Theoretical studies of inorganic compounds. 34 Energy decomposition analysis of E-E bonding in planar and perpendicular X₂E-EX ₂ (E = B, Al, Ga, In, Tl; X = H, F, Cl, Br, I)

The nature of E-E bonding in group 13 compounds X₂E-EX ₂ (E = B, Al, Ga, In, Tl; X = H, F, Cl, Br, I) has been investigated by means of an energy decomposition analysis (EDA) at the BP86/TZ2P level of theory. The calculated equilibrium geometries of all molecules B ₂X₄-Tl₂X₄ have a perpendicular (D_2d) geometry. The largest energy barriers for rotation about the E-E bond are predicted for the hydrogen species B₂H ₄-Tl₂H₄. The EDA shows that the rotational barriers of B₂X₄-Tl₂X₄ may not be used for an estimate of the hyperconjugative strength in the D_2d structures except for the tetrahydrides. The values for the planar (D _2h) transition states reveals that π conjugation of the halogen lone-pair electrons stabilizes the transition states. The bonding analysis shows that hyperconjugation in B₂I₄ is stronger than in B ₂H₄ although the latter compound has a higher rotational barrier than the former. In B₂F₄, hyperconjugative stabilization of the perpendicular structure and conjugative stabilization of the planar structure nearly cancel each other yielding a nearly vanishing rotational barrier. The heavier analogues Al₂X₄-Tl ₂X₄ have low rotational barriers and rather weak hyperconjugative interactions. The larger rotational barriers of the hydrogen systems Al₂H₄-Tl₂H₄ compared with the tetrahalogen compounds is explained with the cooperation of the relatively large hyperconjugation in the perpendicular form and the relatively weak conjugation in the planar transition structures. The EDA also indicates that the electrostatic (ΔE_elstat) and molecular orbital (ΔE _orb) components of the E-E bonding are similar in magnitude. The calculated B-B bond dissociation energies of B₂X₄ (D _e = 93.0-108.4 kcal/mol) show that the bonds are rather strong. The heavier analogues Al₂X₄-Tl₂X₄ have weaker bonds (D_e = 16.6-61.7 kcal/mol). In general, the X ₂E-EX₂ bond dissociation energies follow the trend for atoms E: B ≫ Al > Ga > In > Tl and for atoms X: H > F > Cl > I.