Protactinium compounds

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Protactinium compounds are compounds containing the element protactinium (symbol: Pa). These compounds usually have protactinium in the +5 oxidation state, although these compounds can also exist in the +2, +3 and +4 oxidation states.

Contents

Properties of protactinium compounds

Formulacolorsymmetry space group No Pearson symbol a (pm)b (pm)c (pm)Zdensity (g/cm3)
Pasilvery-gray tetragonal [1] I4/mmm139tI2392.5392.5323.8215.37
PaOrocksalt [2] Fm3m225cF8496.1413.44
PaO2 blackfcc [2] Fm3m225cF12550.5410.47
Pa2O5 whiteFm3m [2] 225cF16547.6547.6547.6410.96
Pa2O5whiteorthorhombic [2] 692402418
PaH3blackcubic [2] Pm3n223cP32664.8664.8664.8810.58
PaF4brown-redmonoclinic [2] C2/c15mS602
PaCl4green-yellow tetragonal [3] I41/amd141tI20837.7837.7748.144.72
PaBr4browntetragonal [4] [5] I41/amd141tI20882.4882.4795.7
PaCl5 yellow monoclinic [6] C2/c15mS24797113583643.74
PaBr5redmonoclinic [5] [7] P21/c14mP24838.51120.51214.644.98
PaOBr3monoclinic [5] C21691.1387.1933.4
Pa(PO3)4orthorhombic [8] 696.9895.91500.9
Pa2P2O7cubic [8] Pa3865865865
Pa(C8H8)2golden-yellowmonoclinic [9] 7098751062

Here a, b and c are lattice constants in picometers, No is space group number and Z is the number of formula units per unit cell; fcc stands for the face-centered cubic symmetry. Density was not measured directly but calculated from the lattice parameters.

Oxides and oxygen-containing salts


Protactinium oxides are known for the metal oxidation states +2, +4 and +5. The most stable is white pentoxide Pa2O5, which can be produced by igniting protactinium(V) hydroxide in air at a temperature of 500 °C. [10] Its crystal structure is cubic, and the chemical composition is often non-stoichiometric, described as PaO2.25. Another phase of this oxide with orthorhombic symmetry has also been reported. [2] [11] The black dioxide PaO2 is obtained from the pentoxide by reducing it at 1550 °C with hydrogen. It is not readily soluble in either dilute or concentrated nitric, hydrochloric or sulfuric acids, but easily dissolves in hydrofluoric acid. [2] The dioxide can be converted back to pentoxide by heating in oxygen-containing atmosphere to 1100 °C. [11] The monoxide PaO has only been observed as a thin coating on protactinium metal, but not in an isolated bulk form. [2]

Protactinium forms mixed binary oxides with various metals. With alkali metals A, the crystals have a chemical formula APaO3 and perovskite structure, or A3PaO4 and distorted rock-salt structure, or A7PaO6 where oxygen atoms form a hexagonal close-packed lattice. In all these materials, protactinium ions are octahedrally coordinated. [12] [13] The pentoxide Pa2O5 combines with rare-earth metal oxides R2O3 to form various nonstoichiometric mixed-oxides, also of perovskite structure. [14]

Protactinium oxides are basic; they easily convert to hydroxides and can form various salts, such as sulfates, phosphates, nitrates, etc. Protactinyl nitrate is usually white but can be brown due to radiolytic decomposition. Heating the nitrate in air at 400 °C converts it to the white protactinium pentoxide. [15] The polytrioxophosphate Pa(PO3)4 can be produced by reacting difluoride sulfate PaF2SO4 with phosphoric acid (H3PO4) under inert gas atmosphere. Heating the product to about 900 °C eliminates the reaction by-products such as hydrofluoric acid, sulfur trioxide and phosphoric anhydride. Heating it to higher temperatures in an inert atmosphere decomposes Pa(PO3)4 into the diphosphate PaP2O7, which is analogous to diphosphates of other actinides. In the diphosphate, the PO3 groups form pyramids of C2v symmetry. Heating PaP2O7 in air to 1400 °C decomposes it into the pentoxides of phosphorus and protactinium. [8]

Halides

Protactinium(V) fluoride forms white crystals where protactinium ions are arranged in pentagonal bipyramids and coordinated by 7 other ions. The coordination is the same in protactinium(V) chloride, but the color is yellow. The coordination changes to octahedral in the brown protactinium(V) bromide and is unknown for protactinium(V) iodide. The protactinium coordination in all its tetrahalides is 8, but the arrangement is square antiprismatic in protactinium(IV) fluoride and dodecahedral in the chloride and bromide. Brown-colored protactinium(III) iodide has been reported where protactinium ions are 8-coordinated in a bicapped trigonal prismatic arrangement. [16]

Coordination of protactinium (solid circles) and halogen atoms (open circles) in protactinium(V) fluoride or chloride. PaCl5.svg
Coordination of protactinium (solid circles) and halogen atoms (open circles) in protactinium(V) fluoride or chloride.

Protactinium(V) fluoride and protactinium(V) chloride have a polymeric structure of monoclinic symmetry. There, within one polymeric chain, all the halide atoms lie in one graphite-like plane and form planar pentagons around the protactinium ions. The coordination 7 of protactinium originates from the 5 halide atoms and two bonds to protactinium atoms belonging to the nearby chains. These compounds easily hydrolyze in water. [17] The pentachloride melts at 300 °C and sublimates at even lower temperatures.

Protactinium(V) fluoride can be prepared by reacting protactinium oxide with either bromine pentafluoride or bromine trifluoride at about 600 °C, and protactinium(IV) fluoride is obtained from the oxide and a mixture of hydrogen and hydrogen fluoride at 600 °C; a large excess of hydrogen is required to remove atmospheric oxygen leaks into the reaction. [2]

Protactinium(V) chloride is prepared by reacting protactinium oxide with carbon tetrachloride at temperature of 200–300 °C. [2] The by-products (such as PaOCl3) are removed by fractional sublimation. [6] Reduction of protactinium(V) chloride with hydrogen at about 800 °C yields protactinium(IV) chloride – a yellow-green solid which sublimes in vacuum at 400 °C; it can also be obtained directly from protactinium dioxide by treating it with carbon tetrachloride at 400 °C. [2]

Protactinium bromides are produced by the action of aluminium bromide, hydrogen bromide, carbon tetrabromide or a mixture of hydrogen bromide and thionyl bromide on protactinium oxide. An alternative reaction is between protactinium pentachloride and hydrogen bromide or thionyl bromide. [2] Protactinium(V) bromide has two similar monoclinic forms, one is obtained by sublimation at 400–410 °C and another by sublimation at slightly lower temperature of 390–400 °C. [5] [7]

Protactinium iodides result from the oxides and aluminium iodide or ammonium iodide heated to 600 °C. [2] Protactinium(III) iodide was obtained by heating protactinium(V) iodide in vacuum. [17] As with oxides, protactinium forms mixed halides with alkali metals. Among those, most remarkable is Na3PaF8 where protactinium ion is symmetrically surrounded by 8 F− ions which form a nearly perfect cube. [18]

More complex protactinium fluorides are also known such as Pa2F9 [17] and ternary fluorides of the types MPaF6 (M = Li, Na, K, Rb, Cs or NH4), M2PaF7 (M = K, Rb, Cs or NH4) and M3PaF8 (M = Li, Na, Rb, Cs), all being white crystalline solids. The MPaF6 formula can be represented as a combination of MF and PaF5. These compounds can be obtained by evaporating a hydrofluoric acid solution containing these both complexes. For the small alkali cations like Na, the crystal structure is tetragonal, whereas it lowers to orthorphombic for larger cations K+, Rb+, Cs+ or NH4+. A similar variation was observed for the M2PaF7 fluorides, namely the crystal symmetry was dependent on the cation and differed for Cs2PaF7 and M2PaF7 (M = K, Rb or NH4). [19]

Other inorganic compounds

Oxyhalides and oxysulfides of protactinium are known. PaOBr3 has a monoclinic structure composed of double-chain units where protactinium has coordination 7 and is arranged into pentagonal bipyramids. The chains are interconnected through oxygen and bromine atoms, and each oxygen atom is related to three protactinium atoms. [5] PaOS is a light-yellow non-volatile solid with a cubic crystal lattice isostructural to that of other actinide oxysulfides. It is obtained by reacting protactinium(V) chloride with a mixture of hydrogen sulfide and carbon disulfide at 900 °C. [2]

In hydrides and nitrides, protactinium has a low oxidation state of about +3. The hydride is obtained by direct action of hydrogen on the metal at 250 °C, and the nitride is a product of ammonia and protactinium tetrachloride or pentachloride. This bright yellow solid is stable to heating to 800 °C in vacuum. Protactinium carbide PaC is formed by reduction of protactinium tetrafluoride with barium in a carbon crucible at a temperature of about 1400 °C. [2] Protactinium forms borohydrides which include Pa(BH4)4. It has an unusual polymeric structure with helical chains where the protactinium atom has coordination number of 12 and is surrounded by six BH4− ions. [20]

Organometallic compounds

The proposed structure of the protactinocene (Pa(C8H8)2) molecule Uranocene-3D-balls.png
The proposed structure of the protactinocene (Pa(C8H8)2) molecule

Protactinium(IV) forms a tetrahedral complex tetrakis(cyclopentadienyl)protactinium(IV) (or Pa(C5H5)4) with four cyclopentadienyl rings, which can be synthesized by reacting protactinium(IV) chloride with molten Be(C5H5)2. One ring can be substituted with a halide atom. [21] Another organometallic complex is golden-yellow bis(π-cyclooctatetraene) protactinium, or protactinocene, Pa(C8H8)2, which is analogous in structure to uranocene. There, the metal atom is sandwiched between two cyclooctatetraene ligands. Similar to uranocene, it can be prepared by reacting protactinium tetrachloride with dipotassium cyclooctatetraenide, K2C8H8, in tetrahydrofuran. [9]

See also

Related Research Articles

<span class="mw-page-title-main">Protactinium</span> Chemical element with atomic number 91 (Pa)

Protactinium is a chemical element; it has symbol Pa and atomic number 91. It is a dense, radioactive, silvery-gray actinide metal which readily reacts with oxygen, water vapor, and inorganic acids. It forms various chemical compounds, in which protactinium is usually present in the oxidation state +5, but it can also assume +4 and even +3 or +2 states. Concentrations of protactinium in the Earth's crust are typically a few parts per trillion, but may reach up to a few parts per million in some uraninite ore deposits. Because of its scarcity, high radioactivity, and high toxicity, there are currently no uses for protactinium outside scientific research, and for this purpose, protactinium is mostly extracted from spent nuclear fuel.

<span class="mw-page-title-main">Haloalkane</span> Group of chemical compounds derived from alkanes containing one or more halogens

The haloalkanes are alkanes containing one or more halogen substituents. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalkanes are widely used commercially. They are used as flame retardants, fire extinguishants, refrigerants, propellants, solvents, and pharmaceuticals. Subsequent to the widespread use in commerce, many halocarbons have also been shown to be serious pollutants and toxins. For example, the chlorofluorocarbons have been shown to lead to ozone depletion. Methyl bromide is a controversial fumigant. Only haloalkanes that contain chlorine, bromine, and iodine are a threat to the ozone layer, but fluorinated volatile haloalkanes in theory may have activity as greenhouse gases. Methyl iodide, a naturally occurring substance, however, does not have ozone-depleting properties and the United States Environmental Protection Agency has designated the compound a non-ozone layer depleter. For more information, see Halomethane. Haloalkane or alkyl halides are the compounds which have the general formula "RX" where R is an alkyl or substituted alkyl group and X is a halogen.

In chemistry, a halide is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, astatide, or theoretically tennesside compound. The alkali metals combine directly with halogens under appropriate conditions forming halides of the general formula, MX. Many salts are halides; the hal- syllable in halide and halite reflects this correlation. All Group 1 metals form halides that are white solids at room temperature.

A silver halide is one of the chemical compounds that can form between the element silver (Ag) and one of the halogens. In particular, bromine (Br), chlorine (Cl), iodine (I) and fluorine (F) may each combine with silver to produce silver bromide (AgBr), silver chloride (AgCl), silver iodide (AgI), and four forms of silver fluoride, respectively.

<span class="mw-page-title-main">Tantalum(V) chloride</span> Chemical compound

Tantalum(V) chloride, also known as tantalum pentachloride, is an inorganic compound with the formula TaCl5. It takes the form of a white powder and is commonly used as a starting material in tantalum chemistry. It readily hydrolyzes to form tantalum(V) oxychloride (TaOCl3) and eventually tantalum pentoxide (Ta2O5); this requires that it be synthesised and manipulated under anhydrous conditions, using air-free techniques.

<span class="mw-page-title-main">Silver compounds</span> Chemical compounds containing silver

Silver is a relatively unreactive metal, although it can form several compounds. The common oxidation states of silver are (in order of commonness): +1 (the most stable state; for example, silver nitrate, AgNO3); +2 (highly oxidising; for example, silver(II) fluoride, AgF2); and even very rarely +3 (extreme oxidising; for example, potassium tetrafluoroargentate(III), KAgF4). The +3 state requires very strong oxidising agents to attain, such as fluorine or peroxodisulfate, and some silver(III) compounds react with atmospheric moisture and attack glass. Indeed, silver(III) fluoride is usually obtained by reacting silver or silver monofluoride with the strongest known oxidizing agent, krypton difluoride.

Uranium compounds are compounds formed by the element uranium (U). Although uranium is a radioactive actinide, its compounds are well studied due to its long half-life and its applications. It usually forms in the +4 and +6 oxidation states, although it can also form in other oxidation states.

Bromine compounds are compounds containing the element bromine (Br). These compounds usually form the -1, +1, +3 and +5 oxidation states. Bromine is intermediate in reactivity between chlorine and iodine, and is one of the most reactive elements. Bond energies to bromine tend to be lower than those to chlorine but higher than those to iodine, and bromine is a weaker oxidising agent than chlorine but a stronger one than iodine. This can be seen from the standard electrode potentials of the X2/X− couples (F, +2.866 V; Cl, +1.395 V; Br, +1.087 V; I, +0.615 V; At, approximately +0.3 V). Bromination often leads to higher oxidation states than iodination but lower or equal oxidation states to chlorination. Bromine tends to react with compounds including M–M, M–H, or M–C bonds to form M–Br bonds.

Iodine compounds are compounds containing the element iodine. Iodine can form compounds using multiple oxidation states. Iodine is quite reactive, but it is much less reactive than the other halogens. For example, while chlorine gas will halogenate carbon monoxide, nitric oxide, and sulfur dioxide, iodine will not do so. Furthermore, iodination of metals tends to result in lower oxidation states than chlorination or bromination; for example, rhenium metal reacts with chlorine to form rhenium hexachloride, but with bromine it forms only rhenium pentabromide and iodine can achieve only rhenium tetraiodide. By the same token, however, since iodine has the lowest ionisation energy among the halogens and is the most easily oxidised of them, it has a more significant cationic chemistry and its higher oxidation states are rather more stable than those of bromine and chlorine, for example in iodine heptafluoride.

The thallium halides include monohalides, where thallium has oxidation state +1, trihalides in which thallium generally has oxidation state +3, and some intermediate halides containing thallium with mixed +1 and +3 oxidation states. These salts find use in specialized optical settings, such as focusing elements in research spectrophotometers. Compared to the more common zinc selenide-based optics, materials such as thallium bromoiodide enable transmission at longer wavelengths. In the infrared, this allows for measurements as low as 350 cm−1 (28 μm), whereas zinc selenide is opaque by 21.5 μm, and ZnSe optics are generally only usable to 650 cm−1 (15 μm).

There are three sets of Indium halides, the trihalides, the monohalides, and several intermediate halides. In the monohalides the oxidation state of indium is +1 and their proper names are indium(I) fluoride, indium(I) chloride, indium(I) bromide and indium(I) iodide.

<span class="mw-page-title-main">Protactinium(V) chloride</span> Chemical compound

Protactinium(V) chloride is the chemical compound composed of protactinium and chlorine with the formula PaCl5. It forms yellow monoclinic crystals and has a unique structure composed of chains of 7 coordinate, pentagonal bipyramidal, protactinium atoms sharing edges.

<span class="mw-page-title-main">Berkelium compounds</span> Chemical compounds

Berkelium forms a number of chemical compounds, where it normally exists in an oxidation state of +3 or +4, and behaves similarly to its lanthanide analogue, terbium. Like all actinides, berkelium easily dissolves in various aqueous inorganic acids, liberating gaseous hydrogen and converting into the trivalent oxidation state. This trivalent state is the most stable, especially in aqueous solutions, but tetravalent berkelium compounds are also known. The existence of divalent berkelium salts is uncertain and has only been reported in mixed lanthanum chloride-strontium chloride melts. Aqueous solutions of Bk3+ ions are green in most acids. The color of the Bk4+ ions is yellow in hydrochloric acid and orange-yellow in sulfuric acid. Berkelium does not react rapidly with oxygen at room temperature, possibly due to the formation of a protective oxide surface layer; however, it reacts with molten metals, hydrogen, halogens, chalcogens and pnictogens to form various binary compounds. Berkelium can also form several organometallic compounds.

<span class="mw-page-title-main">Metal halides</span>

Metal halides are compounds between metals and halogens. Some, such as sodium chloride are ionic, while others are covalently bonded. A few metal halides are discrete molecules, such as uranium hexafluoride, but most adopt polymeric structures, such as palladium chloride.

In chemistry, molecular oxohalides (oxyhalides) are a group of chemical compounds in which both oxygen and halogen atoms are attached to another chemical element A in a single molecule. They have the general formula AOmXn, where X is a halogen. Known oxohalides have fluorine (F), chlorine (Cl), bromine (Br), and/or iodine (I) in their molecules. The element A may be a main group element, a transition element, a rare earth element or an actinide. The term oxohalide, or oxyhalide, may also refer to minerals and other crystalline substances with the same overall chemical formula, but having an ionic structure.

<span class="mw-page-title-main">Lead compounds</span> Type of compound

Compounds of lead exist with lead in two main oxidation states: +2 and +4. The former is more common. Inorganic lead(IV) compounds are typically strong oxidants or exist only in highly acidic solutions.

<span class="mw-page-title-main">Thorium compounds</span> Chemical compounds

Many compounds of thorium are known: this is because thorium and uranium are the most stable and accessible actinides and are the only actinides that can be studied safely and legally in bulk in a normal laboratory. As such, they have the best-known chemistry of the actinides, along with that of plutonium, as the self-heating and radiation from them is not enough to cause radiolysis of chemical bonds as it is for the other actinides. While the later actinides from americium onwards are predominantly trivalent and behave more similarly to the corresponding lanthanides, as one would expect from periodic trends, the early actinides up to plutonium have relativistically destabilised and hence delocalised 5f and 6d electrons that participate in chemistry in a similar way to the early transition metals of group 3 through 8: thus, all their valence electrons can participate in chemical reactions, although this is not common for neptunium and plutonium.

Einsteinium compounds are compounds that contain the element einsteinium (Es). These compounds largely have einsteinium in the +3 oxidation state, or in some cases in the +2 and +4 oxidation states. Although einsteinium is relatively stable, with half-lives ranging from 20 days upwards, these compounds have not been studied in great detail.

Actinium compounds are compounds containing the element actinium (Ac). Due to actinium's intense radioactivity, only a limited number of actinium compounds are known. These include: AcF3, AcCl3, AcBr3, AcOF, AcOCl, AcOBr, Ac2S3, Ac2O3, AcPO4 and Ac(NO3)3. Except for AcPO4, they are all similar to the corresponding lanthanum compounds. They all contain actinium in the oxidation state +3. In particular, the lattice constants of the analogous lanthanum and actinium compounds differ by only a few percent.

Neptunium compounds are compounds containing the element neptunium (Np). Neptunium has five ionic oxidation states ranging from +3 to +7 when forming chemical compounds, which can be simultaneously observed in solutions. It is the heaviest actinide that can lose all its valence electrons in a stable compound. The most stable state in solution is +5, but the valence +4 is preferred in solid neptunium compounds. Neptunium metal is very reactive. Ions of neptunium are prone to hydrolysis and formation of coordination compounds.

References

  1. ↑ Donohue, J. (1959). "On the crystal structure of protactinium metal". Acta Crystallographica . 12 (9): 697–698. Bibcode:1959AcCry..12..697D. doi:10.1107/S0365110X59002031.
  2. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Sellers, Philip A.; Fried, Sherman; Elson, Robert E.; Zachariasen, W. H. (1954). "The Preparation of Some Protactinium Compounds and the Metal". Journal of the American Chemical Society . 76 (23): 5935. doi:10.1021/ja01652a011.
  3. ↑ Brown D.; Hall T.L.; Moseley P.T (1973). "Structural parameters and unit cell dimensions for the tetragonal actinide tetrachlorides(Th, Pa, U, and Np) and tetrabromides (Th and Pa)". Journal of the Chemical Society, Dalton Transactions (6): 686–691. doi:10.1039/DT9730000686.
  4. ↑ Tahri, Y.; Chermette, H.; El Khatib, N.; Krupa, J.; et al. (1990). "Electronic structures of thorium and protactinium halide clusters of [ThX8]4− type". Journal of the Less Common Metals . 158: 105–116. doi:10.1016/0022-5088(90)90436-N.
  5. 1 2 3 4 5 Brown, D.; Petcher, T. J.; Smith, A. J. (1968). "Crystal Structures of some Protactinium Bromides". Nature . 217 (5130): 737. Bibcode:1968Natur.217..737B. doi:10.1038/217737a0. S2CID   4264482.
  6. 1 2 Dodge, R. P.; Smith, G. S.; Johnson, Q.; Elson, R. E. (1967). "The crystal structure of protactinium pentachloride". Acta Crystallographica . 22 (1): 85–89. Bibcode:1967AcCry..22...85D. doi:10.1107/S0365110X67000155.
  7. 1 2 Brown, D.; Petcher, T. J.; Smith, A. J. (1969). "The crystal structure of β-protactinium pentabromide". Acta Crystallographica B . 25 (2): 178. Bibcode:1969AcCrB..25..178B. doi:10.1107/S0567740869007357.
  8. 1 2 3 Brandel, V.; Dacheux, N. (2004). "Chemistry of tetravalent actinide phosphates—Part I". Journal of Solid State Chemistry . 177 (12): 4743. Bibcode:2004JSSCh.177.4743B. doi:10.1016/j.jssc.2004.08.009.
  9. 1 2 Starks, David F.; Parsons, Thomas C.; Streitwieser, Andrew; Edelstein, Norman (1974). "Bis(π-cyclooctatetraene) protactinium". Inorganic Chemistry . 13 (6): 1307. doi:10.1021/ic50136a011.
  10. ↑ Greenwood, p. 1268
  11. 1 2 Elson, R.; Fried, Sherman; Sellers, Philip; Zachariasen, W. H. (1950). "The tetravalent and pentavalent states of protactinium". Journal of the American Chemical Society . 72 (12): 5791. doi:10.1021/ja01168a547.
  12. ↑ Greenwood, p. 1269
  13. ↑ Iyer, P. N.; Smith, A. J. (1971). "Double oxides containing niobium, tantalum or protactinium. IV. Further systems involving alkali metals". Acta Crystallographica B . 27 (4): 731. Bibcode:1971AcCrB..27..731I. doi:10.1107/S056774087100284X.
  14. ↑ Iyer, P. N.; Smith, A. J. (1967). "Double oxides containing niobium, tantalum, or protactinium. III. Systems involving the rare earths". Acta Crystallographica . 23 (5): 740. Bibcode:1967AcCry..23..740I. doi:10.1107/S0365110X67003639.
  15. ↑ Grossmann, R.; Maier, H.; Szerypo, J.; Friebel, H. (2008). "Preparation of 231Pa targets". Nuclear Instruments and Methods in Physics Research A . 590 (1–3): 122. Bibcode:2008NIMPA.590..122G. doi:10.1016/j.nima.2008.02.084.
  16. ↑ Greenwood, p. 1270
  17. 1 2 3 Greenwood, p. 1271
  18. ↑ Greenwood, p. 1275
  19. ↑ Asprey, L. B.; Kruse, F. H.; Rosenzweig, A.; Penneman, R. A. (1966). "Synthesis and X-Ray Properties of Alkali Fluoride-Protactinium Pentafluoride Complexes". Inorganic Chemistry . 5 (4): 659. doi:10.1021/ic50038a034.
  20. ↑ Greenwood, p. 1277
  21. ↑ Greenwood, pp. 1278–1279
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