Dihydrogen hexachloroplatinate (IV) hexahydrate

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Dihydrogen hexachloroplatinate (IV) hexahydrate

General
Systematic name	Dihydrogen hexachloroplatinate (IV) hexahydrate
Other names	Chloroplatinic Acid
Molecular formula	H₂PtCl₆*(H₂O)₆
Molar mass	517.891 g/mol
Appearance	Reddish brown solid
CAS number	[16941-12-1]
Properties
Density and phase	2.431 g/cm³, solid
Solubility in water	? g/100 ml (? °C)
Melting point	60 °C (333 K)
Boiling point	115 °C (388 K)
Acidity (pK_a)	?
Basicity (pK_b)	?
Chiral rotation [α]_D	?°
Viscosity	? cP at ? °C
Structure
Molecular shape	octahedral
Coordination geometry	octahedral
Crystal structure	Anti-fluorite.
Dipole moment	? D
Hazards
MSDS	External MSDS
Main hazards	Caustic. Sensitizer!
NFPA 704	0 3 0
Flash point	? °C
R/S statement	R: R34, R42, R43 S: S22, S26, S36, S37, S39, S45
RTECS number	TP1510000
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other anions	H₂PdCl₆ (unstable) hexachloropalladic Acid
Other cations	K₂PtCl₆ Potassium Chloroplatinate (NH₄)₂PtCl₆ Ammonium Chloroplatinate, Rb₂PtCl₆ Rubidium Chloroplatinate, Cs₂PtCl₆ Cæsium Chloroplatinate
Related ?	?
Related compounds	?
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Dihydrogen hexachloroplatinate (IV) hexahydrate (H₂PtCl₆*(H₂O)₆), also known as hexachlorplatinic acid or chloroplatinic acid, is one of the most readily available compounds of platinum. The commercial product is the oxonium salt of the hexachloroplatinate(IV) anion. Therefore, the correct formula is [H₃O]₂[PtCl₆].2H₂O.(5,6) The related palladium compound,[H₃0]₂[PdCl₆] is extremely unstable and has not been isolated in pure form.(6)

1 Use in determination of potassium
2 Production
3 Chemical properties
4 Safety information
5 References
6 Suppliers
7 External links

[edit] Use in determination of potassium

Chloroplatinic acid was popularlized for the determination of potassium. The potassium is selectively precipitated as potassium chloroplatinate. Determinations were done in 85% (v/v)alcohol solutions with excess platinate ions, and the precipitated product was weighed. Potassium could be detected for solutions as dilute as 0.02 to 0.2% (m/v).

$2K^+_{(aq)} + PtCl^{2-}_{6(aq)}\ \rightarrow\ K_2PtCl_{6(s)}$ .

This method for determination of potassium was advantagous vs. the cobaltinitrite method used previously, since it required a single precipitation reaction.(2)

While this determination might have saved time, any chemical reaction requiring a precious metal is rather expensive. So, this method of determination of potassium is much more expensive than the potassium-sodium cobaltinitrite method.

While the final determination itself is much faster using chloroplatinic acid. Producing the chemical is rather involved and tempermental. This draw back adds cost to the determination of potassium due to the removal of nitrosoplatinic chloride. Unless the nitrosoplatinic chloride is removed from the crude chloroplatinic acid, the potassium content of a sample will be systematically low.

Today, the concentation of ions such as potassium is determined in solution with an ion-selective electrode. These modern methods still suffer from error, however, caused by the similar chemical nature of many ions.

[edit] Production

Chloroplatinic acid is produced by dissolving platinum metal sponge in a 1:3 (v:v) solution of nitric acid and hydrochloric acid called aqua regia.(1) This reaction produces a mixture of chloroplatinous acid (H₂PtCl₄ and nitrosoplatinic chloride ([NO]₂PtCl₄). The nitrosoplatinic chloride is removed by repeatedly extracting and evaporating HCl from the solid mixture produced in the reaction with platinum and aqua regia. The remaining chloroplatinous acid is then oxidized to choroplatinic acid by saturating a solution with chlorine and heating to remove any excess. (4) The resulting chloroplatinic acid is brownish-red. (1)

$2HNO_{3(aq)} + 4HCl_{(aq)} + Pt_{(s)}\ \rightarrow\ (NO)_2PtCl_{4(s)} + 3H_2O + ^1/_2O_2$ .

$(NO)_2PtCl_{4(s)} + 2HCl_{(aq)}\ \rightarrow\ H_2PtCl_{4(aq)} + NOCl$ .

$H_2PtCl_{4(aq)} + Cl_{2(aq)}\ \rightarrow\ H_2PtCl_{6(aq)}$ .

Alternatively, the nitric acid in aqua regia can be replaced with 30% solution of hydrogen peroxide. However, this preparation requires the use of finely divided platinum powder known as platinum black. This approach avoids the formation of the nitrosoplatinic chloride (NO]₂PtCl₄), and the use of chlorine gas in the traditional synthesis. (4)

[edit] Chemical properties

Like many platinum compounds, chloroplatinic acid is used in catalysis. This compound was first used by John Speier and his colleagues to catalyze the reaction of silyl hydrides with olefins.(3) Typical of his reactions, Speier used isopropanol solutions containing trichlorosilane (SiHCl₃), and methyldichlorosilane (CH₃SiCl₂), with pentenes. Speier found that chloroplatinic acid avoided polymerization, a problem that plagued previous attempts at hydrosilylation.

Much speculation has surrounded the actual site of catalysis in chloroplatinic acid. Speier himself proposed that catalysis centered on the platinate ion (7), and possibly involved an oxidative addition reductive elimination process with the olefinic substrate. This process seems plausible since the chlorine ligands of the complex would be laible.

However, more recent evidence has suggested that catalysis with chloroplatinic acid involves low valent platinum. The II and IV oxidation states of platinum could not afford the highly catalytic nature of the reactions observed. Instead Lewis and his coworkers have suggested a mechanism involving Pt⁽⁰⁾ with olefins as ligands.(8)

[edit] Safety information

Stable. Hygroscopic. Avoid water, acids, bases, light. (A)

[edit] References

1. Earnshaw, Greenwood. Chemistry of the Elements. Second Edition. Elsevier Butterworth-Heinemann. New York. 2005

2. G. F. Smith, J. L. Gring. "The Separation and Determination of the Alkali Metals Using Perchloric Acid. V. Perchloric Acid and Chloroplatinic Acid in the Determination of Small Amounts of Potassium in the Presence of Large Amounts of Sodium" J. Am. Chem. Soc.; 1933; 55(10); 3957-3961.

3. J. L. Speier, J. A. Webster, G. H. Barnes "The Addition of Silicon Hydrides to Olefinic Double Bonds. Part II. The Use of Group VIII Metal Catalysts" J. Am. Chem. Soc.; 1957; 79(4); 974-979.

4. P. Rudnick, R. D. Cooke. THE PREPARATION OF HYDROCHLOROPLATINIC ACID BY MEANS OF HYDROGEN PEROXIDE. J. Am. Chem. Soc.; 1917; 39(4); 633-635.

5. A. E. Schweizer, G. T. Kerr. Thermal Decomposition of Hexachloroplatinic Acid. Inorg. Chem.; 1978; 17(8); 2326-2327.

6. Hollemann, Wiberg. Inorganic Chemistry First Edition. Academic Press. New York, 2001

7. John C. Saam, John L. Speier. "The Addition of Silicon Hydrides to Olefinic Double Bonds. Part 111. The Addition to Non-terminal Olefins in the Presence of Chloroplatinic Acid" J. Am. Chem. Soc.; 1958; 80; 4104

8. L.N. Lewis, K.G. Sy, G.L. Bryant, Jr. and P.E. Donahue. Organometallics 10 (1991), p. 3750.