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; π donor groups R′, while the substituents R may either be σ or; π-bonded groups. Calculations of CI₃B-BR′ [R′ = CI, 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 CI₃B-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 σ-bonded bulky substituent. Possible candidates that are suggested for synthetic work are the borane complexes (C₆F₅)₃B-E′R′ and ^tBu₃B-E′R′ (E′= AI-TI) and the alane complexes CI₃AI-E′R′ (E′= Ga-TI).