Palladium

MELTING POINT: 1,552°C
BOILING POINT: 3,760°C
DENSITY: 12.0 g/cm³
MOST COMMON IONS: Pd²⁺

The element palladium was isolated and identified by William Wollaston in 1803. Its name comes from the asteroid Pallas. (Pallas was another name for Athena, the Greek goddess of wisdom.) Palladium in pure form is not found in nature. The preparation of the element is via a series of reactions. Platinum metal ore concentrates (65% of which come from the Merensky Reef in South Africa) are treated with aqua regia (giving copper and nickel as byproducts). The solutions, containing H₂PdCl₄ with platinum and gold complexes, are treated with FeCl₂ (which precipitates gold) and then with excess of NH₄OH followed by HCl to precipitate the impure [Pd(NH₃)₂Cl₂]. This compound is purified by dissolution in NH₄OH and precipitation with HCl. The pure [Pd(NH₃)₂Cl₂] is ignited to palladium metal.

Palladium metal, like platinum metal, is silvery-white and lustrous and has malleable and ductile properties. It has the face-centered cubic crystal structure. It forms a fluoride, PdF₄ (brick-red), and other halides: PdF₂ (pale violet), α-PdCl₂ (dark red), PbBr₂ (red black), and PdI₂ (black). Pd metal can absorb up to 935 times its own volume of hydrogen molecules. When the composition reaches about PdH_0.5, the substance becomes a semiconductor.

Palladium can form complexes in a variety of oxidation states. Table 1 contains some examples.

Palladium has extensive use as a catalyst in hydrogenation and dehydrogenation reactions, due to its capacity of combination with hydrogen. Palladium films are used as electrical contacts in connectors. Palladium-silver and palladium-nickel alloys are used to substitute for gold in jewelry.

Table 1. Some palladium complexes.

Lea B. Zinner

Bibliography

Allred, A. L. (1961). Journal of Inorganic Nuclear Chemistry 17:215. Greenwood, Norman N., and Earnshaw, A. (1984). Chemistry of the Elements. New York: Pergamon Press.

Livingstone, Stanley E. (1973). "The Platinum Metals." In Comprehensive Inorganic Chemistry, Vol. 3, ed. J. C. Bailar Jr.; H. J. Emeléus; Sir Ronald Nyholm; and A.F. Trotman-Dickenson. Oxford, U.K.: Pergamon Press.

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