Sulfur tetrafluoride

Sulfur in SF₄ is in the +4 oxidation state, with one lone pair of electrons. The atoms in SF₄ are arranged in a see-saw shape, with the sulfur atom at the center. One of the three equatorial positions is occupied by a nonbonding lone pair of electrons. Consequently, the molecule has two distinct types of F ligands, two axial and two equatorial. The relevant bond distances are S–F_ax = 164.3 pm and S–F_eq = 154.2 pm. It is typical for the axial ligands in hypervalent molecules to be bonded less strongly.

The ¹⁹F NMR spectrum of SF₄ reveals only one signal, which indicates that the axial and equatorial F atom positions rapidly interconvert via pseudorotation.^[4]

At the laboratory scale, sulfur tetrafluoride is prepared from elemental sulfur and cobaltic fluoride^[5]

S + 4CoF₃ → SF₄ + 4CoF₂

SF₄ is industrially produced by the reaction of SCl₂ and NaF with acetonitrile as a catalyst^[6]

3 SCl₂ + 4 NaF → SF₄ + S₂Cl₂ + 4 NaCl

At higher temperatures (e.g. 225–450 °C), the solvent is superfluous. Moreover, sulfur dichloride may be replaced by elemental sulfur (S) and chlorine (Cl₂).^[7][8]

A low-temperature (e.g. 20–86 °C) alternative to the chlorinative process above uses liquid bromine (Br₂) as oxidant and solvent:^[9]

S(s) + 2 Br₂(l; excess) + 4KF(s) → SF₄↑ + 4 KBr(brom)

In organic synthesis, SF₄ is used to convert COH and C=O groups into CF and CF₂ groups, respectively.^[10] The efficiency of these conversions are highly variable.

In the laboratory, the use of SF₄ has been superseded by the safer and more easily handled diethylaminosulfur trifluoride, (C₂H₅)₂NSF₃, "DAST":^[11] This reagent is prepared from SF₄:^[12]

SF₄ + (CH₃)₃SiN(C₂H₅)₂ → (C₂H₅)₂NSF₃ + (CH₃)₃SiF

Sulfur chloride pentafluoride (SF
₅Cl), a useful source of the SF₅ group, is prepared from SF₄.^[13]

SF₄ + Cl₂ + CsF → SF₅Cl + CsCl

Hydrolysis of SF₄ gives sulfur dioxide:^[14]

SF₄ + 2 H₂O → SO₂ + 4 HF

This reaction proceeds via the intermediacy of thionyl fluoride, which usually does not interfere with the use of SF₄ as a reagent.^[6]

When amines are treated with SF₄ and a base, aminosulfur difluorides result.^[15]

SF
₄ reacts inside the lungs with moisture, forming sulfur dioxide and hydrogen fluoride which forms highly toxic and corrosive hydrofluoric acid ^[16]