Low-Valent Group-13 Chemistry. Theoretical Investigation of the Structures and Relative Stabilities of Donor−Acceptor Complexes R3E−E‘R‘ and Their Isomers R2E−E‘RR‘

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Abstract
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−Tl 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 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 Do = 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‘ = Al−Tl) and the alane complexes Cl3Al−E‘R‘ (E‘ = Ga−Tl).