Tin(II) chloride

Tin(II) chloride, also known as stannous chloride, is a white crystalline solid with the formula SnCl₂. It forms a stable dihydrate, but aqueous solutions tend to undergo hydrolysis, particularly if hot. SnCl₂ is widely used as a reducing agent (in acid solution), and in electrolytic baths for tin-plating. Tin(II) chloride should not be confused with the other chloride of tin; tin(IV) chloride or stannic chloride (SnCl₄).

Tin(II) chloride

Names
IUPAC names Tin(II) chloride Tin dichloride
Other names Stannous chloride Tin salt Tin protochloride
Identifiers
CAS Number	7772-99-8^Y 10025-69-1 (dihydrate)^Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:78067^N
ChemSpider	22887^N
ECHA InfoCard	100.028.971
E number	E512 (acidity regulators, ...)
PubChem CID	24479
RTECS number	XP8700000 (anhydrous) XP8850000 (dihydrate)
UNII	R30H55TN67^Y 1BQV3749L5 (dihydrate)^Y
UN number	3260
CompTox Dashboard (EPA)	DTXSID8021351
InChI InChI=1S/2ClH.Sn/h2*1H;/q;;+2/p-2^N Key: AXZWODMDQAVCJE-UHFFFAOYSA-L^N InChI=1/2ClH.Sn/h2*1H;/q;;+2/p-2 Key: AXZWODMDQAVCJE-NUQVWONBAJ
SMILES Cl[Sn]Cl
Properties
Chemical formula	SnCl2
Molar mass	189.60 g/mol (anhydrous) 225.63 g/mol (dihydrate)
Appearance	White crystalline solid
Odor	odorless
Density	3.95 g/cm^3 (anhydrous) 2.71 g/cm^3 (dihydrate)
Melting point	247 °C (477 °F; 520 K) (anhydrous) 37.7 °C (dihydrate)
Boiling point	623 °C (1,153 °F; 896 K) (decomposes)
Solubility in water	83.9 g/100 ml (0 °C) Hydrolyses in hot water
Solubility	soluble in ethanol, acetone, ether, Tetrahydrofuran insoluble in xylene
Magnetic susceptibility (χ)	−69.0·10^−6 cm^3/mol
Structure
Crystal structure	Layer structure (chains of SnCl3 groups)
Coordination geometry	Trigonal pyramidal (anhydrous) Dihydrate also three-coordinate
Molecular shape	Bent (gas phase)
Hazards
Main hazards	Irritant, dangerous for aquatic organisms
Safety data sheet	See: data page ICSC 0955 (anhydrous) ICSC 0738 (dihydrate)
NFPA 704 (fire diamond)	0 3 0
Lethal dose or concentration (LD, LC):
LD50 (median dose)	700 mg/kg (rat, oral) 10,000 mg/kg (rabbit, oral) 250 mg/kg (mouse, oral)[1]
Related compounds
Other anions	Tin(II) fluoride Tin(II) bromide Tin(II) iodide
Other cations	Germanium dichloride Tin(IV) chloride Lead(II) chloride
Supplementary data page
Structure and properties	Refractive index (n), Dielectric constant (εr), etc.
Thermodynamic data	Phase behaviour solid–liquid–gas
Spectral data	UV, IR, NMR, MS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is ^YN ?)
Infobox references

Chemical structure

SnCl₂ has a lone pair of electrons, such that the molecule in the gas phase is bent. In the solid state, crystalline SnCl₂ forms chains linked via chloride bridges as shown. The dihydrate is also three-coordinate, with one water coordinated on to the tin, and a second water coordinated to the first. The main part of the molecule stacks into double layers in the crystal lattice, with the "second" water sandwiched between the layers.

Structures of tin(II) chloride and related compounds

Ball-and-stick models of the crystal structure of SnCl₂[2]

Chemical properties

Tin(II) chloride can dissolve in less than its own mass of water without apparent decomposition, but as the solution is diluted, hydrolysis occurs to form an insoluble basic salt:

SnCl₂ (aq) + H₂O (l) ⇌ Sn(OH)Cl (s) + HCl (aq)

Therefore, if clear solutions of tin(II) chloride are to be used, it must be dissolved in hydrochloric acid (typically of the same or greater molarity as the stannous chloride) to maintain the equilibrium towards the left-hand side (using Le Chatelier's principle). Solutions of SnCl₂ are also unstable towards oxidation by the air:

6 SnCl₂ (aq) + O₂ (g) + 2 H₂O (l) → 2 SnCl₄ (aq) + 4 Sn(OH)Cl (s)

This can be prevented by storing the solution over lumps of tin metal.[3]

There are many such cases where tin(II) chloride acts as a reducing agent, reducing silver and gold salts to the metal, and iron(III) salts to iron(II), for example:

SnCl₂ (aq) + 2 FeCl₃ (aq) → SnCl₄ (aq) + 2 FeCl₂ (aq)

It also reduces copper(II) to copper(I).

Solutions of tin(II) chloride can also serve simply as a source of Sn²⁺ ions, which can form other tin(II) compounds via precipitation reactions. For example, reaction with sodium sulfide produces the brown/black tin(II) sulfide:

SnCl₂ (aq) + Na₂S (aq) → SnS (s) + 2 NaCl (aq)

If alkali is added to a solution of SnCl₂, a white precipitate of hydrated tin(II) oxide forms initially; this then dissolves in excess base to form a stannite salt such as sodium stannite:

SnCl₂(aq) + 2 NaOH (aq) → SnO·H₂O (s) + 2 NaCl (aq)

SnO·H₂O (s) + NaOH (aq) → NaSn(OH)₃ (aq)

Anhydrous SnCl₂ can be used to make a variety of interesting tin(II) compounds in non-aqueous solvents. For example, the lithium salt of 4-methyl-2,6-di-tert-butylphenol reacts with SnCl₂ in THF to give the yellow linear two-coordinate compound Sn(OAr)₂ (Ar = aryl).[4]

Tin(II) chloride also behaves as a Lewis acid, forming complexes with ligands such as chloride ion, for example:

SnCl₂ (aq) + CsCl (aq) → CsSnCl₃ (aq)

Most of these complexes are pyramidal, and since complexes such as SnCl₃ have a full octet, there is little tendency to add more than one ligand. The lone pair of electrons in such complexes is available for bonding, however, and therefore the complex itself can act as a Lewis base or ligand. This seen in the ferrocene-related product of the following reaction :

SnCl₂ + Fe(η⁵-C₅H₅)(CO)₂HgCl → Fe(η⁵-C₅H₅)(CO)₂SnCl₃ + Hg

SnCl₂ can be used to make a variety of such compounds containing metal-metal bonds. For example, the reaction with dicobalt octacarbonyl:

SnCl₂ + Co₂(CO)₈ → (CO)₄Co-(SnCl₂)-Co(CO)₄

Preparation

Anhydrous SnCl₂ is prepared by the action of dry hydrogen chloride gas on tin metal. The dihydrate is made by a similar reaction, using hydrochloric acid:

Sn (s) + 2 HCl (aq) → SnCl₂ (aq) + H
₂ (g)

The water then carefully evaporated from the acidic solution to produce crystals of SnCl₂·2H₂O. This dihydrate can be dehydrated to anhydrous using acetic anhydride.[5]

Uses

A solution of tin(II) chloride containing a little hydrochloric acid is used for the tin-plating of steel, in order to make tin cans. An electric potential is applied, and tin metal is formed at the cathode via electrolysis.

Tin(II) chloride is used as a mordant in textile dyeing because it gives brighter colours with some dyes e.g. cochineal. This mordant has also been used alone to increase the weight of silk.

In recent years, an increasing number of tooth paste brands has been adding Tin(II) chloride as protection against enamel erosion to their formula , e. g. Oral-B or Elmex.

It is used as a catalyst in the production of the plastic polylactic acid (PLA).

It also finds a use as a catalyst between acetone and hydrogen peroxide to form the tetrameric form of acetone peroxide.

Tin(II) chloride also finds wide use as a reducing agent. This is seen in its use for silvering mirrors, where silver metal is deposited on the glass:

Sn²⁺ (aq) + 2 Ag⁺ → Sn⁴⁺ (aq) + 2 Ag (s)

A related reduction was traditionally used as an analytical test for Hg²⁺(aq). For example, if SnCl₂ is added dropwise into a solution of mercury(II) chloride, a white precipitate of mercury(I) chloride is first formed; as more SnCl₂ is added this turns black as metallic mercury is formed. Stannous chloride can be used to test for the presence of gold compounds. SnCl₂ turns bright purple in the presence of gold (see Purple of Cassius).

When mercury is analyzed using atomic absorption spectroscopy, a cold vapor method must be used, and tin (II) chloride is typically used as the reductant.

In organic chemistry, SnCl₂ is mainly used in the Stephen reduction, whereby a nitrile is reduced (via an imidoyl chloride salt) to an imine which is easily hydrolysed to an aldehyde.[6]

The reaction usually works best with aromatic nitriles Aryl-CN. A related reaction (called the Sonn-Müller method) starts with an amide, which is treated with PCl₅ to form the imidoyl chloride salt.

The Stephen reduction

The Stephen reduction is less used today, because it has been mostly superseded by diisobutylaluminium hydride reduction.

Additionally, SnCl₂ is used to selectively reduce aromatic nitro groups to anilines.[7]

Aromatic nitro group reduction using SnCl₂

SnCl₂ also reduces quinones to hydroquinones.

Stannous chloride is also added as a food additive with E number E512 to some canned and bottled foods, where it serves as a color-retention agent and antioxidant.

SnCl₂ is used in radionuclide angiography to reduce the radioactive agent technetium-99m-pertechnetate to assist in binding to blood cells.

Aqueous stannous chloride is used by many precious metals refining hobbyists and professionals as an indicator of gold and platinum group metals in solutions.

Molten SnCl₂ can be oxidised to form highly crystalline SnO₂ nanostructures.[8][9]

Notes

1. "Tin (inorganic compounds, as Sn)". Immediately Dangerous to Life and Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
2. J. M. Leger; J. Haines; A. Atouf (1996). "The high pressure behaviour of the cotunnite and post-cotunnite phases of PbCl₂ and SnCl₂". J. Phys. Chem. Solids. 57 (1): 7–16. Bibcode:1996JPCS...57....7L. doi:10.1016/0022-3697(95)00060-7.
3. H. Nechamkin (1968). The Chemistry of the Elements. New York: McGraw-Hill.
4. B. Cetinkaya, I. Gumrukcu, M. F. Lappert, J. L. Atwood, R. D. Rogers and M. J. Zaworotko (1980). "Bivalent germanium, tin, and lead 2,6-di-tert-butylphenoxides and the crystal and molecular structures of M(OC6H2Me-4-But2-2,6)2 (M = Ge or Sn)". J. Am. Chem. Soc. 102 (6): 2088–2089. doi:10.1021/ja00526a054.CS1 maint: multiple names: authors list (link)
5. W. L. F. Armarego; C. L. L. Chai (2009). Purification of laboratory chemicals (6 ed.). The United States of America: Butterworth-Heinemann.
6. Williams, J. W. (1955). "β-Naphthaldehyde". Organic Syntheses.; Collective Volume, 3, p. 626
7. F. D. Bellamy & K. Ou (1984). "Selective reduction of aromatic nitro compounds with stannous chloride in non acidic and non aqueous medium". Tetrahedron Letters. 25 (8): 839–842. doi:10.1016/S0040-4039(01)80041-1.
8. A.R.Kamali, Thermokinetic characterisation of tin(11) chloride, J Therm Anal Calorim 118(2014) 99-104.
9. A.R.Kamali et al.Transformation of molten SnCl₂ to SnO₂ nano-single crystals, Ceram Intern 40 (2014)8533-8538.

References

• N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.
• Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990.
• The Merck Index, 7th edition, Merck & Co, Rahway, New Jersey, USA, 1960.
• A. F. Wells, 'Structural Inorganic Chemistry, 5th ed., Oxford University Press, Oxford, UK, 1984.
• J. March, Advanced Organic Chemistry, 4th ed., p. 723, Wiley, New York, 1992.