Stability of Alkaline Earth Monofluoride Complexes in Methanol

Fluoride complexes occupy a central role in ligand-acceptor classification schemes. Depending upon the nature of the acceptor, a given aqueous fluoride complex is either much stronger or much weaker than the other corresponding halide complexes. The extension of these classifications to other solvent systems is made difficult by the dearth of data concerning the stability of nonaqueous fluoride complexes. In this paper, the use of fluoride ion-selective-electrode potentiometry to obtain such data is illustrated with results obtained for alkaline earth monofluorides in methanol and methanol-water mixtures. The weak alkaline earth monofluorides show the stability trend MgF+ > CaF+ > SrF+ > BaF+ in water. The same trend is observed in methanol but the complex stabilities are greatly enhanced compared to the aqueous cases. This enhancement decreases as the atomic number of the alkaline earth increases. The effect of the dielectric constant upon complex stability is illustrated and SrF+ is shown to obey the Born equation for all water and/or methanol solvent systems, while MgF+ shows a deviation arising from the strong hydration of Mg2+. Enthalpy and entropy information for MgF+ and SrF+ was estimated from temperature variation experiments. The results show that both reactions are entropy-controlled in methanol, as is characteristic for aqueous monofluoride complexes. The variation of the electrode system reference potential as a function of solvent composition is illustrated. Experimental procedure is briefly outlined and difficulties encountered in the nonaqueous measurements using a fluoride ion-selective electrode are described.