Structures and bonding situation of the allyl systems and cyclic isomers [H₂E-E(H)-EH₂]^-,·,+ (E = C, Si, Ge, Sn)

Quantum chemical calculations using DFT and ab initio methods were carried out on the structures of the title compounds. The nature of the bonding was investigated with an energy decomposition analysis. The calculations predict that the planar C_2v allyl structures of the neutral and charged heavier group-14 homologues [H₂E-E(H)-EH₂] ^-,·,+ (E = Si - Sn) are no minima on the potential energy surface. Energy minima for allyl-type structures of the latter systems possess C_s symmetry and pyramidal EH₂ groups. The energetically lowest lying form of [H₂E-E(H)-EH₂]^-,·,+ (E = Si - Sn) has a cyclic structure for the neutral molecules and the anion and a quasi-cyclic equilibrium arrangement for the cations. In contrast, the cyclic isomers of the carbon molecules [H₂C-C(H)-CH₂] ^-,·,+are significantly higher in energy than the allyl structures. Energy decomposition analyses show that the lower stability of the planar C_2v allyl structures of [H₂E-E(H)-EH ₂]^-,·,+ (E = Si - Sn) does not come from weak π conjugation. The relative contribution of π conjugation in the latter species is even higher than in the allyl system of carbon. The cyclic form of [H ₂E-E(H)-EH₂]^-,·,+ (E = Si - Sn) is lower in energy than the allyl form, because the σ bonding in the former structures is much stronger than in the latter. This overcompensates the higher Pauli repulsion in the cyclic form. In the carbon systems, the Pauli repulsion of the cyclic structures is very strong, because the bonds are much shorter than in the heavier homologues. Consequently, the stronger Pauli repulsion in the cyclic isomers is not compensated by the stronger attraction compared with the allyl system.