Specific interactions between anions and cations in protophobic aprotic solvents. Triple-ion and higher aggregate formation from lithium and tributylammonium thiocyanates

Abstract

Triple-ion and quadrupole formation in addition to ion-pair formation from lithium and tributylammonium thiocyanates has been examined by means of conductometry, in several protophobic aprotic solvents: nitrobenzene, benzonitrile, acetonitrile, nitromethane and propylene carbonate. The formation constants of the ion pair (M⁺+ SCN^–⇌ MSCN, K^a₁; where M⁺= Li⁺ or n-Bu₃NH⁺), symmetrical triple ions [2M⁺+ SCN^–⇌(M⁺)₂SCN^–, K^a₂; M⁺+ 2 SCN^–⇌ M⁺(SCN^–)₂, K^a₃; K^a₂=K^a₃] and the quadrupole [(M⁺)₂SCN^–+ SCN^–⇌(MSCN)₂, K^a₄; M⁺+ M⁺(SCN^–)₂⇌(MSCN)₂, K^a₅=(K^a₅) were evaluated after correction of the activity coefficients of the ions. Effects of the ratio Λ_T/Λ₀ on the conductivities were examined, where Λ_T and Λ₀ are the molar conductivities at infinite dilution of the triple ions and simple ions, respectively. A remarkable enhancement of the triple-ion formation from LiSCN was observed in nitromethane: K^a′₁= 5.5 × 10⁴, K^a′₂=K^a′₃= 8.0 × 10⁷ and Λ_T/Λ₀= 0.4 (the effects of the ionic atmosphere and viscosity changes are taken into account). At higher salt concentrations, quadrupole formation was observed, which caused the disappearance of the minimum in the Λ–C^1/2 curve. In contrast, the degree of triple-ion formation from n-Bu₃NHSCN was much smaller than that from LiSCN and quadrupole formation was not observed in all the solvents. The formation of (Li⁺)₂SCN^– was suggested by spectrophotometry. Both the donor and acceptor numbers of the solvents were concerned with the formation of higher ion aggregates.