The results of quantum chemical calculations at the gradient-corrected density functional theory (DFT) level with the B3LYP functional of the donor-acceptor complexes R3E-E'R' and their isomers R2E-E'RR', where E, E' = B-TI and R, R' = H, Cl, or CH3, are reported. The theoretically predicted energy differences between the donor-acceptor form R3E-E'R' and the classical isomer R2E-E'RR' and the bond dissociation energies of the E-E' bonds are given. The results are discussed in order to show which factors stabilize the isomers R3E-E'R'. There is no simple correlation of the nature of the group-13 elements E, E' and the substituents R, R' with the stability of the complexes. The isomers R3E-E'R' come stabilized by pi donor groups R', while the substituents R may either be sigma- or pi-bonded groups. Calculations of Cl3B-BR' [R' = Cl, cyclopentadienyl (Cp), or Cp*] indicate that the Cp* group has a particularly strong effect on the complex form. The calculations show that the experimentally known complex Cl3B-BCp* is the strongest bonded donor-acceptor complex of main-group elements that has been synthesized until now. The theoretically predicted B-B bond energy is D-o = 50.6 kcal/mol. However, the calculations indicate that it should also be possible to isolate donor-acceptor complexes R3E-E'R' where R' is a sigma-bonded bulky substituent. Possible candidates that are suggested for synthetic work are the borane complexes (C6F5)(3)B-E'R' and (Bu3B)-Bu-t-E'R' (E' = Al-Tl) and the alane complexes Cl3Al-E'R' (E' = Ga-Tl).