Rhodium
MELTING POINT: 1,960°C
BOILING POINT: 3,700°C
DENSITY: 12.41 g/cm3
MOST COMMON IONS: +, +2, +3
Rhodium was discovered in 1804 by the English chemist William Wollaston. Its name derives from the Greek word rhodos, meaning rose—the color of solutions containing rhodium salts. The abundance of rhodium in Earth's crust is approximately 0.0004 ppm. Its purification requires its separation from other platinum metals. Treatment of a crude platinum metal concentrate with aqua regia leaves an insoluble portion that is then fused with bisulfate; this dissolves only the rhodium component, which is eventually converted to a solution of chloro complexes, whose treatment with H2 precipitates the metal.
The extremely unreactive, silvery-white element is used in jewelry plating and is part of anodic and thermocouple materials. However, compounds containing Rh in oxidation states from (VI) to (−I) can be produced, the most common being those of Rh(III). Developments in Rh chemistry since the late 1960s have focused on various catalytic processes that use Rh(I) complexes, particularly those containing phosphines such as PR3 (R is an aryl or alkyl group) and/or CO, because such ligands stabilize this oxidation state.
The catalytic processes, which operate via mechanisms that cycle between Rh(I) and Rh(III) intermediates, include: (1) hydrogenation (the activation of H2 for the reduction of unsaturated organic compounds), (2) hydroformylation (the activation of H2 and CO for their addition to olefins to generate aldehydes or alcohols); and (3) carbonylation (the activation of CO for its addition to organics). Some of the processes have been developed commercially.
- An Rh(I) complex containing a chiral (optically active) phosphine ligand (i.e., one with three different substituents on the pyramidal P-atom) can catalyze a reaction such as R(R′)C=CHR′ + H2 → R(R′)C*HCH2R (where R, R′, and R′ are different substituents); the C* atom of the product has four different substituents and is therefore chiral, and thus the use of a small amount of a chiral Rh catalyst generates large amounts of chiral product. Such a catalytic asymmetric hydrogenation has been used for the production of L-dopa, a drug used to treat Parkinson's disease.
- An Rh(I) complex containing CO and PR3 ligands can catalyze the formation of butan-1-al from propene (CH2=CHCH3 + H2 + CO → CH3CH2CH2CHO), a commercially operated process.
- An Rh(I) complex containing CO is used industrially to synthesize acetic acid from methanol (CH3OH + CO → CH3COOH).
SEE ALSO PLATINUM.
Brian R. James
Bibliography
Griffith, W. P. (1967). The Chemistry of the Rarer Platinum Metals (Os, Ru, Ir, and Rh). New York: Interscience Publishers.
Parshall, George W., and Ittel, Steven D. (1992). Homogeneous Catalysis: The Applications and Chemistry of Catalysis by Soluble Transition Metal Complexes, 2nd edition. New York: Wiley.
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