FLUORIDE COMPLEXES OF BERYLLIUM(2) IN AQUEOUS MEDIA

The stability of the beryllium fluoride complexes has been measured potentiometrically in 1 m NaCl at 0, 25, and 60° and in 1 m NaClO4 at 25°. The potentiometric cell consisted of a solid-state fluoride electrode and two hydrogen electrodes. The HF and HF2- equilibria were determined in both media in order to demonstrate the reversibility of the electrode as well as to interpret beryllium fluoride equilibria in acidic solutions. Efforts were made to identify species of the type BexFy-(OH)z(2x-y-z)+ which might exist by varying the beryllium ion, fluoride ion, and hydrogen ion concentrations over wide limits. Only mononuclear species were observed and no mixed complexes were identified. The small differences seen in chloride and in perchlorate media are attributed to changes in activity coefficients. Four mononuclear complexes with y = 1-4 give an excellent fit to the data which cover ligand numbers up to 3.4. Omission of any one of the stepwise equilibria leads to large systematic deviations in the calculated ligand numbers. The over-all log Q values in 1 m NaCl at 25° are 4.900 ± 0.014, 8.662 ± 0.015, 11.450 ± 0.018, and 12.876 ± 0.022. The differences in successive stability constants are interpreted in terms of the statistical effects and the electrostatic interactions in a series of tetrahedral complexes. The overall beryllium fluoride formation equilibria are slightly exothermic with enthalpies of -0.35 ± 0.32, -1.51 ± 0.28, -1.80 ± 0.34, and -2.26 ± 0.35 kcal/mol, respectively. The stability of the complexes results from the large positive entropies of 21.2 ± 1.1, 34.6 ± 1.0, 46.4 ± 1.1, and 51.3 ± 1.2 eu. The stability data confirm some previously reported nmr results and the Raman spectrum observed for a (NH4)2BeF4 solution is consistent with a tetrahedral structure. A general correlation was found for the log Q values for the fluoride complexes of a large number of metal ions in terms of Z+ and γ+, the charge and radius of the cation. The log Q for the over-all formation reaction for metal fluoride complexes is given by log Q1,ν = -0.46 - 1.1y + (0.48Z+2/γ+). © 1969, American Chemical Society. All rights reserved.