Plutonyl
Encyclopedia
The plutonyl ion is an oxycation
of plutonium
in the oxidation state
+6, with the chemical formula
[PuO2]2+. It is isostructural
with the uranyl
ion, compared to which it has a slightly shorter M-O bond. It is easily reduced to plutonium(III). The plutonyl ion forms many complexes
, particularly with ligands that have oxygen donor atoms. Complexes of the plutonyl ion are important in nuclear fuel reprocessing.
ion very closely. Both ions are linear with the metal atom midway between the two oxygen atoms. Many compounds of the two ions are isostructural
. The ν (Pu-O) asymmetric stretching frequency is some 20 cm−1 lower, at ca. 910 cm−1, in complexes with the same ligand set. From this it can be inferred that the Pu-O bond is only a little weaker than the U-O bond. The electronic structures are also similar.
In aqueous solution there are some differences in hydrolysis behaviour, not only in the log β* values (definition of β*) but in nature of the polymeric species that can be formed. In the table below, 1,2 stoichiometry means a species with one actinyl ion and two hydroxide
ions, etc.. This is one of the few instances of notable differences between plutonyl and uranyl.
Distinct optical absorbance bands at 842 and 845 nm were observed for the mononuclear and dinuclear hydrolysis species. Hydrolysis of plutonyl is important for an understanding of pollution of natural waters.
Another significant difference is that plutonyl is a much stronger oxidizing agent
than uranyl. The standard reduction potentials for aqueous solutions are shown in the next table.
Conversely, plutonyl is more easily reduced than uranyl. This difference is utilized in the separation of plutonium from uranium in the PUREX
process, as described below.
The plutonyl ion is always associated with other ligands. The most common arrangement is for the so-called equatorial ligands to lie in a plane perpendicular to the O-Pu-O line and passing through the plutonium atom. With four ligands, as in [PuO2Cl4]2- the plutonium has a distorted octahedral environment, with a square of ligand atoms in the equatorial plane. In plutonyl nitrate, PuO2(NO3)22H2O, as in uranyl nitrate
there is a hexagon of six ligand atoms in the equatorial plane, four oxygen atoms from bidentate nitrate ions and two oxygens from the water molecules. Plutonyl nitrate, like uranyl nitrate, is soluble in diethyl ether
. The complex that is extracted has no electrical charge. This is the most important factor in making the complex soluble in organic solvents. Also the water molecules are replaced by ether molecules. Replacing the water molecules that are bound to the plutonyl ion in aqueous solution by a second, hydrophobic, ligand increases the solubility of the neutral complex in the organic solvent. This has been called a synergic effect.
The solubility of plutonyl nitrate in organic solvents is utilized in the PUREX
process. Plutonyl nitrate is extracted with tributyl phosphate
, (CH3CH2CH2CH2O)3PO, TBP, as the preferred second ligand, and kerosene the preferred organic solvent. It is recovered by treatment with aqueous ferrous
sulfamate
which selectively reduces the plutonium to the +3 oxidation state
in the aqueous solution, leaving the uranium in the organic phase. Plutonyl complex chemistry is an active research area, for dealing with environmental contamination.
Oxycation
An oxycation is a polyatomic ion with a positive charge that contains oxygen.-Examples:* Nitrosonium ion, NO+* Nitronium ion, NO2+* Vanadyl ion, VO2+, a very stable oxycationSee category for a bigger list....
of plutonium
Plutonium
Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...
in the oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
+6, with the chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
[PuO2]2+. It is isostructural
Isostructural
The term isostructural is used for chemical compounds that have similar chemical structures, and also with the meaning of isomorphous when used in relation to crystal structures...
with the uranyl
Uranyl
The uranyl ion is an oxycation of uranium in the oxidation state +6, with the chemical formula [UO2]2+. It has a linear structure with short U-O bonds, indicative of the presence of multiple bonds between uranium and oxygen. Four or more ligands are bound to the uranyl ion in an equatorial plane...
ion, compared to which it has a slightly shorter M-O bond. It is easily reduced to plutonium(III). The plutonyl ion forms many complexes
Complex (chemistry)
In chemistry, a coordination complex or metal complex, is an atom or ion , bonded to a surrounding array of molecules or anions, that are in turn known as ligands or complexing agents...
, particularly with ligands that have oxygen donor atoms. Complexes of the plutonyl ion are important in nuclear fuel reprocessing.
Chemical properties
The chemistry of the plutonyl ion resembles the chemistry of the uranylUranyl
The uranyl ion is an oxycation of uranium in the oxidation state +6, with the chemical formula [UO2]2+. It has a linear structure with short U-O bonds, indicative of the presence of multiple bonds between uranium and oxygen. Four or more ligands are bound to the uranyl ion in an equatorial plane...
ion very closely. Both ions are linear with the metal atom midway between the two oxygen atoms. Many compounds of the two ions are isostructural
Isostructural
The term isostructural is used for chemical compounds that have similar chemical structures, and also with the meaning of isomorphous when used in relation to crystal structures...
. The ν (Pu-O) asymmetric stretching frequency is some 20 cm−1 lower, at ca. 910 cm−1, in complexes with the same ligand set. From this it can be inferred that the Pu-O bond is only a little weaker than the U-O bond. The electronic structures are also similar.
In aqueous solution there are some differences in hydrolysis behaviour, not only in the log β* values (definition of β*) but in nature of the polymeric species that can be formed. In the table below, 1,2 stoichiometry means a species with one actinyl ion and two hydroxide
Hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...
ions, etc.. This is one of the few instances of notable differences between plutonyl and uranyl.
| Stoichiometry | uranyl | plutonyl |
|---|---|---|
| 1,1 | ||
| −5.76 | ||
| 1,2 | ||
| −11.69 | ||
| 2,2 | −7.79 | |
| 2,4 | −19.3 | |
| 3,4 | ||
| 3,5 | ||
Distinct optical absorbance bands at 842 and 845 nm were observed for the mononuclear and dinuclear hydrolysis species. Hydrolysis of plutonyl is important for an understanding of pollution of natural waters.
Another significant difference is that plutonyl is a much stronger oxidizing agent
Oxidizing agent
An oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...
than uranyl. The standard reduction potentials for aqueous solutions are shown in the next table.
| Couple | uranyl | plutonyl |
|---|---|---|
| MO22+/M4+ | 0.38 | 1.04 |
| M4+/M3+ | ||
| 1.01 |
Conversely, plutonyl is more easily reduced than uranyl. This difference is utilized in the separation of plutonium from uranium in the PUREX
PUREX
PUREX is an acronym standing for Plutonium - URanium EXtraction — de facto standard aqueous nuclear reprocessing method for the recovery of uranium and plutonium from used nuclear fuel. It is based on liquid-liquid extraction ion-exchange.The PUREX process was invented by Herbert H. Anderson and...
process, as described below.
The plutonyl ion is always associated with other ligands. The most common arrangement is for the so-called equatorial ligands to lie in a plane perpendicular to the O-Pu-O line and passing through the plutonium atom. With four ligands, as in [PuO2Cl4]2- the plutonium has a distorted octahedral environment, with a square of ligand atoms in the equatorial plane. In plutonyl nitrate, PuO2(NO3)22H2O, as in uranyl nitrate
Uranyl nitrate
Uranyl nitrate is a water soluble yellow uranium salt. The yellow-green crystals of uranium nitrate hexahydrate are triboluminescent.Uranyl nitrate can be prepared by reaction of uranium salts with nitric acid...
there is a hexagon of six ligand atoms in the equatorial plane, four oxygen atoms from bidentate nitrate ions and two oxygens from the water molecules. Plutonyl nitrate, like uranyl nitrate, is soluble in diethyl ether
Diethyl ether
Diethyl ether, also known as ethyl ether, simply ether, or ethoxyethane, is an organic compound in the ether class with the formula . It is a colorless, highly volatile flammable liquid with a characteristic odor...
. The complex that is extracted has no electrical charge. This is the most important factor in making the complex soluble in organic solvents. Also the water molecules are replaced by ether molecules. Replacing the water molecules that are bound to the plutonyl ion in aqueous solution by a second, hydrophobic, ligand increases the solubility of the neutral complex in the organic solvent. This has been called a synergic effect.
The solubility of plutonyl nitrate in organic solvents is utilized in the PUREX
PUREX
PUREX is an acronym standing for Plutonium - URanium EXtraction — de facto standard aqueous nuclear reprocessing method for the recovery of uranium and plutonium from used nuclear fuel. It is based on liquid-liquid extraction ion-exchange.The PUREX process was invented by Herbert H. Anderson and...
process. Plutonyl nitrate is extracted with tributyl phosphate
Tributyl phosphate
Tributyl phosphate, known commonly as TBP, is an organophosphorus compound with the formula 3PO. This colourless, odorless liquid finds some applications as an extractant and a plasticizer. It is an ester of orthophosphoric acid with n-butanol.- Production :Tributyl phosphate is manufactured by...
, (CH3CH2CH2CH2O)3PO, TBP, as the preferred second ligand, and kerosene the preferred organic solvent. It is recovered by treatment with aqueous ferrous
Ferrous
Ferrous , in chemistry, indicates a divalent iron compound , as opposed to ferric, which indicates a trivalent iron compound ....
sulfamate
Sulfamic acid
Sulfamic acid, also known as amidosulfonic acid, amidosulfuric acid, aminosulfonic acid, and sulfamidic acid, is a molecular compound with the formula H3NSO3...
which selectively reduces the plutonium to the +3 oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
in the aqueous solution, leaving the uranium in the organic phase. Plutonyl complex chemistry is an active research area, for dealing with environmental contamination.