Platinum family

Iridium, a metal of the platinum family, is white, similar to platinum, but with a slight yellowish cast. Because iridium is very hard and brittle, it is hard to machine, form, or work. 

Metals in the platinum family are recognized for their ability to promote combustion at lowtemperatures. Other catalysts include various oxides of copper, chromium, vanadium, nickel, and cobalt. These catalysts are subject to poisoning, particularly from halogens, halogen and sulfur compounds, zinc, arsenic, lead, mercury, and particulates. It is therefore important that catalyst surfaces be clean and active to ensure optimum performance. 

The most successful class of active ingredient for both oxidation and reduction is that of the noble metals silver, gold, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Platinum and palladium readily oxidize carbon monoxide, all the hydrocarbons except methane, and the partially oxygenated organic compounds such as aldehydes and alcohols. Under reducing conditions, platinum can convert NO to N2 and to NH3. Platinum and palladium are used in small quantities as promoters for less active base metal oxide catalysts. Platinum is also a candidate for simultaneous oxidation and reduction when the oxidant/re-ductant ratio is within 1% of stoichiometry. The other four elements of the platinum family are in short supply. Ruthenium produces the least NH3 concentration in NO reduction in comparison with other catalysts, but it forms volatile toxic oxides. 

An interesting, peculiar laboratory preparative reaction may finally be mentioned. This is based on the very high stability of the intermetallic compounds of actinides (and lanthanides) with the platinum family metals. The combined reduction capability of Pt with H2 (coupled reduction, see 6.7.2 fi) can be used to obtain, from its oxide, the platinide of the actinide metal. The An-Pt intermetallic compound can then be decomposed by heating in vacuum and the actinide can be obtained by distillation. 

On the basis of their behaviour and of their applications, however, it may be useful to describe these metals subdividing them into two families iron family and platinum family. 

Metals of the platinum family. These metals pertain to the 2nd and 3rd transition rows. They represent, together with silver and gold, the metals also known with the common name of noble metals. 

Platinum family metals preparation. The platinum metals (with gold and silver) are commonly found together, and a number of schemes are in current use for their separation. Platinum metals not only are extracted from ores but, due to their high value, are also recovered from a wide range of industrial residues such as Pt, Rh gauze catalysts, defective components from glass industries, etc. 

Table 5.43. Platinum family metals selected physical properties.

Table 5.45. Platinum family metals crystal structures, lattice parameters and calculated densities.

Platinum family metals and alloys. The platinum family (platinum group) metals (Ru, Os, Rh, Ir, Pd, Pt) belong (together with Au and possibly Ag and Re) to the class of precious metals. This characteristic together with their inertness to the environment (with regard to corrosion and oxidation) is the basis for a number of uses such as in the following. 

The powder and dust of rhodium metal are flammable in air. Some of the compounds may cause skin irritations. It is best to use approved laboratory procedures when handling any of the six elements in the platinum family of metals. 

One of palladiums unique characteristics is its abihty to absorb 900 times its own volume of hydrogen gas. When the surface of the pure metal is exposed to hydrogen gas (H ), the gas molecules break into atomic hydrogen. These hydrogen atoms then seep into the holes in the crystal structure of the metal. The result is a metallic hydride (PdH that changes palladium from an electrical conductor to a semiconductor. The compound palladium dichloride (PdCl ) also has the ability to absorb large quantities of carbon monoxide (CO). These characteristics are useful for many commercial applications. Palladium is the most reactive of all the platinum family of elements (Ru, Rh, Pd, Os, Is, and Pt.)... 

The earliest scientific descriptions of platinum, are those of Dr. Brownrigg and Don Antonio de UUoa in the middle of the eighteenth century. Rhodium, palladium, osmium, and iridium were discovered in 1803 and 1804, the first two by Dr. Wollaston and the others by his friend, Smithson Tennant. Thomsons History of Chemistry and Berzelius correspondence and diary present a pleasing picture of these two great English chemists. Ruthenium, the Russian member of the platinum family, was discovered much later by Karl Karlovich Klaus, whose life story was beautifully told by Professor B. N Menschutkin of the Polytechnic Institute of Leningrad. 

The element ruthenium is the little Benjamin of the platinum family. It did not see the light until more than a century after the discovery of platinum, but, to avoid separating it too far from its older brothers, its story will be told here. 

The bottom line on monocyclic aromatic compounds with re-electron sextet is that so far, among six-membered systems only benzene, the azines with 1 through 4 nitrogen atoms, phosphabenzene and arsaben-zene, pyrylium, azapyrylium, chalcogenopyrylium cations (with or without exocyclic groups such as hydroxy, amino and corresponding tautomeric or prototropic forms), and the metallabenzenes with platinum family metals have been proved to afford stable molecules under normal conditions. The list is richer for five-membered systems. 

Iridium is in Group 9 (VIIIB) of the periodic table. The periodic table is a chart that shows how elements are related to one another. Iridium is a transition metal that is also part of the platinum family. 

The metals in the platinum family are also known as the noble metals. They have this name because they do not react well with other elements and compounds. They appear to be too superior to react with most other substances. 

Platinum family (noble metals) A group of elements that occur close to platinum in the periodic table and in Earth s surface. 

Osmium is an element in Group 8 (VIIIB) of the periodic table. The periodic table is a chart showing how chemical elements are related to one another. Osmium is also a member of the platinum family. This family consists of five other elements ruthenium, rhodium, palladium, iridium, and platinum. These elements often occur together in Earth s cmst. They also have similar physical and chemical properties, and they are used in alloys. 

Like other members of the platinum family, ruthenium occurs in platinum ores. It is obtained from those ores and from the mineral osmir-idium by purification of the natural material. 

Ruthenium is most often combined with platinum or palladium in alloys. Electrical contacts, devices for measuring very high and very low temperatures, and medical instruments are often made from ruthenium alloys. Ruthenium is also used in alloys with other platinum family metals to make jewelry and art objects. This use is limited, however, because of the high cost of ruthenium metal. 

Oxidation of hydrocarbons and alcohols If reasonably effective oxidation catalysts can be identified for aqueous electrolytes, hydrocarbon and alcohol oxidation processes would make possible promising fuel cells operating directly on quite practical fuels at moderate temperatures. The currently used platinum and platinum-family metals and alloys have substantial activity, but it is not sufficient for practical fuel cells with aqueous electrolytes. With the many electrons involved in the complete oxidation, the detailed mechanisms for the oxidation are likely to be quite complex. To avoid incomplete oxidation it is probably necessary to have the reactants remain adsorbed on the electrode surface through the complete oxidation to C02 and H20. Here again, new promising catalysts and new experimental approaches are needed. 

Iridium — (L. iris, rainbow), Ir at. wt. 192.217(3) at. no. 77 m.p. 2446°C b.p. 4428°C sp. gr. 22.562 (20°C) valence 3 or 4. Discovered in 1803 by Tennant in the residue left when crude platinum is dissolved by aqua regia. The name iridium is appropriate, for its salts are highly colored. Iridium, a metal of the platinum family, is white, similar to platinum, but with a slight yellowish cast. It is very hard and brittle, making it very hard to machine, form, or work. It is the most corrosion-resistant metal known, and was used in making the standard meter... 

New Applications of TCNE in Organometallic Chemistry, A. J. Fatiadi (1987). Selected reactions used in organometallic synthesis are reviewed. 311 references are given. Structure and bonding of metal-TCNE complexes as well as reactions of TCNE with main-group organometallics, with transition-metal complexes, with metal-coordinated alkenes and alkynes, and reactions of platinum-family complexes are discussed. 

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