Which kind of binuclear sandwich-like compound is more stable in M2(η4 − E4)2 (M = B, Al, Ga; E = XYZ = BCO, BNN, CBO), the metal centered or the non-metal centered sandwich-like complex?

► There exist MM single bonds in M2(η4 − E4)2 (M = B, Al, Ga; E = XYZ = BCO, BNN, CBO). ► The MX bond in B2(η4 − E4)2 is covalent, while in M2(η4 − E4)2 (M = Al, Ga) is ionic. ► E42- ligands of M2(η4 − E4)2 possess aromaticity. ► M2(η4 − E4)2 are substantial stable and predicted to be synthesized in the future. ► The non-metal centered species are more stable than the metal centered ones.