Ruthenium

Ruthenium (pronounced /ruːˈθiːniəm/ roo-THEE-nee-əm) is the chemical element with the atomic number 44, and is represented by the symbol Ru. It is a rare transition metal of the platinum group of the periodic table; and like the other metals of the platinum group, ruthenium is chemically inert to most other chemicals. The Russian scientist Karl Klaus discovered the element in 1844 an named it after Ruthenia, the Latin word for Rus'. Ruthenium is found associated with platinum ores. Ruthenium is a minor component in these ores and therefore is a relatively rare element. Most of the ruthenium is used for wear resistant electrical contacts and the production of thick film resistors. A minor application is the use in some platinum alloys.

Characteristics
Physical
An irregular bar of lustrous silvery metal. One end is rough, as though broken, while the other, cigar-shaped end is relatively smooth.
A half of a pure, electron-beam remelted ruthenium bar

A polyvalent hard white metal, ruthenium is a member of the platinum group, and is in group 8 of the periodic table:
Z Element No. of electrons/shell
26 iron 2, 8, 14, 2
44 ruthenium 2, 8, 18, 15, 1
76 osmium 2, 8, 18, 32, 14, 2
108 hassium 2, 8, 18, 32, 32, 14, 2

However, it has an atypical configuration in its outermost electron shells compared to the rest of the members: 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 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, but does oxidize readily on exposure to air to form ruthenium tetroxide, RuO4, a strong oxidizing agent with properties analogous to those of osmium tetroxide. 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.[3]

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.[3]
Chemical
See also: category:Ruthenium compounds
A small amount of a slightly clumpy, vivid red powder
Tris(bipyridine)ruthenium(II)-chloride

The oxidation states of ruthenium range from +1 to +8, and −2 is known. Ruthenium compounds are often similar in properties to those of osmium and exhibit at least eight oxidation states, but the +2, +3, and +4 states are the most common. Examples are ruthenium(IV) oxide (RuO2, oxidation state +4), dipotassium ruthenate (K2RuO4, +6), potassium perruthenate (KRuO4, +7) and ruthenium tetroxide (RuO4, +8). Compounds of ruthenium with chlorine are ruthenium(II) chloride (RuCl2) and ruthenium(III) chloride (RuCl3).[4]
Organometallic chemistry

Ruthenium is a versatile metal that can easily form compounds with carbon-ruthenium bonds; these compounds tend to be darker in color and react more quickly than the osmium compounds. The organometallic ruthenium compound that is easiest to make is RuHCl(CO)(PPh3)3 (chlorohydrotris(triphenylphosphine)ruthenium (II)). This compound has two forms (yellow and pink) that are identical once they are dissolved but different in the solid state.[4]

An organometallic compound similar to ruthenocene, bis(2,4-dimethylpentadienyl)ruthenium, is readily synthesized in near quantitative yields and has applications in vapor-phase deposition of metallic ruthenium, as well as in catalysis, including Fischer-Tropsch synthesis of transportation fuels.[5] Important catalysts based on ruthenium are Grubbs' catalyst[6] and Roper's complex[7].
Isotopes
Main article: isotopes of ruthenium

Naturally occurring ruthenium is composed of seven stable isotopes. Additionally, 34 radioactive isotopes have been discovered. Of these radioisotopes, the most stable are 106Ru with a half-life of 373.59 days, 103Ru with a half-life of 39.26 days and 97Ru with a half-life of 2.9 days.[8][9]

Fifteen other radioisotopes have been characterized with atomic weights ranging from 89.93 u (90Ru) to 114.928 u (115Ru). Most of these have half-lives that are less than five minutes except 95Ru (half-life: 1.643 hours) and 105Ru (half-life: 4.44 hours).[8][9]

The primary decay mode before the most abundant isotope, 102Ru, is electron capture and the primary mode after is beta emission. The primary decay product before 102Ru is technetium and the primary mode after is rhodium.[8][9]
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.[10] 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.[11] Residues of platinum production for minting where 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. His work was never confirmed, however, and he later withdrew his claim of discovery.[12] Jöns Berzelius and Gottfried Osann nearly discovered ruthenium in 1827.[13] 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.[14]

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.[10] Klaus isolated ruthenium from the platinum residues of the rouble production while he was working in Kazan University, Kazan.[14] Klaus showed that ruthenium oxide contained a new metal and obtained 6 grams of ruthenium from the part of crude platinum that is insoluble in aqua regia.[14]

The name derives from Ruthenia, the Latin word for Rus', a historical area which includes present-day western Russia, Ukraine, Belarus, and parts of Slovakia and Poland. Karl Klaus used the name proposed by Gottfried Osann in 1828. He chose the element's name in honor of his birthland, as he was born in Tartu, Estonia, which was at the time a part of the Russian Empire.[10][15]
Occurrence
See also: category:Ruthenium minerals

Ruthenium is exceedingly rare and is the 74th most abundant metal on Earth.[12] 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 ruthenium is very rare mineral (Ir replaces part of Ru in its structure).[16][17]
Production
Mining

Roughly 12 tonnes of Ru is mined each year with world reserves estimated as 5,000 tonnes.[12] The composition of the mined platinum group metal (PGM) mixtures varies in a wide range depending on the geochemical formation. For example, the PGMs mined in South Africa contain on average 11% ruthenium while the PGMs mined in the USSR contain only 2% based on research dating from 1992.[18][19]

Ruthenium, like the other platinum group metals, is obtained commercially as a by-product from nickel and copper mining and processing or by direct 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.[16][17] In order to separate the metals, they must first be brought into solution. Several methods are available depending on the separation process and the composition of the mixture; two representative methods are fusion with sodium peroxide followed by dissolution in aqua regia, and dissolution in a mixture of chlorine with hydrochloric acid.[20][21] 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, RuO4 is separated from OsO4 by precipitation of (NH4)3RuCl6 with ammonium chloride or by distillation or extraction with organic solvents of the volatile osmium tetroxide.[22] Hydrogen is used to reduce ammonium ruthenium chloride yielding a powder.[4] 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.[23]
From used nuclear fuels
Main article: Synthesis of precious metals

Ruthenium is a fission product of uranium-235, therefore each kilo of fission products contains significant amounts of the lighter platinum group metals and therefore 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.[24][25][26]
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,[23] the use as plating material for electric contacts is one of the major applications.[16][27] The thin coatings are either put on by electroplating[28] or sputtering.[29]

Ruthenium dioxide, lead and bismuth[30] ruthenates, the latter with perovskite crystal structure,[31] are used in thick film chip resistors.[32] The first two applications account for 50% of the ruthenium consumption.[12]

There are only a few alloys used other than with elements of the platinum group metals. Ruthenium is always used in small quantities in those alloys to improve certain properties of the alloys. One example is the use of small amounts of ruthenium to increase the stability of gold in jewelry. The beneficial effect on the corrosion resistance of titanium alloys lead to the development of a special alloy containing 0.1% ruthenium .[33] 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,[34] as well as TMS-138[35] and TMS-174.[36][37] 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.[38]

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.[39] The fluorescence of some ruthenium complexes is quenched by oxygen, which has led to their use as optode sensors for oxygen.[40] 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.[41] The beta-decaying isotope 106 of ruthenium is used in radiotherapy of eye tumors, mainly malignant melanomas of the uvea.[42] Ruthenium-centered complexes are being researched for possible anticancer properties.[43] 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.

Laboratory uses

Ruthenium is also 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 from oil refineries and from other industrial processes.[44] Organometallic ruthenium carbene and allenylidene complexes have recently been found as highly efficient catalysts for olefin metathesis with important applications in organic and pharmaceutical chemistry.[45] Some ruthenium complexes absorb light throughout the visible spectrum and are being actively researched in various, potential, solar energy technologies. Ruthenium-based dyes have been used as the electron providers in dye-sensitized solar cells, a promising new low-cost solar cell system.[46] Ruthenium chemical vapor deposition (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.[47] Ruthenium was also suggested as a possible material for microelectronics because it's use is compatible with semiconductor processing techniques.[48]

Precautions

The compound ruthenium tetroxide, RuO4, similar to osmium tetroxide, is volatile, highly toxic and may cause explosions if allowed to come into contact with combustible materials.[49] Ruthenium plays no biological role but does strongly stain human skin, may be carcinogenic[50] and bio-accumulates in bone.

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