Ruthenium

Ruthenium is a chemical element; it has 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 unreactive to most chemicals. Karl Ernst Claus, a Russian scientist of Baltic-German ancestry, discovered the element in 1844 at Kazan State University and named it in honor of Russia, using the Latin name Ruthenia. Ruthenium is usually found as a minor component of platinum ores; the annual production has risen from about 19 tonnes in 2009 to 35.5 tonnes in 2017. Most ruthenium produced is used in wear-resistant electrical contacts and thick-film resistors. A minor application for ruthenium is in platinum alloys and as a chemical catalyst. A new application of ruthenium is as the capping layer for extreme ultraviolet photomasks in semiconductor lithography. Ruthenium 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, and in pyroxenite deposits in South Africa.

Characteristics

Physical properties

Ruthenium, a polyvalent hard white metal, 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

While other group 8 elements have two electrons in the outermost shell, in ruthenium the outermost shell has only one electron. This anomaly, which has no effect on chemical properties, is also observed in all other elements in the range of Z = 41-45.

Chemical properties

Ruthenium has four crystal modifications and does not tarnish at ambient conditions; it oxidizes upon heating to. Ruthenium dissolves in fused alkalis to give ruthenates. It is not attacked by acids but is attacked by sodium hypochlorite at room temperature, and halogens at high temperatures. Ruthenium is most readily attacked by oxidizing agents. 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. The metal can be plated by electroplating and by thermal decomposition. A ruthenium–molybdenum alloy is known to be superconductive at temperatures below 10.6 K. Ruthenium is the only 4d transition metal that can assume the oxidation state +8, and even then it is less stable there than the heavier congener osmium: this is the first group from the left of the table where the second and third-row transition metals display notable differences in chemical behavior. Like iron but unlike osmium, ruthenium can form aqueous cations in its lower oxidation +2 and +3 states.
Ruthenium is the first in a downward trend in the melting and boiling points and atomization enthalpy in the 4d transition metals after the maximum seen at molybdenum, because the 4d subshell is more than half full and the electrons are contributing less to metallic bonding. Unlike the lighter congener iron, ruthenium is usually paramagnetic at room temperature, as iron also is above its Curie point. However, the metastable tetragonal phase of ruthenium, created as a thin film on single crystal Mo, is ferromagnetic at room temperature.
The reduction potentials in acidic aqueous solution for some common ruthenium species are shown below:

Isotopes

Naturally occurring ruthenium is composed of seven stable isotopes: 96, 98-102, 104. Additionally, 34 synthetic radioactive isotopes have been discovered. Of these radioisotopes, the most stable are ¹⁰⁶Ru with a half-life of 371.8 days, ¹⁰³Ru with a half-life of 39.245 days and ⁹⁷Ru with a half-life of 2.837 days.
Fifteen other radioisotopes have been characterized ranging from ⁸⁵Ru to ¹²⁵Ru. Most of these have half-lives that are less than five minutes; the exceptions are ⁹⁴Ru, ⁹⁵Ru, and ¹⁰⁵Ru.
The primary decay mode for isotopes lighter than the most abundant isotope, ¹⁰²Ru, is electron capture to produce technetium while the primary mode for heavier isotopes is beta emission to rhodium.
¹⁰⁶Ru is a fission product of uranium or plutonium. High concentrations of this isotope detected in the atmosphere over Europe were associated with an alleged undeclared nuclear accident in Russia in 2017.

Occurrence

Ruthenium is found in about 100 parts per trillion in the Earth's crust, making it the 78th most abundant element. It 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.

Production

Roughly 30 tonnes of ruthenium are mined each year, and world reserves are estimated at 5,000 tonnes. The composition of the mined platinum group metal mixtures varies widely, 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%. Ruthenium, osmium, and iridium are considered the minor platinum group metals.
Ruthenium, like the other platinum group metals, is obtained commercially as a by-product from processing of nickel, copper, and platinum metal ore. During electrorefining of copper and nickel, noble metals such as silver, gold, and the platinum group metals precipitate as anode mud, the feedstock for the extraction. The metals are converted to ionized solutes by any of several methods, depending on the composition of the feedstock. One representative method is fusion with sodium peroxide followed by dissolution in aqua regia, and solution in a mixture of chlorine with hydrochloric acid. Osmium, ruthenium, rhodium, and iridium are insoluble in aqua regia and readily precipitate, leaving the other metals in solution. Rhodium is 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 dissolved Ru and Os salts. After oxidation to the volatile oxides, is separated from by precipitation of 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 product is reduced using hydrogen, yielding the metal as a powder or sponge metal that can be treated with powder metallurgy techniques or argon-arc welding.
Ruthenium is contained in spent nuclear fuel, both as a direct fission product and as a product of neutron absorption by long-lived fission product. After allowing the unstable isotopes of ruthenium to decay, chemical extraction could yield ruthenium for use in all applications of ruthenium.
Ruthenium can also be produced by deliberate nuclear transmutation from. Given its relatively long half-life, high fission product yield and high chemical mobility in the environment, is among the most often proposed non-actinides for commercial-scale nuclear transmutation. has a relatively large neutron cross section, and because technetium has no stable isotopes, there would not be a problem of neutron activation of stable isotopes. Significant amounts of are produced in nuclear fission. They are also produced as a byproduct of the use of in nuclear medicine, because this isomer decays to. Exposing the target to strong enough neutron radiation will eventually yield appreciable quantities of ruthenium, which can be chemically separated while consuming.

Chemical compounds

The oxidation states of ruthenium range from −2 to +8. 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.

Oxides and chalcogenides

Ruthenium can be oxidized to ruthenium oxide, which can, in turn, be oxidized by sodium metaperiodate to the volatile yellow tetrahedral ruthenium tetroxide, RuO₄, an aggressive, strong oxidizing agent with structure and properties analogous to osmium tetroxide. RuO₄ is mostly used as an intermediate in the purification of ruthenium from ores and radiowastes.
Dipotassium ruthenate and potassium perruthenate are also known. Unlike osmium tetroxide, ruthenium tetroxide is less stable, is strong enough as an oxidizing agent to oxidize dilute hydrochloric acid and organic solvents like ethanol at room temperature, and is easily reduced to ruthenate in aqueous alkaline solutions; it decomposes to form the dioxide above 100 °C. Unlike iron but like osmium, ruthenium does not form oxides in its lower +2 and +3 oxidation states. Ruthenium forms dichalcogenides, which are diamagnetic semiconductors crystallizing in the pyrite structure. Ruthenium sulfide occurs naturally as the mineral laurite.
Like iron, ruthenium does not readily form oxoanions and prefers to achieve high coordination numbers with hydroxide ions instead. Ruthenium tetroxide is reduced by cold dilute potassium hydroxide to form black potassium perruthenate, KRuO₄, with ruthenium in the +7 oxidation state. Potassium perruthenate can also be produced by oxidizing potassium ruthenate, K₂RuO₄, with chlorine gas. The perruthenate ion is unstable and is reduced by water to form the orange ruthenate. Potassium ruthenate may be synthesized by reacting ruthenium metal with molten potassium hydroxide and potassium nitrate.
Some mixed oxides are also known, such as M^IIRu^IVO₃, Na₃Ru^VO₄, NaRuO, and MLnRuO.