Ruthenium

Ruthenium ( ) is a chemical element represented by the symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Similar to the other metals of the platinum group, ruthenium is inert to most chemicals. The Russian scientist Karl Ernst Claus discovered the element in 1844 and named it after Ruthenia, the Latin word for Rus'. Ruthenium usually occurs as a minor component of platinum ores and its annual production is only about 12 tonnes worldwide. Most ruthenium is used for wear-resistant electrical contacts and the production of thick-film resistors. A minor application of ruthenium is its use in some platinum alloys.

Characteristics

Physical properties

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A polyvalent hard white metal, ruthenium is a member of the platinum group and is in group 8 of the periodic table:

However, it has an atypical configuration in its outermost electron shells: whereas all other group 8 elements have 2 electrons in the outermost shell, in ruthenium, one of those is transferred to a lower shell. This effect can be observed in the neighborhood of niobium (41), ruthenium (44), rhodium (45), and palladium (46).

Ruthenium has four crystal modifications and does not tarnish at normal temperatures. Ruthenium dissolves in fused alkalis, is not attacked by acids but is attacked by halogens at high temperatures. Small amounts of ruthenium can increase the hardness of platinum and palladium. The corrosion resistance of titanium is increased markedly by the addition of a small amount of ruthenium.

This metal can be plated either by electroplating or by thermal decomposition methods. A ruthenium-molybdenum alloy is known to be superconductive at temperatures below 10.6 K.

Occurrence

Ruthenium is exceedingly rare and is the 74th most abundant metal on Earth. This element is generally found in ores with the other platinum group metals in the Ural Mountains and in North and South America. Small but commercially important quantities are also found in pentlandite extracted from Sudbury, Ontario, Canada, and in pyroxenite deposits in South Africa. The native form of ruthenium is a very rare mineral (Ir replaces part of Ru in its structure).

Production

Mining

Roughly 12 tonnes of Ru is mined each year with world reserves estimated as 5,000 tonnes.

Ruthenium, like the other platinum group metals, is obtained commercially as a by-product from nickel and copper mining and processing as well as by the processing of platinum group metal ores. During electrorefining of copper and nickel, noble metals such as silver, gold and the platinum group metals including selenium and tellurium settle to the bottom of the cell as anode mud, which forms the starting point for their extraction. Osmium, ruthenium, rhodium and iridium can be separated from platinum and gold and base metals by their insolubility in aqua regia, leaving a solid residue. Rhodium can be separated from the residue by treatment with molten sodium bisulfate. The insoluble residue, containing Ru, Os and Ir is treated with sodium oxide, in which Ir is insoluble, producing water-soluble Ru and Os salts. After oxidation to the volatile oxides, is separated from by precipitation of (NH₄)₃RuCl₆ with ammonium chloride or by distillation or extraction with organic solvents of the volatile osmium tetroxide. Hydrogen is used to reduce ammonium ruthenium chloride yielding a powder. The first method to precipitate the ruthenium with ammonium chloride is similar to the procedure that Smithson Tennant and William Hyde Wollaston used for their separation. Several methods are suitable for industrial scale production. In either case, the product is reduced using hydrogen, yielding the metal as a powder or sponge that can be treated using powder metallurgy techniques or by argon-arc welding.

From used nuclear fuels

Ruthenium is a fission product of uranium-235. Each kilo of fission products contains significant amounts of the lighter platinum group metals and also ruthenium. Used nuclear fuel might be a possible source for ruthenium. The complicated extraction is expensive and the also present radioactive isotopes of ruthenium would make a storage for several half-lives of the decaying isotopes necessary. This makes this source of ruthenium unattractive and no large-scale extraction has been started.

Chemical compounds

The oxidation states of ruthenium range from 0 to +8, and −2. The properties of ruthenium and osmium compounds are often similar. The +2, +3, and +4 states are the most common. The most prevalent precursor is ruthenium trichloride, a red solid that is poorly defined chemically but versatile synthetically.

Coordination and organometallic complexes

Ruthenium forms a variety of coordination complexes. Examples are the many pentammine derivatives [Ru(NH₃)₅L]ⁿ⁺ which often exist in both Ru(II) and Ru(III). Derivatives of bipyridine and terpyridine are numerous, best known being the luminiscent tris(bipyridine)ruthenium(II) chloride.

Ruthenium form a wide range compounds with carbon-ruthenium bonds. Ruthenocene is analogous to ferrocene structurally, but exhibits distinctive redox properties. A large number of complexes of carbon monoxide are known, the parent being triruthenium dodecacarbonyl. The analogue of iron pentacarbonyl, ruthenium pentacarbonyl is unstable at ambient conditions. Ruthenium trichloride carbonylates (reacts with carbon monoxide) to give mono- and diruthenium(II) carbonyls from which many derivatives have been prepared such as RuHCl(CO)(PPh₃)₃ and Ru(CO)₂(PPh₃)₃ (Roper's complex). Heating solutions of ruthenium trichloride in alcohols with triphenylphosphine gives tris(triphenylphosphine)ruthenium dichloride (RuCl₂(PPh₃)₃), which converts to the hydride complex chlorohydridotris(triphenylphosphine)ruthenium(II) (RuHCl(PPh₃)₃).

History

Though naturally occurring platinum, containing all six platinum group metals, was used for a long time by pre-Columbian Americans and known as a material to European chemists from the mid-16th century, it took until the mid-18th century for platinum to be identified as a pure element. The discovery that natural platinum contained palladium, rhodium, osmium and iridium took place in the first decade of the 19th century. Platinum in alluvial sands of Russian rivers gave access to raw material for use in plates and medals and for the minting of ruble coins, starting in 1828. Residues of platinum production for minting were available in the Russian Empire, and therefore most of the research on them was done in Eastern Europe.

It is possible that the Polish chemist Jędrzej Śniadecki isolated element 44 (which he called "vestium") from platinum ores in 1807. He published his discovery in Polish language in article "Rosprawa o nowym metallu w surowey platynie odkrytym" in 1808. His work was never confirmed, however, and he later withdrew his claim of discovery. They examined residues that were left after dissolving crude platinum from the Ural Mountains in aqua regia. Berzelius did not find any unusual metals, but Osann thought he found three new metals, pluranium, ruthenium and polinium. This discrepancy led to a long-standing controversy between Berzelius and Osann about the composition of the residues.

In 1844, the Russian scientist Karl Klaus showed that the compounds prepared by Gottfried Osann contained small amounts of ruthenium, which Klaus had discovered the same year.

Applications

Because of its ability to harden platinum and palladium, ruthenium is used in platinum and palladium alloys to make wear-resistant electrical contacts. In this application, only thin plated films are used to achieve the necessary wear-resistance. Because of its lower cost and similar properties compared to rhodium, The thin coatings are either put on by electroplating or sputtering.

Ruthenium dioxide, lead and bismuth ruthenates, the latter with perovskite crystal structure, are used in thick film chip resistors. The first two applications account for 50% of the ruthenium consumption. Ruthenium is also used in some advanced high-temperature single-crystal superalloys, with applications including the turbine blades in jet engines. Several nickel based superalloy compositions are described in the literature. Among them are EPM-102 (with 3 % Ru) and TMS-162 (with 6 % Ru), both containing 6 % rhenium, as well as TMS-138 and TMS-174. Fountain pen nibs are frequently tipped with alloys containing ruthenium. From 1944 onward, the famous Parker 51 fountain pen was fitted with the "RU" nib, a 14K gold nib tipped with 96.2% ruthenium and 3.8% iridium.

Ruthenium is a component of mixed-metal oxide (MMO) anodes used for cathodic protection of underground and submerged structures, and for electrolytic cells for chemical processes such as generating chlorine from salt water. The fluorescence of some ruthenium complexes is quenched by oxygen, which has led to their use as optode sensors for oxygen. Ruthenium red, [(NH₃)₅Ru-O-Ru(NH₃)₄-O-Ru(NH₃)₅]⁶⁺, is a biological stain used to stain polyanionic molecules such as pectin and nucleic acids for light microscopy and electron microscopy. The beta-decaying isotope 106 of ruthenium is used in radiotherapy of eye tumors, mainly malignant melanomas of the uvea. Ruthenium-centered complexes are being researched for possible anticancer properties. Ruthenium, unlike traditional platinum complexes, shows greater resistance to hydrolysis and more selective action on tumors. NAMI-A and KP1019 are two drugs undergoing clinical evaluation against metastatic tumors and colon cancers.

Catalysis

Ruthenium is a versatile catalyst. Hydrogen sulfide can be split by light by using an aqueous suspension of CdS particles loaded with ruthenium dioxide. This may be useful in the removal of H₂S in oil refineries and other industrial processing facilities. Organometallic ruthenium carbene and alkylidene complexes have been found to be highly efficient catalysts for olefin metathesis, a process with important applications in organic and pharmaceutical chemistry.

Solar energy conversion

Some ruthenium complexes absorb light throughout the visible spectrum and are being actively researched in various, potential, solar energy technologies. For example, Ruthenium-based compounds have been used for light absorption in dye-sensitized solar cells, a promising new low-cost solar cell system.

Data storage

Chemical vapor deposition of ruthenium (CVD) is used as a method to produce thin films of pure ruthenium on substrates. These films show promising properties for the use in microchips and for the giant magnetoresistive read element for hard disk drives. Ruthenium was also suggested as a possible material for microelectronics because its use is compatible with semiconductor processing techniques.

Exotic materials

Many ruthenium based oxides show very unusual properties, such as a Quantum Critical Point behavior, exotic superconductivity, and high temperature ferromagnetism.

References

External links

Nano-layer of ruthenium stabilizes magnetic sensors

Category:Chemical elements Category:Transition metals Category:Precious metals Category:Noble metals