- systems are predicted to be unstable with respect to electron autodetachment. However, the analogues silicon systems Si2On2- turn out to be electronically more stable than the corresponding carbon species. For Si2O42- which marks the onset of electronic stability in the SimOn2- series the influence of the basis set and electron correlation on the geometrical parameters as well as on the electron detachment energy are studied in some detail. The possibility of decomposition via fragmentation or adiabatic electron loss is discussed. A simple criterion to determine whether a given structure may represent a promising candidate for a stable dianion of the SimOn2- type is suggested.