Stannous fluoride
Encyclopedia
Tin fluoride, known by the common name stannous fluoride, is a chemical compound with the formula Sn
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...

F
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...

2
. It is a colorless solid used as an ingredient in toothpaste
Toothpaste
Toothpaste is a paste or gel dentifrice used with a toothbrush as an accessory to clean and maintain the aesthetics and health of teeth. Toothpaste is used to promote oral hygiene: it serves as an abrasive that aids in removing the dental plaque and food from the teeth, assists in suppressing...

s that are typically more expensive than those that use sodium fluoride
Sodium fluoride
Sodium fluoride is an inorganic chemical compound with the formula NaF. A colorless solid, it is a source of the fluoride ion in diverse applications. Sodium fluoride is less expensive and less hygroscopic than the related salt potassium fluoride....

. Stannous fluoride converts the calcium mineral apatite
Apatite
Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...

 into fluorapatite
Fluorapatite
Fluorapatite, often with the alternate spelling of fluoroapatite, is a mineral with the formula Ca53F . Fluorapatite is a hard crystalline solid. Although samples can have various color , the pure mineral is colorless as expected for a material lacking transition metals...

, which makes tooth enamel
Tooth enamel
Tooth enamel, along with dentin, cementum, and dental pulp is one of the four major tissues that make up the tooth in vertebrates. It is the hardest and most highly mineralized substance in the human body. Tooth enamel is also found in the dermal denticles of sharks...

 more resistant to bacteria generated acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

 attacks. Sodium fluoride
Sodium fluoride
Sodium fluoride is an inorganic chemical compound with the formula NaF. A colorless solid, it is a source of the fluoride ion in diverse applications. Sodium fluoride is less expensive and less hygroscopic than the related salt potassium fluoride....

 and sodium fluorophosphate, on the other hand, become biologically inactive when combined with calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

. Used in combination with calcium minerals, sodium fluoride is ineffective while stannous fluoride remains effective in strengthening tooth enamel. Stannous fluoride has also been shown to be more effective than sodium fluoride
Sodium fluoride
Sodium fluoride is an inorganic chemical compound with the formula NaF. A colorless solid, it is a source of the fluoride ion in diverse applications. Sodium fluoride is less expensive and less hygroscopic than the related salt potassium fluoride....

 in controlling gingivitis
Gingivitis
Gingivitis is a term used to describe non-destructive periodontal disease. The most common form of gingivitis is in response to bacterial biofilms adherent to tooth surfaces, termed plaque-induced gingivitis, and is the most common form of periodontal disease...

.

Stannous fluoride was used, under the trade name "Fluoristan," in the original formulation of the toothpaste Crest
Crest (brand)
Crest is a brand of toothpaste made by Procter & Gamble in Germany and in United States of America and sold worldwide. In many countries in Europe, such as Germany, Bulgaria, Serbia, Ukraine, Russia, Poland, Hungary and Romania, it is sold as Blend-A-Med, the name of an established German...

,
though it was later replaced with sodium monofluorophosphate
Sodium monofluorophosphate
Sodium monofluorophosphate, commonly abbreviated MFP, is the inorganic compound with the formula Na2PO3F. Typical for a salt, MFP is odourless, colourless, and water-soluble. This salt is an ingredient in some toothpastes.-Uses of MFP:...

, or "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. Any stannous fluoride staining that occurs due to improper brushing is not permanent.

Stannous fluoride is also readily available in over-the-counter rinses.

SnF2 can be prepared by evaporating a solution of SnO in 40% HF
Hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....

.

Aqueous solutions

Readily soluble in water SnF2 is hydrolysed forming at low concentration species such as SnOH+, Sn(OH)2 and Sn(OH)3 and at higher concentrations, predominantly polynuclear species, 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 is a Lewis acid
Lewis acid
]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...

 forming, for example, a 1:1 complex (CH3)3NSnF2 and 2:1 complex [(CH3)3N]2SnF2 with trimethylamine
Trimethylamine
Trimethylamine is an organic compound with the formula N3. This colorless, hygroscopic, and flammable tertiary amine has a strong "fishy" odor in low concentrations and an ammonia-like odor at higher concentrations...

, and a 1:1 complex with dimethylsulfoxide, (CH3)2SO.SnF2.

In solutions containing fluoride ion, F it forms fluoride complexes SnF3, Sn2F5, SnF2(OH2). Crystallization from an aqueous solution containing NaF
Sodium fluoride
Sodium fluoride is an inorganic chemical compound with the formula NaF. A colorless solid, it is a source of the fluoride ion in diverse applications. Sodium fluoride is less expensive and less hygroscopic than the related salt potassium fluoride....

 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 however be produced from a pyridine – water solution.
Other compounds containing the pyramidal SnF3 anion are known for example Ca(SnF3)2

Reducing properties

SnF2 is a reducing agent
Reducing agent
A reducing agent is the element or compound in a reduction-oxidation reaction that donates an electron to another species; however, since the reducer loses an electron we say it is "oxidized"...

, with a standard reduction potential 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 but in each case there are three nearest neighbours with Sn at the apex of a trigonal pyramid and the lone pair of electrons is sterically active. Other forms reported have the GeF2 and TeO2 structures.

Molecular SnF2

In the vapour phase SnF2 forms monomers as well as 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
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