![Uranyl-ion-structure](https://upload.wikimedia.org/wikipedia/commons/6/66/Uranyl-ion-structure.svg)
The uranyl ion is an
oxycation
In chemistry, an oxycation is a polyatomic ion with a positive charge that contains oxygen.
Examples
* Dioxygenyl ion,
* Nitrosonium ion,
* Nitronium ion,
* Vanadyl ion, VO2+, a very stable oxycation
* Uranyl ion, , all natural U6+ occurs in t ...
of
uranium
Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...
in the
oxidation state
In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
+6, with the
chemical formula
In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...
. It has a linear structure with short U–O bonds, indicative of the presence of
multiple bond
In chemistry, bond order, as introduced by Linus Pauling, is defined as the difference between the number of bonds and anti-bonds.
The bond order itself is the number of electron pairs (covalent bonds) between two atoms. For example, in diatomi ...
s between uranium and oxygen. Four or more
ligand
In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electr ...
s may be bound to the uranyl ion in an equatorial plane around the uranium atom. The uranyl ion forms many
complexes, particularly with ligands that have oxygen donor atoms. Complexes of the uranyl ion are important in the extraction of uranium from its ores and in
nuclear fuel reprocessing
Nuclear reprocessing is the chemical separation of fission products and actinides from spent nuclear fuel. Originally, reprocessing was used solely to extract plutonium for producing nuclear weapons. With commercialization of nuclear power, the ...
.
Structure and bonding
![F4M0](https://upload.wikimedia.org/wikipedia/commons/e/e7/F4M0.png)
The uranyl ion is linear and symmetrical, with both U–O bond lengths of about 180 pm. The bond lengths are indicative of the presence of multiple bonding between the uranium and oxygen atoms. Since uranium(VI) has the
electronic configuration
In atomic physics and quantum chemistry, the electron configuration is the distribution of electrons of an atom or molecule (or other physical structure) in atomic or molecular orbitals. For example, the electron configuration of the neon atom ...
of the preceding
noble gas
The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low chemi ...
,
radon
Radon is a chemical element with the symbol Rn and atomic number 86. It is a radioactive, colourless, odourless, tasteless noble gas. It occurs naturally in minute quantities as an intermediate step in the normal radioactive decay chains through ...
, the electrons used in forming the U–O bonds are supplied by the oxygen atoms. The electrons are donated into empty
atomic orbitals
In atomic theory and quantum mechanics, an atomic orbital is a function describing the location and wave-like behavior of an electron in an atom. This function can be used to calculate the probability of finding any electron of an atom in any spe ...
on the uranium atom. The empty orbitals of lowest energy are 7s, 5f and 6d. In terms of
valence bond theory
In chemistry, valence bond (VB) theory is one of the two basic theories, along with molecular orbital (MO) theory, that were developed to use the methods of quantum mechanics to explain chemical bonding. It focuses on how the atomic orbitals of ...
, the
sigma bond
In chemistry, sigma bonds (σ bonds) are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals. Sigma bonding is most simply defined for diatomic molecules using the language and tools of s ...
s may be formed using d
''z''2 and f
''z''3 to construct sd, sf and df
hybrid orbitals
In chemistry, orbital hybridisation (or hybridization) is the concept of mixing atomic orbitals to form new ''hybrid orbitals'' (with different energies, shapes, etc., than the component atomic orbitals) suitable for the pairing of electrons to f ...
(the ''z''-axis passes through the oxygen atoms). (d
''xz'', d
''yz'') and (f
''xz''2 and f
''yz''2) may be used to form
pi bond
In chemistry, pi bonds (Ï€ bonds) are covalent chemical bonds, in each of which two lobes of an orbital on one atom overlap with two lobes of an orbital on another atom, and in which this overlap occurs laterally. Each of these atomic orbitals ...
s. Since the pair of d or f orbitals used in bonding are
doubly degenerate, this equates to an overall
bond order
In chemistry, bond order, as introduced by Linus Pauling, is defined as the difference between the number of bonds and anti-bonds.
The bond order itself is the number of electron pairs (covalent bonds) between two atoms. For example, in diat ...
of three.
![UO2(aq)2(NO3)2wDistancess](https://upload.wikimedia.org/wikipedia/commons/5/52/UO2%28aq%292%28NO3%292wDistancess.svg)
The uranyl 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–U–O line and passing through the uranium atom. With four ligands, as in
2Cl4">O2Cl4sup>2−, the uranium has a distorted
octahedral
In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at ea ...
environment. In many cases more than four ligands occupy the equator.
In
uranyl fluoride
Uranyl fluoride is the inorganic compound with the formula UO2F2. As shown by x-ray crystallography, the uranyl (UO22+) centers are complemented by six fluoride ligands.
This salt is very soluble in water as well as hygroscopic. It is forms in th ...
, UO
2F
2, the uranium atom achieves a
coordination number
In chemistry, crystallography, and materials science, the coordination number, also called ligancy, of a central atom in a molecule or crystal is the number of atoms, molecules or ions bonded to it. The ion/molecule/atom surrounding the central i ...
of 8 by forming a layer structure with two oxygen atoms in a uranyl configuration and six fluoride ions bridging between uranyl groups. A similar structure is found in α-
uranium trioxide
Uranium trioxide (UO3), also called uranyl oxide, uranium(VI) oxide, and uranic oxide, is the hexavalent oxide of uranium. The solid may be obtained by heating uranyl nitrate to 400 °C. Its most commonly encountered polymorph, γ-UO3, is a ...
, with oxygen in place of fluoride, except that in that case the layers are connected by sharing oxygen atom from "uranyl groups", which are identified by having relatively short U–O distances. A similar structure occurs in some
uranate
A uranate is a ternary oxide involving the element uranium in one of the oxidation states 4, 5 or 6. A typical chemical formula is MxUyOz, where M represents a cation. The uranium atom in uranates(VI) has two short collinear U–O bonds and either ...
s, such as calcium uranate, CaUO
4, which may be written as Ca(UO
2)O
2 even though the structure does not contain isolated uranyl groups.
Spectroscopy
The colour of uranyl compounds is due to ligand-to-metal
charge transfer transitions at ca. 420 nm, on the blue edge of the
visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visual perception, visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' or simply light. A typical human eye wil ...
. The exact location of the absorption band and
NEXAFS
X-ray absorption near edge structure (XANES), also known as near edge X-ray absorption fine structure (NEXAFS), is a type of absorption spectroscopy that indicates the features in the X-ray absorption spectra (XAS) of condensed matter due to the p ...
bands depends on the nature of the equatorial ligands. Compounds containing the uranyl ion are usually yellow, though some compounds are red, orange or green.
Uranyl compounds also exhibit
luminescence
Luminescence is spontaneous emission of light by a substance not resulting from heat; or "cold light".
It is thus a form of cold-body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions or stress on a cryst ...
. The first study of the green luminescence of
uranium glass
Uranium glass is glass which has had uranium, usually in oxide diuranate form, added to a glass mix before melting for colouration. The proportion usually varies from trace levels to about 2% uranium by weight, although some 20th-century pieces ...
, by
Brewster in 1849, began extensive studies of the spectroscopy of the uranyl ion. Detailed understanding of this spectrum was obtained 130 years later. It is now well-established that the uranyl luminescence is more specifically a
phosphorescence
Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. Unlike fluo ...
, as it is due to a transition from the lowest triplet excited state to the singlet ground state. The luminescence from K
2UO
2(SO
4)
2 was involved in the discovery of
radioactivity.
The uranyl ion has characteristic ''ν''
U–O stretching
vibrations
Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin ''vibrationem'' ("shaking, brandishing"). The oscillations may be periodic, such as the motion of a pendulum—or random, such ...
at ca. 880 cm
−1 (
Raman spectrum
Raman spectroscopy () (named after Indian physicist C. V. Raman) is a Spectroscopy, spectroscopic technique typically used to determine vibrational modes of Molecule, molecules, although rotational and other low-frequency modes of systems may als ...
) and 950 cm
−1 (
infrared spectrum
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
). These frequencies depend somewhat on which ligands are present in the equatorial plane. Correlations are available between the stretching frequency and U–O bond length. It has also been observed that the stretching frequency correlates with the position of the equatorial ligands in the
spectrochemical series
A spectrochemical series is a list of ligands ordered by ligand "strength", and a list of metal ions based on oxidation number, group and element. For a metal ion, the ligands modify the difference in energy Δ between the d orbitals, called the l ...
.
Aqueous chemistry
![Uranium fraction diagram with no carbonate](https://upload.wikimedia.org/wikipedia/commons/a/ac/Uranium_fraction_diagram_with_no_carbonate.png)
The aqueous uranyl ion is a
weak acid
Acid strength is the tendency of an acid, symbolised by the chemical formula HA, to dissociate into a hydron (chemistry), proton, H+, and an anion, A-. The Dissociation (chemistry), dissociation of a strong acid in solution is effectively comple ...
.
:
2(H2O)4">O2(H2O)4sup>2+
2(H2O)3(OH)">O2(H2O)3(OH)sup>+ + H
+;
p''K''a = ca. 4.2
As pH increases polymeric species with stoichiometry
2)2(OH)2">UO2)2(OH)2sup>2+ and
2)3(OH)5">UO2)3(OH)5sup>+ are formed before the hydroxide UO
2(OH)
2 precipitates. The hydroxide dissolves in strongly alkaline solution to give hydroxo complexes of the uranyl ion.
The uranyl ion can be
reduced by mild reducing agents, such as zinc metal, to the oxidation state +4. Reduction to uranium(III) can be done using a
Jones reductor
A Jones reductor is a device used to reduce aqueous solutions of metal ions. The active component is a zinc amalgam. Jones reductors have been used for preparing solutions of titanium(III), vanadium(II), chromium(II), molybdenum(III), niobium(III), ...
.
Complexes
![Uranium fraction diagram with carbonate present](https://upload.wikimedia.org/wikipedia/commons/7/76/Uranium_fraction_diagram_with_carbonate_present.png)
The uranyl ion behaves as a
hard
Hard may refer to:
* Hardness, resistance of physical materials to deformation or fracture
* Hard water, water with high mineral content
Arts and entertainment
* ''Hard'' (TV series), a French TV series
* Hard (band), a Hungarian hard rock supe ...
acceptor and forms weaker complexes with nitrogen-donor ligands than with fluoride and oxygen donor ligands, such as hydroxide,
carbonate
A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...
,
nitrate
Nitrate is a polyatomic ion
A polyatomic ion, also known as a molecular ion, is a covalent bonded set of two or more atoms, or of a metal complex, that can be considered to behave as a single unit and that has a net charge that is not zer ...
,
sulfate
The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ar ...
and
carboxylate
In organic chemistry, a carboxylate is the conjugate base of a carboxylic acid, (or ). It is an ion with negative charge.
Carboxylate salts are salts that have the general formula , where M is a metal and ''n'' is 1, 2,...; ''carboxylat ...
. There may be 4, 5 or 6 donor atoms in the equatorial plane. In uranyl nitrate,
2(NO3)2">O2(NO3)2·2H
2O, for example, there are six donor atoms in the equatorial plane, four from
bidentate nitrato ligands and two from water molecules. The structure is described as
hexagonal bipyramid
A hexagonal bipyramid is a polyhedron formed from two hexagonal pyramid (geometry), pyramids joined at their bases. The resulting solid has 12 triangular face (geometry), faces, 8 vertex (geometry), vertices and 18 edges. The 12 faces are identic ...
al. Other oxygen-donor ligands include
phosphine oxide
Phosphine oxides are phosphorus compounds with the formula OPX3. When X = alkyl or aryl, these are organophosphine oxides. Triphenylphosphine oxide is an example. An inorganic phosphine oxide is phosphoryl chloride (POCl3).
Structure and bonding ...
s and
phosphate ester
In organic chemistry, organophosphates (also known as phosphate esters, or OPEs) are a class of organophosphorus compounds with the general structure , a central phosphate molecule with alkyl or Aryl, aromatic substituents. They can be conside ...
s.
Uranyl nitrate, UO
2(NO
3)
2, can be
extracted from aqueous solution into
diethyl ether
Diethyl ether, or simply ether, is an organic compound in the ether class with the formula , sometimes abbreviated as (see Pseudoelement symbols). It is a colourless, highly volatile, sweet-smelling ("ethereal odour"), extremely flammable liq ...
. The complex that is extracted has two nitrato ligands bound to the uranyl ion, making a complex with no electrical charge and also the water molecules are replaced by ether molecules, giving the whole complex notable
hydrophobic
In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, th ...
character. Electroneutrality is the most important factor in making the complex soluble in organic solvents. The nitrate ion forms much stronger complexes with the uranyl ion than it does with
transition metal
In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...
and
lanthanide
The lanthanide () or lanthanoid () series of chemical elements comprises the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium. These elements, along with the chemically similar elements scandium and yttr ...
ions. For this reason only uranyl and other actinyl ions, including the plutonyl ion, , can be extracted from mixtures containing other ions. Replacing the water molecules that are bound to the uranyl 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 complexes formed by the uranyl ion in aqueous solution are of major importance both in the extraction of uranium from its ores and in nuclear fuel reprocessing. In industrial processes, uranyl nitrate is extracted with
tributyl phosphate
Tributyl phosphate, known commonly as TBP, is an organophosphorus compound with the chemical formula (CH3CH2CH2CH2O)3PO. This colourless, odorless liquid finds some applications as an extractant and a plasticizer. It is an ester of phosphoric aci ...
(TBP, (CH
3CH
2CH
2CH
2O)
3PO) as the preferred second ligand and kerosene the preferred organic solvent. Later in the process, uranium is stripped from the organic solvent by treating it with strong nitric acid, which forms complexes such as
2(NO3)4">O2(NO3)4sup>2− which are more soluble in the aqueous phase. Uranyl nitrate is recovered by evaporating the solution.
[
]
Minerals
The uranyl ion occurs in minerals derived from uranium ore
Uranium ore deposits are economically recoverable concentrations of uranium
Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 9 ...
deposits by water-rock interactions that occur in uranium-rich mineral seams. Examples of uranyl containing minerals include:
* silicates: uranophane
Uranophane ( Ca( U O2)2( SiO3O H)2·5 H2O), also known as uranotile, is a rare calcium uranium silicate hydrate mineral that forms from the oxidation of other uranium-bearing minerals. It has a yellow color and is radioactive.
Alice Mary Weeks, a ...
(H3O)2Ca(UO2)2(SiO4)·3H2O)
* phosphates: autunite
Autunite (hydrated calcium uranyl phosphate), with formula Ca(UO2)2(PO4)2·10–12H2O, is a yellow-greenish fluorescent phosphate mineral with a hardness of 2–. Autunite crystallizes in the orthorhombic system and often occurs as tabular square ...
(Ca(UO2)2(PO4)2·8–12H2O), torbernite
Torbernite is a radioactive, hydrated green copper uranyl phosphate mineral, found in granites and other uranium-bearing deposits as a secondary mineral. Its name derives from the Swedish chemist Torbern Bergman (1735–1784), It is also known a ...
(Cu(UO2)2(PO4)·8–12H2O)
* arsenates: arsenuranospathite (Al(UO2)2(AsO4)2F·20H2O)
* vanadates: carnotite
Carnotite is a potassium uranium vanadate radioactive mineral with chemical formula K2( U O2)2( VO4)2·3 H2O. The water content can vary and small amounts of calcium, barium, magnesium, iron, and sodium are often present.
Occurrence
Carnotite i ...
(K2(UO2)2(VO4)2·3H2O), tyuyamunite
Tyuyamunite (pronounced tuh-YOO-ya-moon-ite) is a very rare uranium mineral with formula Ca(UO2)2V2O8·(5-8)H2O. It is a member of the carnotite group. It is a bright, canary-yellow color because of its high uranium content. Also, because of tyuyam ...
(Ca(UO2)2V2O8·8H2O)
* carbonates: schröckingerite
Schröckingerite is a radioactive yellow uranium-containing carbonate mineral, hydrated sodium calcium uranyl sulfate carbonate fluoride.http://www.webmineral.com/data/Schrockingerite.shtml Webmineralhttp://www.mindat.org/min-3584.html Mindat Sc ...
NaCa3(UO2)(CO3)3(SO4)F·10H2O
* oxalates: uroxite 2)2(C2O4)(OH)2(H2O)2">UO2)2(C2O4)(OH)2(H2O)2·H2O.
These minerals are of little commercial value as most uranium is extracted from pitchblende
Uraninite, formerly pitchblende, is a radioactive, uranium-rich mineral and ore with a chemical composition that is largely UO2 but because of oxidation typically contains variable proportions of U3O8. Radioactive decay of the uranium causes th ...
.
Uses
Uranyl salts are used to stain samples for electron and electromagnetic microscopy studies of DNA.
Health and environmental issues
Uranyl salts are toxic and can cause severe chronic kidney disease
Chronic kidney disease (CKD) is a type of kidney disease in which a gradual loss of kidney function occurs over a period of months to years. Initially generally no symptoms are seen, but later symptoms may include leg swelling, feeling tired, vo ...
and acute tubular necrosis
Acute tubular necrosis (ATN) is a medical condition involving the death of tubular epithelial cells that form the renal tubules of the kidneys. Because necrosis is often not present, the term acute tubular injury (ATI) is preferred by pathologists ...
. Target organs include the kidney
The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blood ...
s, liver
The liver is a major Organ (anatomy), organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of proteins and biochemicals necessary for ...
, lungs
The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of th ...
and brain
A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...
. Uranyl ion accumulation in tissues including gonocytes produces congenital disorder
A birth defect, also known as a congenital disorder, is an abnormal condition that is present at birth regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities can ...
s, and in white blood cells causes immune system damage. Uranyl compounds are also neurotoxin
Neurotoxins are toxins that are destructive to nerve tissue (causing neurotoxicity). Neurotoxins are an extensive class of exogenous chemical neurological insultsSpencer 2000 that can adversely affect function in both developing and mature ner ...
s. Uranyl ion contamination has been found on and around depleted uranium
Depleted uranium (DU; also referred to in the past as Q-metal, depletalloy or D-38) is uranium with a lower content of the fissile isotope than natural uranium.: "Depleted uranium possesses only 60% of the radioactivity of natural uranium, hav ...
targets.
All uranium compounds are radioactive
Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...
. However, uranium is usually in depleted form, except in the context of the nuclear industry. Depleted uranium consists mainly of 238U which decays by alpha decay
Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or 'decays' into a different atomic nucleus, with a mass number that is reduced by four and an atom ...
with a half-life of . Even if the uranium contained 235U which decays with a similar half-life of about , both of them would still be regarded as weak alpha emitters and their radioactivity is only hazardous with direct contact or ingestion.
References
{{reflist
Oxycations