Ruthenium tetroxide is the
inorganic compound with the formula RuO
4. It is a yellow volatile solid that melts near room temperature.
It has the odor of ozone.
Samples are typically black due to impurities. The analogous
OsO4 is more widely used and better known. One of the few solvents in which RuO
4 forms stable solutions is
CCl4.
Preparation
RuO
4 is prepared by oxidation of
ruthenium(III) chloride with
NaIO4.
:8 Ru
3+(aq) + 5 IO
4−(aq) + 12 H
2O(l) → 8 RuO
4(s) + 5 I
−(aq) + 24 H
+(aq)
Due to its challenging reactivity, RuO
4 it is always generated ''in situ'' and used in catalytic quantities, at least in organic reactions. Typically a solution of ruthenium trichloride is oxidized by
sodium metaperiodate.
[
]
Structure
RuO4 forms two crystal structures, one with cubic symmetry and another with monoclinic
In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic s ...
symmetry, isotypic to OsO4. The molecule adopts a tetrahedral
In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the ...
geometry, with the Ru–O distances ranging from 169 to 170 pm.
Uses
Isolation of ruthenium from ores
The main commercial value of RuO4 is as an intermediate in the production of ruthenium compounds and metal from ores. Like other platinum group metals
The platinum-group metals (abbreviated as the PGMs; alternatively, the platinoids, platinides, platidises, platinum group, platinum metals, platinum family or platinum-group elements (PGEs)) are six noble, precious metallic elements clustered t ...
(PGMs), ruthenium
Ruthenium is a chemical element with the 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 chemical ...
occurs at low concentrations and often mixed with other PGMs. Together with OsO4, it is separated from other PGMs by distillation of a chlorine-oxidized extract. Ruthenium is separated from OsO4 by reducing RuO4 with hydrochloric acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid
Acid strength is the tendency of an acid, symbol ...
, a process that exploits the highly positive reduction potential for the 4">uO4sup>0/- couple.
Organic chemistry
RuO4 is of specialized value in organic chemistry because it oxidizes virtually any hydrocarbon. For example, it will oxidize adamantane
Adamantane is an organic compound with a formula C10H16 or, more descriptively, (CH)4(CH2)6. Adamantane molecules can be described as the fusion of three cyclohexane rings. The molecule is both rigid and virtually stress-free. Adamantane is the ...
to 1-adamantanol. Because it is such an aggressive oxidant, reaction conditions must be mild, generally room temperature. Although a strong oxidant, RuO4 oxidations do not perturb stereocenter
In stereochemistry, a stereocenter of a molecule is an atom (center), axis or plane that is the focus of stereoisomerism; that is, when having at least three different groups bound to the stereocenter, interchanging any two different groups c ...
s that are not oxidized. Illustrative is the oxidation of the following diol to a carboxylic acid:
:
Oxidation of epoxy alcohols also occurs without degradation of the epoxide ring:
:
Under milder conditions, oxidative reaction yields aldehyde
In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...
s instead. RuO4 readily converts secondary alcohols into ketones. Although similar results can be achieved with other cheaper oxidant
An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxi ...
s such as PCC
PCC may refer to:
Science and technology
* Pearson correlation coefficient (''r''), in statistics
* Periodic counter-current chromatography, a type of affinity chromatography
* Portable C Compiler, an early compiler for the C programming language
...
- or DMSO
Dimethyl sulfoxide (DMSO) is an organosulfur compound with the formula ( CH3)2. This colorless liquid is the sulfoxide most widely used commercially. It is an important polar aprotic solvent that dissolves both polar and nonpolar compounds ...
-based oxidants, RuO4 is ideal when a very vigorous oxidant is needed, but mild conditions must be maintained. It is used in organic synthesis to oxidize internal alkynes
\ce
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Acetylene
\ce
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\ce
Propyne
\ce
\ce
\ce
\ce
1-Butyne
In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and no ...
to 1,2-diketone
In organic chemistry, a dicarbonyl is a molecule containing two carbonyl () groups. Although this term could refer to any organic compound containing two carbonyl groups, it is used more specifically to describe molecules in which both carbonyls ...
s, and terminal alkynes along with primary alcohols to carboxylic acids. When used in this fashion, the ruthenium(VIII) oxide is used in catalytic amounts and regenerated by the addition of sodium periodate
Sodium periodate is an inorganic salt, composed of a sodium cation and the periodate anion. It may also be regarded as the sodium salt of periodic acid. Like many periodates, it can exist in two different forms: sodium ''meta''periodate (formula ...
to ruthenium(III) chloride and a solvent
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
mixture of acetonitrile, water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
and carbon tetrachloride. RuO4 readily cleaves double bonds to yield carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups. A compound containi ...
products, in a manner similar to ozonolysis
In organic chemistry, ozonolysis is an organic reaction where the unsaturated bonds of alkenes (), alkynes (), or azo compounds () are cleaved with ozone (). Alkenes and alkynes form organic compounds in which the multiple carbon–carbon b ...
. OsO4, a more familiar oxidant that is structurally similar to RuO4, does not cleave double bonds, instead producing vicinal diol products. However, with short reaction times and carefully controlled conditions, RuO4 can also be used for dihydroxylation.
Because RuO4 degrades the "double bonds" of arenes (especially electron-rich ones) by dihydroxylation and cleavage of the C-C bond in a way few other reagents can, it is useful as a "deprotection" reagent for carboxylic acids that are masked as aryl groups (typically phenyl or ''p''-methoxyphenyl). Because the fragments formed are themselves readily oxidizable by RuO4, a substantial fraction of the arene carbon atoms undergo exhaustive oxidation to form carbon dioxide. Consequently, multiple equivalents of the terminal oxidant (often in excess of 10 equivalents per aryl ring) are required to achieve complete conversion to the carboxylic acid, limiting the practicality of the transformation.
Although used as a direct oxidant
An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxi ...
, due to the relatively high cost of RuO4 it is also used catalytically with a cooxidant. For an oxidation of cyclic alcohols with RuO4 as a catalyst and bromate
The bromate anion, BrO, is a bromine-based oxoanion. A bromate is a chemical compound that contains this ion. Examples of bromates include sodium bromate, (), and potassium bromate, ().
Bromates are formed many different ways in municipal drin ...
as oxidant under basic conditions, RuO4 is first activated by hydroxide:
:RuO4 + OH− → HRuO5−
The reaction proceeds via a glycolate complex.
Other uses
Ruthenium tetroxide is a potential staining agent. It 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.
Gaseous release by nuclear accidents
Because of the very high volatility of ruthenium tetroxide () ruthenium radioactive isotopes with their relative short half-life are considered as the second most hazardous gaseous isotopes after iodine-131
Iodine-131 (131I, I-131) is an important radioisotope of iodine discovered by Glenn Seaborg and John Livingood in 1938 at the University of California, Berkeley. It has a radioactive decay half-life of about eight days. It is associated with nu ...
in case of release by a nuclear accident.[Backman, U., Lipponen, M., Auvinen, A., Jokiniemi, J., & Zilliacus, R. (2004)]
Ruthenium behaviour in severe nuclear accident conditions
Final report (No. NKS–100). Nordisk Kernesikkerhedsforskning.[Beuzet, E., Lamy, J. S., Perron, H., Simoni, E., & Ducros, G. (2012)]
Ruthenium release modelling in air and steam atmospheres under severe accident conditions using the MAAP4 code
Nuclear Engineering and Design, 246, 157-162. The two most important radioactive isotopes of ruthenium
Naturally occurring ruthenium (44Ru) is composed of seven stable isotopes. Additionally, 27 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 3 ...
are 103Ru and 106Ru. They have half-lives of 39.6 days and 373.6 days, respectively.
References
Further reading
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{{Oxides
Ruthenium compounds
Electron microscopy stains
Transition metal oxides