The nature of metal−metal bonding in group 13 dimetallenes REER (E = Al, Ga, In, Tl; R = H, Me, tBu, Ph) was investigated by use of quantum chemical methods that include HF, second order Møller−Plesset perturbation theory (MP2), coupled cluster (CCSD(T)), complete active space with (CASPT2) and without (CAS) second order perturbation theory, and two density functionals, namely, B3LYP and M06-2X. The results show that the metal−metal interaction in group 13 dimetallenes stems almost exclusively from static and dynamic electron correlation effects: both dialuminenes and digallenes have an important singlet diradical component in their wave function, whereas the bonding in the heavier diindenes and, in particular, dithallenes is dominated by closed shell metallophilic interactions. The reported calculations represent a systematic attempt to determine the metal and ligand dependent bonding changes in these systems.