Plutonium compounds

Plutonium compounds are compounds containing the element plutonium. At room temperature, pure plutonium is silvery in color but gains a tarnish when oxidized. The element displays four common ionic oxidation states in aqueous solution and one rare one:

Pu, as Pu³⁺
Pu, as Pu⁴⁺
Pu, as
Pu, as
Pu, as -the heptavalent ion is rare.

The color shown by plutonium solutions depends on both the oxidation state and the nature of the acid anion. It is the acid anion that influences the degree of complexing—how atoms connect to a central atom—of the plutonium species. Additionally, the formal +2 oxidation state of plutonium is known in the complex , Cp″ = C₅H₃₂.
A +8 oxidation state has been reported as well in the volatile tetroxide. Though it readily decomposes via a reduction mechanism similar to, it is stated that can be stabilized in alkaline solutions, tetrachloromethane, and chloroform. However, a more recent study showed that Pu does not form in aqueous alkaline solutions.
Metallic plutonium is produced by reacting plutonium tetrafluoride with barium, calcium or lithium at 1200 °C. Metallic plutonium is attacked by acids, oxygen, and steam but not by alkalis and dissolves easily in concentrated hydrochloric, hydroiodic and perchloric acids. Molten metal must be kept in a vacuum or an inert atmosphere to avoid reaction with air. At 135 °C the metal will ignite in air and will explode if placed in carbon tetrachloride.
Plutonium is a reactive metal. In moist air or moist argon, the metal oxidizes rapidly, producing a mixture of oxides and hydrides. If the metal is exposed long enough to a limited amount of water vapor, a powdery surface coating of PuO₂ is formed. Also formed is plutonium hydride but an excess of water vapor forms only PuO₂.
Plutonium shows enormous, and reversible, reaction rates with pure hydrogen, forming plutonium hydride. It also reacts readily with oxygen, forming PuO and PuO₂ as well as intermediate oxides; plutonium oxide fills 40% more volume than plutonium metal. The metal reacts with the halogens, see below. The following oxyhalides are observed: PuOF, PuOCl, PuOBr, and PuOI. It will react with carbon to form PuC, nitrogen to form PuN, and silicon to form PuSi₂.
The organometallic chemistry of plutonium complexes is typical for organoactinide species; a characteristic example of an organoplutonium compound is plutonocene. Computational chemistry methods indicate an enhanced covalent character in the plutonium-ligand bonding.
Powders of plutonium, its hydrides and certain oxides like Pu₂O₃
are pyrophoric, meaning they can ignite spontaneously at ambient temperature and are therefore handled in an inert, dry atmosphere of nitrogen or argon. Bulk plutonium ignites only when heated above 400 °C. Pu₂O₃ spontaneously heats up and transforms into PuO₂, which is stable in dry air, but reacts with water vapor when heated.
Crucibles used to contain plutonium need to be able to withstand its strongly reducing properties. Refractory metals such as tantalum and tungsten along with the more stable oxides, borides, carbides, nitrides and silicides can tolerate this. Melting in an electric arc furnace can be used to produce small ingots of the metal without the need for a crucible.
Cerium is used as a chemical simulant of plutonium for development of containment, extraction, and other technologies.

Halides

Plutonium reacts readily with halogens, forming halides of composition PuX₃ or PuX₄. The higher plutonium fluorides and and the higher plutonium chloride are also known, though and are only known in the gas phase. Several of these compounds form hydrates, with compositions,,,,, and.

Fluorides

The compounds,, and are well-characterized. The plutonium fluoride hydrate can be prepared from adding hydrofluoric acid to a plutonium solution in nitric acid, from which it precipitates. The hydrate can then be converted to the anhydrous form by heating in hydrofluoric acid stream. However, this method for production is not favored as it can lead to problems with filtration after the product is precipitated. Alternatively, it can be produced by reacting plutonium compounds, such as or with gaseous fluorinating agents, such as,,,, or. This method also produces other plutonium fluorides, but these can be reduced to the trifluoride by action of reducing agents such as hydrogen gas. Due to the toxicity of these fluorinating agents, other methods have also been proposed, such as the reaction of plutonium dioxide with ammonium bifluoride. is used to prepare plutonium metal, forming the metal on action by calcium and iodine.
As with, a plutonium fluoride hydrate,, can be precipitated by hydrofluoric acid in aqueous solution. Surprisingly, however, decomposition of this compound in vacuum yields instead of. Like with, though, it can be converted to by heating in a stream of HF. It is most commonly produced by reacting plutonium dioxide with hydrofluoric acid at high temperatures with oxygen gas. Several other materials can be used in place of plutonium dioxide, like plutonium nitrate or plutonium peroxide. Anhydrous adopts a polymeric structure, isostructural with uranium tetrafluoride. In addition to the anhydrous form, two distinct hydrates of are known: the aforementioned, and . The hydrates are also isostructural to the corresponding uranium compounds. adopts a cubic structure, while adopts an orthorhombic structure. has been found to slowly age over time; a 30 year old sample of was found to contain 88% anhydrous, 8%, and 4%. It is an important intermediate in plutonium metal production.
Upon reaction with fluorine gas at high temperatures, is oxidized to plutonium hexafluoride,. It is also readily produced upon reaction with powerful fluorinating agents, like krypton difluoride and dioxygen difluoride. Unlike the related compound uranium hexafluoride, is unstable and highly reactive. solid is prone to radiolysis from alpha particles produced in the radioactive decay of plutonium, forming and fluorine gas; however, gas phase reaches a stable equilibrium. It undergoes slow hydrolysis to plutonyl fluoride. In hydrofluoric acid solution, forms a hydrate,, and a solid incorporating HF,.
Plutonium also forms a few fluorides intermediate between and, namely plutonium fluoride, as well as the mixed-valent. is a brick-red solid formed during the production of. is a weakly-characterized compound only known in the gas phase.

Chlorides

The only stable binary plutonium chloride in the solid phase is plutonium chloride,. It can be produced in several ways. For medium-scale reactions, the best method of production is the action of hydrochloric acid on plutonium oxalate. Plutonium oxalate can also be converted to plutonium chloride by action of hexachloropropene. Analytically pure samples of can be created by reacting plutonium oxide prepared by calcination of plutonium oxalate with phosgene or carbon tetrachloride at elevated temperatures. has been determined to adopt the uranium chloride-type structure. In addition to anhydrous, multiple hydrates are also known, with compositions and., which can be synthesized by evaporating HCl solutions, adopts the -type structure, consisting of cations which are linked by chloride anions. It melts in its own waters of crystallization at 94 °C. is isostructural with.
While it does not exist as a solid, dissociating into plutonium chloride and chlorine gas, binary plutonium tetrachloride is known in the gas phase, and it forms several stable adducts, such as with dimethoxyethane or diphenylsulfoxide. Plutonium tetrachloride gas is formed from the reaction of plutonium chloride with chlorine gas at high temperatures, enhancing the volatility of the product. The dimethoxyethane adduct, , can be prepared by evaporating a plutonium solution in HCl, adding the residue to a solution of DME, and then adding. This adduct can be used to prepare other compounds. For example, when it is dissolved in tetrahydrofuran, it forms the mixed-valence complex .
Despite solid being unknown, several stable solid plutonium chlorides derived from, such as the hexachloroplutonates, are well-characterized. The hexachloroplutonate ion is the dominant plutonium species in concentrated hydrochloric acid, and several compounds are known containing this ion. Dicaesium hexachloroplutonate can be precipitated from concentrated HCl solution by the addition of caesium chloride. can be used to prepare other plutonium compounds, such as the cyclopentadienide complex. Other hexachloroplutonates are also known, such as,,, and.

Bromides

The only stable compound in the plutonium-bromine system is PuBr₃. It is formed via several methods, the two best ones being direct synthesis from plutonium and bromine, and reaction of plutonium hydride with hydrobromic acid. It is also prepared by the hydrobromination of various plutonium compounds, like,, and, vacuum decomposition of, or reaction of with bromine. Its structure, termed the structure, consists of polyhedra, where plutonium has a coordination geometry of bicapped trigonal prismatic, which link together to form infinite chains. A hydrate of plutonium bromide,, is also known, and is formed from plutonium bromide solutions containing water. Its structure consists of the complex, and is isostructural with. It also forms a complex with tetrahydrofuran, , which can be formed by reacting plutonium and bromine in THF solution, though the hexahydrate is formed if water is present in the THF solution. The THF complex is used in the synthesis of other plutonium compounds.
A few plutonium bromide complexes are known containing plutonium. Plutonium tetrabromide forms stable complexes with hexamethylphosphoramide triphenylphosphine oxide, and tricyclohexylphosphine oxide, as well as an unstable complex with acetonitrile, which decomposes to a plutonium compound. The plutonium complex ion is also known, found in the tetraethylammonium salt.