Mass-Spectrometric Studies of Bonding in the Group IIA Fluorides

Abstract

A mass spectrometer was used to study high‐temperature gaseous equilibria of the type M(g) + MF2(g) = 2MF(g), where M = Mg, Sr, and Ba. The heat of this reaction gives directly the difference between bonddissociation energies D ( MF − F ) − D ( M − F ) . Results obtained for the above equilibrium were ΔH 298 = 25.6, 2.1, and −6.7 kcal (all ±2 kcal) for Mg, Sr, and Ba, respectively. In addition, the dissociation energies, D° 298 , of MgF, SrF, and BaF were determined as 110.4 ± 1.2, 129.5 ± 1.6, and 140.3 ± 1.6. Although the bonddissociation energies of the lighter Group II fluorides are in qualitative accord with the simple s – p valence state scheme of covalent bonding, this model breaks down rather badly for the heavier metal fluorides. It appears, however, that the bonddissociation energies of these heavier fluorides can be accounted for quite well by an electrostatic model which considers each molecule as an assembly of polarizable ions. The measured dissociation energies of the metal monofluorides can be correlated reasonably well with the spectroscopic values of D 0 ° if the degree of ionic character is considered.