10.1 Compounds of fluorine

Elemental fluorine (F ₂ ) is the most reactive element. Fluorine combines directly with all other elements, except nitrogen and the lighter noble gases. It also reacts with many compounds forming fluorides, and many organic compounds inflame and burn in the gas. The highly reactive nature is due to the weak F-F bond (thermodynamically unstable), which provides a low activation energy to reactions (kinetically unstable). The ΔG for reactions is often large due to the strength of the resulting X-F bonds. The weak F-F bond (158 kJ/mol) is due to the small size (0.5 Å) and high nuclear charge of fluorine that result in a small overlap of the bonding orbitals and a repulsion between the non-bonding orbitals (lone pairs) on the two fluorine atoms.

Ionic salts

The ease of formation of F ^- anion is due to the high electron affinity of fluorine (-322 kJ/mol). Since the fluoride ion is small (1.33 Å) and the least polarizable anion (i.e., hard ) it is stable in ionic lattices with metal cations in a high oxidation state (high charge), e.g., MnF ₄ and CrF ₅ . In general the highest oxidation states for any metal are found with the fluoride salts. The large ionization energies needed to produce the cations are recovered by the high lattice energies.

Covalent compounds

The high electronegativity of fluorine means that it forms a single electron pair bond polar bond with a high ionic character. The polar nature of the bond means that there is a large inductive effect within a molecule. For example, perfluoroethanol (CF ₃ CF ₂ OH) has an acidity comparable to acetic acid.

The high strength of X-F bonds ( [link] ) is also due to the high ionic character (up to 50%) that results in a high activation energy for bond breaking. In contrast, the low polarizability of the fluorine means that the inter-molecular van der Waals bonds are very weak. Thus, even with very high molecular weights the boiling point can be very low, e.g, WF ₆ , Bp = 17 °C, Mw = 297.84 g/mol.

A wide range of fluoride complexes may be prepared from both metal (FeF ₆ ^3- , RuF ₆ ^- , PtF ₆ ^2- , and SnF ₆ ^2- ) and non-metal (BF ₄ ^- , SiF ₆ ^2- , and PF ₆ ^- ) fluorides. While many fluorides are salts, when the metal is in its higher oxidation states (e.g., OsF ₆ and WF ₆ ), the formation of an ionic lattice with the appropriate cation (i.e., Os ⁶⁺ and W ⁶⁺ respectively) is energetically unfavorable.

Hydrogen fluoride

Hydrogen fluoride is synthesized by the reaction of a fluoride salt with a concentrated acid, [link] . The HF vapor may be condensed, and then subsequently purified by distillation.

The H-F bonding in hydrogen fluoride involves an electron pair bond with a high degree of ionic character. This results in a very polar H-F bond and a large dipole moment (1.86 D).