Towards understanding the stability of the N₅⁺-containing salts: The role of counterions

The thermal stabilities of the N₅⁺M^- species (M = Sb(OH)₆, Sb(OH)₄F₂, AlF₆, AlF₄, BF₄, B(CF₃)₄, PF₆, AsF₆ and SbF₆) have been studied by means of density functional theory. The present calculations indicate that their thermal stabilities (represented by activation enthalpy, ΔH^≠ in kcal mol^-1) decrease in the order N₅⁺ (47.2) > N₅B(CF₃)₄ (34.1) ≈ N₅SbF₆ (31.6) ≈ N₅AsF₆ (31.5) > N₅PF₆ (30.5) > N₅AlF₄ (27.1) ≈ N₅BF₄ (27.1) > N₅AlF₆ (8.5). Only N₅SbF₆ has a small positive reaction enthalpy, Δ_rH. The thermal stability of the N₅⁺ salt depends on the amount of electron transfer from the counterion to N₅⁺ in the reaction. The high electronegativite atoms or groups in the counterion are essential. Another crucial factor is bond strength between the central ion and the ligand. Studies of the N₅Sb(OH)₄F₂ isomers indicate that the OH rather than the fluorine ions in the axial coordination positions could stabilize the decomposition transition structure through partial dissociation of the Sb-OH bond, which is used to explain the reason why there is an obvious difference in the decomposition activation enthalpies of the three isomers.