Formula SnF2 Melting point 215 °C Density 4.57 g/cm³ | Molar mass 156.69 g/mol Boiling point 850 °C Appearance colorless solid | |
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Tin(II) fluoride, commonly referred to commercially as stannous fluoride (from Latin stannum, 'tin') is a chemical compound with the formula SnF2. It is a colorless solid used as an ingredient in toothpastes that are typically more expensive than those that use sodium fluoride. Stannous fluoride converts the calcium mineral apatite into fluorapatite, which makes tooth enamel more resistant to bacteria-generated acid attacks. In toothpastes containing calcium minerals, sodium fluoride becomes ineffective over time, while stannous fluoride remains effective in strengthening tooth enamel. Stannous fluoride has been shown to be more effective than sodium fluoride in reducing the incidence of dental caries and controlling gingivitis.
Contents
Stannous fluoride was used under the trade name Fluoristan in the original formulation of the toothpaste brand Crest, though it was later replaced with sodium monofluorophosphate under the trade name Fluoristat. It is the active ingredient in Crest Pro Health brand toothpaste. Crest Pro Health issues a warning on the tube that stannous fluoride may cause staining, which can be avoided by proper brushing, and that its particular formulation is resistant to staining. However, any stannous fluoride staining that occurs due to improper brushing is not permanent. Stannous fluoride is also used in Oral-B Pro-Expert. Stannous fluoride is also readily available in over-the-counter rinses.
Production
SnF2 can be prepared by evaporating a solution of SnO in 40% HF.
Aqueous solutions
Readily soluble in water, SnF2 is hydrolysed. At low concentration, it forms species such as SnOH+, Sn(OH)2 and Sn(OH)3−. At higher concentrations, predominantly polynuclear species are formed, including Sn2(OH)22+ and Sn3(OH)42+. Aqueous solutions readily oxidise to form insoluble precipitates of SnIV, which are ineffective as a dental prophylactic. Studies of the oxidation using Mössbauer spectroscopy on frozen samples suggests that O2 is the oxidizing species.
Lewis acidity
SnF2 acts as a Lewis acid. For example, it forms a 1:1 complex (CH3)3NSnF2 and 2:1 complex [(CH3)3N]2SnF2 with trimethylamine, and a 1:1 complex with dimethylsulfoxide, (CH3)2SO.SnF2.
In solutions containing the fluoride ion, F−, it forms the fluoride complexes SnF3−, Sn2F5−, and SnF2(OH2). Crystallization from an aqueous solution containing NaF produces compounds containing polynuclear anions, e.g. NaSn2F5 or Na4Sn3F10 depending on the reaction conditions, rather than NaSnF3. The compound NaSnF3, containing the pyramidal SnF3− anion, can be produced from a pyridine – water solution. Other compounds containing the pyramidal SnF3− anion are known, such as Ca(SnF3)2.
Reducing properties
SnF2 is a reducing agent, with a standard reduction potential of Eo (SnIV/ SnII) = +0.15V. Solutions in HF are readily oxidised by a range of oxidizing agents (O2, SO2 or F2) to form the mixed valence compound, Sn3F8 (containing SnII and SnIV and no Sn – Sn bonds).
Structure
The monoclinic form contains tetramers, Sn4F8, where there are two distinct coordination environments for the Sn atoms. In each case, there are three nearest neighbours, with Sn at the apex of a trigonal pyramid, and the lone pair of electrons sterically active. Other forms reported have the GeF2 and paratellurite structures.
Molecular SnF2
In the vapour phase, SnF2 forms monomers, dimers, and trimers. Monomeric SnF2 is a non-linear molecule with an Sn-F bond length of 206 pm. Complexes of SnF2, sometimes called difluorostannylene, with an alkyne and aromatic compounds deposited in an argon matrix at 12 K have been reported.
Safety
SnF2 can cause redness and irritation if inhaled or comes into contact with the eyes. At acute levels (over 2 mg/m3), if ingested, it can cause abdominal pains and shock. Rare but serious allergic reactions are possible (symptoms include itching, swelling, and difficulty breathing). When used in dental products, mild tooth discoloration may also occur; this can be removed by brushing.