Covalent versus ionic bonding in alkalimetal fluoride oligomers

The most polar bond in chemistry is that between a fluorine and an alkalimetal atom. Inspired by our recent finding that other polar bonds (C - M and H - M) have important covalent contributions (i.e., stabilization due to bond overlap), we herein address the question if covalency is also essential in the F - M bond. Thus, we have theoretically studied the alkalimetal fluoride monomers, FM, and (distorted) cubic tetramers, (FM)4, with M = Li, Na, K, and Rb, using density functional theory at the BP86/TZ2P level. Our objective is to determine how the structure and thermo-chemistry (e.g., F - M bond lengths and strengths, oligomerization energies, etc.) of alkalimetal fluorides depend on the metal atom, and to understand the emerging trends in terms of quantitative Kohn-Sham molecular orbital theory. The analyses confirm the extreme polarity of the F - M bond (dipole moment, Voronoi deformation density and Hirshfeld atomic charges), and they reveal that bond overlap-derived stabilization (ca. -6, -6, and -2 kcal/mol) contributes only little to the bond strength (-136, -112, and -114 kcal/mol) and the trend therein along Li, Na, and K. According to this and other criteria, the F - M bond is not only strongly polar, but also has a truly ionic bonding mechanism. Interestingly, the polarity is reduced on tetramerization. For the lithium and sodium fluoride tetramers, the F ₄ tetrahedron is larger than and surrounds the M₄ cluster (i.e., F - F > M - M). But in the potassium and rubidium fluoride tetramers, the F₄ tetrahedron is smaller than and inside the M₄ cluster (i.e., F - F < M - M).