Ruthenium

Ruthenium,  ₄₄Ru

General properties
Name, symbol	ruthenium, Ru
Appearance	silvery white metallic
Pronunciation	/ruːˈθiːniəm/ roo-THEE-nee-əm
Ruthenium in the periodic table
Fe ↑ Ru ↓ Os technetium ← ruthenium → rhodium
Atomic number (Z)	44
Group, block	group 8, d-block
Period	period 5
Element category	transition metal
Standard atomic weight (±) (Ar)	101.07(2)[1]
Electron configuration	[Kr] 4d^7 5s^1
per shell	2, 8, 18, 15, 1
Physical properties
Melting point	2607 K ​(2334 °C, ​4233 °F)
Boiling point	4423 K ​(4150 °C, ​7502 °F)
Density near r.t.	12.45 g/cm^3
when liquid, at m.p.	10.65 g/cm^3
Heat of fusion	38.59 kJ/mol
Heat of vaporization	619 kJ/mol
Molar heat capacity	24.06 J/(mol·K)
vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 2588 2811 3087 3424 3845 4388
Atomic properties
Oxidation states	-4, -2, 1,[2] 2, 3, 4, 5, 6, 7, 8 ​(a mildly acidic oxide)
Electronegativity	Pauling scale: 2.2
Ionization energies	1st: 710.2 kJ/mol 2nd: 1620 kJ/mol 3rd: 2747 kJ/mol
Atomic radius	empirical: 134 pm
Covalent radius	146±7 pm
Miscellanea
Crystal structure	​hexagonal close-packed (hcp)
Speed of sound thin rod	5970 m/s (at 20 °C)
Thermal expansion	6.4 µm/(m·K) (at 25 °C)
Thermal conductivity	117 W/(m·K)
Electrical resistivity	71 nΩ·m (at 0 °C)
Magnetic ordering	paramagnetic[3]
Young's modulus	447 GPa
Shear modulus	173 GPa
Bulk modulus	220 GPa
Poisson ratio	0.30
Mohs hardness	6.5
Brinell hardness	2160 MPa
CAS Number	7440-18-8
History
Naming	after Ruthenia (Latin for: medieval Kyivska Rus' region)
Discovery and first isolation	Karl Ernst Claus (1844)
Most stable isotopes of ruthenium
iso NA half-life DM DE (MeV) DP ^96Ru 5.52% >6.7×10^16 y (β^+β^+) 2.7188 ^96Mo ^97Ru syn 2.9 d ε – ^97Tc γ 0.215, 0.324 – ^98Ru 1.88% – (SF) <11.690 – ^99Ru 12.7% – (SF) <12.368 – ^100Ru 12.6% – (SF) <13.634 – ^101Ru 17.0% – (SF) <13.205 – ^102Ru 31.6% – (SF) <12.049 – ^103Ru syn 39.26 d β^− 0.226 ^103Rh γ 0.497 – ^104Ru 18.7% – (β^−β^−) 1.2997 ^104Pd ^106Ru syn 373.59 d β^− 0.039 ^106Rh
Decay modes in parentheses are predicted, but have not yet been observed

Ruthenium is a chemical element with symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to most other chemicals. The Baltic German scientist Karl Ernst Claus discovered the element in 1844, and named it after his homeland, the Russian Empire (one of Russia's Latin names is Ruthenia). Ruthenium usually occurs as a minor component of platinum ores; annual production is about 20 tonnes.^[4] Most ruthenium produced 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, and as a catalyst.

Characteristics

Physical properties

Gas phase grown crystals of ruthenium metal.

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 (predicted)

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 neighboring metals niobium (41), rhodium (45), and palladium (46).

Ruthenium has four crystal modifications and does not tarnish unless subject to high 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.^[5] The 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.^[5]

Isotopes

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.^[6][7]

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).^[6][7]

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 decay product after is rhodium.^[6][7]

Occurrence

Ruthenium is exceedingly rare, only the 74th most abundant element in Earth's crust.^[8] 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).^[9][10]

Production

Mining

Roughly 12 tonnes of ruthenium is mined each year with world reserves estimated as 5,000 tonnes.^[8] 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 former USSR contain only 2% based on research dating from 1992.^[11][12]

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 settle to the bottom of the cell as anode mud, which forms the starting point for their extraction.^[9][10] 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.^[13][14] 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, RuO
4 is separated from OsO
4 by precipitation of (NH₄)₃RuCl₆ with ammonium chloride or by distillation or extraction with organic solvents of the volatile osmium tetroxide.^[15] Hydrogen is used to reduce ammonium ruthenium chloride yielding a powder.^[16] 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 metal that can be treated using powder metallurgy techniques or by argon-arc welding.^[17]

From used nuclear fuels

Fission products of uranium-235 contain significant amounts of ruthenium and the lighter platinum group metals and therefore used nuclear fuel might be a possible source of ruthenium. The complicated extraction is expensive and the radioactive isotopes of ruthenium that are present would make 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.^[18][19][20]

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.^[16]

Oxides

Ruthenium can be oxidized to ruthenium(IV) oxide (RuO₂, oxidation state +4) which can in turn be oxidized by sodium metaperiodate to ruthenium tetroxide, RuO₄, a strong oxidizing agent with structure and properties analogous to osmium tetroxide. Like osmium tetroxide, ruthenium tetroxide is a potent fixative and stain for electron microscopy of organic materials, and is mostly used to reveal the structure of polymer samples.^[21] Dipotassium ruthenate (K₂RuO₄, +6), and potassium perruthenate (KRuO₄, +7) are also known.^[22]

Coordination and organometallic complexes

Tris(bipyridine)ruthenium(II) chloride.

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 luminescent tris(bipyridine)ruthenium(II) chloride.

Ruthenium forms 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).^[16]

In the area of fine chemical synthesis, Grubbs' catalyst is used for alkene metathesis.^[23]

Ruthenides

Metal ruthenides (Ru²⁻) are very rare, but are commonly found in superconductor applications, especially with regard to lanthanide metals such as cerium ruthenide (CeRu₂).^[24]

History

Though naturally occurring platinum alloys containing all six platinum-group metals were 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 occurred in the first decade of the 19th century.^[25] 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.^[26] 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" after the asteroid Vesta discovered shortly before) from South American platinum ores in 1807. He published an announcement of his discovery in 1808.^[27] His work was never confirmed, however, and he later withdrew his claim of discovery.^[8]

Jöns Berzelius and Gottfried Osann nearly discovered ruthenium in 1827.^[28] 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, which he called pluranium, ruthenium and polinium. This discrepancy led to a long-standing controversy between Berzelius and Osann about the composition of the residues.^[29] As Osann was not able to repeat his isolation of ruthenium, he eventually relinquished his claims.^[29][30] The name "ruthenium" was chosen by Osann because the analysed samples stemmed from the Ural Mountains in Russia.^[31] The name itself derives from Ruthenia, the Latin word for Rus', a historical area that included present-day western Russia, Ukraine, Belarus, and parts of Slovakia and Poland.

In 1844, the Russian scientist of Baltic German descent Karl Ernst Claus showed that the compounds prepared by Gottfried Osann contained small amounts of ruthenium, which Claus had discovered the same year.^[25] Claus isolated ruthenium from the platinum residues of the rouble production while he was working in Kazan University, Kazan.^[29] Claus 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.^[29] Choosing the name for the new element, Claus stated: "I named the new body, in honour of my Motherland, ruthenium. I had every right to call it by this name because Mr. Osann relinquished his ruthenium and the word does not yet exist in chemistry."^[29][32]

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,^[17] the use as plating material for electric contacts is one of the major applications.^[9][33] The thin coatings are either applied by electroplating^[34] or sputtering.^[35]

Ruthenium dioxide and lead and bismuth ruthenates are used in thick-film chip resistors.^[36][37][38] These two electronic applications account for 50% of the ruthenium consumption.^[8]

Only a few ruthenium alloys are used other than those with other platinum group metals. Ruthenium is often used in small quantities in those alloys to improve some of their properties. The beneficial effect on the corrosion resistance of titanium alloys led to the development of a special alloy containing 0.1% ruthenium.^[39] 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), as well as TMS-138^[40] and TMS-174.^[41][42] both containing 6% rhenium.^[43] 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.^[44]

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.^[45] The fluorescence of some ruthenium complexes is quenched by oxygen, which has led to their use as optode sensors for oxygen.^[46] 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.^[47] The beta-decaying isotope 106 of ruthenium is used in radiotherapy of eye tumors, mainly malignant melanomas of the uvea.^[48] Ruthenium-centered complexes are being researched for possible anticancer properties.^[49] Compared with platinum complexes, those of ruthenium show 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.

Ruthenium tetroxide is used to expose latent fingerprints by turning to the brown/black ruthenium dioxide when in contact with fatty oils or fats contained in sebaceous contaminants of the print.^[50]

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.^[51] 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.^[52] Ruthenium-promoted cobalt catalysts are used in Fischer-Tropsch synthesis.

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.^[53]

Data storage

Chemical vapor deposition of ruthenium 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.^[54] Ruthenium was also suggested as a possible material for microelectronics because its use is compatible with semiconductor processing techniques.^[55]

Exotic materials

Many ruthenium-based oxides show very unusual properties, such as a quantum critical point behavior,^[56] exotic superconductivity,^[57] and high-temperature ferromagnetism.^[58]

Cultural references

Ruthenium plays an important role in the novel Arctic Drift by Clive Cussler. Ruthenium is found to be a catalyst in a very important new industrial process, but very little of the metal is known to remain on Earth, and the search for a new deposit in Canada is a vital part of the plot.

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External links

• Ruthenium at The Periodic Table of Videos (University of Nottingham)
• Nano-layer of ruthenium stabilizes magnetic sensors