Dihydrogen hexachloroplatinate (IV) hexahydrate

Dihydrogen hexachloroplatinate (IV) hexahydrate is the chemical compound with the formula H₂PtCl₆•(H₂O)₆). Also known as hexachlorplatinic acid or chloroplatinic acid, is one of the most readily available soluble compounds of platinum, but it is also unusual in that it is rarely obtained in a highly pure state. The commercial product is the oxonium salt of the hexachloroplatinate(IV) anion. Therefore, the correct formula is [H₃O]₂[PtCl₆]•4H₂O.(5,6) The related palladium compound,[H₃O]₂[PdCl₆] is extremely unstable and has not been isolated in pure form.^[1]

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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).^[2]

This method for determination of potassium was advantageous vs. the cobaltinitrite method used previously, since it required a single precipitation reaction.^{[citation needed]} Today, the concentration of potassium is determined with an ion-selective electrode. These modern methods remain subject to interference.

Production

Chloroplatinic acid is produced by dissolving platinum metal sponge in a 1:4 (v:v) solution of nitric acid and hydrochloric acid called aqua regia. This reaction is rumored to produce nitrogen-containing platinum compounds, but the product is H₂PtCl₆. Chloroplatinic acid is brownish-red, and can be isolated by evaporating this solution to a syrup.^[3]

Pt + 4 HNO₃ + 6 HCl → H₂PtCl₆ + 4 NO₂ + 4 H₂O

Alternative methods have been heavily investigated, but the older literature can be unreliable.^[4]

Chemical properties

Like many platinum compounds, chloroplatinic acid is used in catalysis. This compound was first used by John Speier and his colleagues from Dow Corning Corporation to catalyze the reaction of silyl hydrides with olefins.(3) Typical of his reactions, Speier used isopropanol solutions containing trichlorosilane (SiHCl₃), and methyldichlorosilane (CH₃HSiCl₂), 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, and possibly involved an oxidative addition reductive elimination process with the olefinic substrate.^[5][6] This process seems plausible since the chlorine ligands of the complex would be labile.

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. Lewis and his coworkers have suggested a mechanism involving Pt⁽⁰⁾ with olefins as ligands.^[7]

Related compounds

Chloroplatinic acid is occasionally contaminated with (NO)₂PtCl₆, and this problem has been the source of considerable discussion. This species is obtained by the reaction of nitrosyl chloride and Pt metal.^[8]

References

1. ^ Earnshaw, Greenwood, N. N. Chemistry of the Elements. Second Edition. Elsevier Butterworth-Heinemann. New York. 2005.
2. ^ G. F. Smith, J. L. Gring (1933). "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. 55 (10): 3957-3961. doi:10.1021/ja01337a007.
3. ^ Kauffman, G. B. "Ammonium Hexachloroplatinate" Inorganic Syntheses McGraw-Hill: New York, 1967, IX, pages 182-4.
4. ^ Rudnick, P.; Cooke, R. D. "THE PREPARATION OF HYDROCHLOROPLATINIC ACID BY MEANS OF HYDROGEN PEROXIDE" Journal of the American Chemical Society 1917, volume 39, pages 633-635.
5. ^ 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.
6. ^ 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" Journal of the American Chemical Society 1958, volume 80, page 4104.
7. ^ L.N. Lewis, K.G. Sy, G.L. Bryant, Jr. and P.E. Donahue. Organometallics 10 (1991), p. 3750.
8. ^ Moravek, R. T.; Kauffman, G. T. "Nitrosyl Hexachloroplatinate" Inorganic Syntheses, 1986, volume 24, p 217-219. ISBN 0-471-83441-6

Other reading

• Schweizer, G. T. Kerr. Thermal Decomposition of Hexachloroplatinic Acid. Inorg. Chem.; 1978; 17(8); 2326-2327.
• Holleman, Wiberg. Inorganic Chemistry First Edition. Academic Press. New York, 2001