Editing Protactinium (section)

==Chemical compounds==
{{Main|Protactinium compounds}}
{| Class = "wikitable" style = "text-align: center"
! Formula
! color
! symmetry
! [[space group]]
! No
! [[Pearson symbol]]
! ''a'' (pm)
! ''b'' (pm)
! ''c'' (pm)
! ''Z''
! density (g/cm<sup>3</sup>)
|-
| Pa
| silvery-gray
| [[Tetragonal crystal system|tetragonal]]<ref name="str" />
| I4/mmm
| 139
| tI2
| 392.5
| 392.5
| 323.8
| 2
| 15.37
|-
| PaO
|
| rocksalt<ref name="pao2">{{cite journal|doi = 10.1021/ja01652a011|date = 1954|last1 = Sellers|first1 = Philip A.|last2 = Fried|first2 = Sherman|last3 = Elson|first3 = Robert E.|last4 = Zachariasen|first4 = W. H.|journal = [[Journal of the American Chemical Society]]|volume = 76|pages = 5935|title = The Preparation of Some Protactinium Compounds and the Metal|issue = 23| bibcode=1954JAChS..76.5935S |url = https://digital.library.unt.edu/ark:/67531/metadc172625/|url-access = subscription}}</ref>
| Fm{{overline|3}}m
| 225
| cF8
| 496.1
|
|
| 4
| 13.44
|-
| [[Protactinium(IV) oxide|PaO<sub>2</sub>]]
| black
| ''fcc''<ref name="pao2" />
| Fm{{overline|3}}m
| 225
| cF12
| 550.5
|
|
| 4
| 10.47
|-
| [[Protactinium(V) oxide|Pa<sub>2</sub>O<sub>5</sub>]]
| white
|
| Fm{{overline|3}}m<ref name="pao2" />
| 225
| cF16
| 547.6
| 547.6
| 547.6
| 4
| 10.96
|-
| Pa<sub>2</sub>O<sub>5</sub>
| white
| orthorhombic<ref name="pao2" />
|
|
|
| 692
| 402
| 418
|
|
|-
| PaH<sub>3</sub>
| black
| cubic<ref name="pao2" />
| Pm{{overline|3}}n
| 223
| cP32
| 664.8
| 664.8
| 664.8
| 8
| 10.58
|-
| PaF<sub>4</sub>
| brown-red
| monoclinic<ref name="pao2" />
| C2/c
| 15
| mS60
|
|
|
| 2
|
|-
| PaCl<sub>4</sub>
| green-yellow
| [[Tetragonal crystal system|tetragonal]]<ref>{{cite journal|journal=[[Journal of the Chemical Society, Dalton Transactions]]|date=1973|title=Structural parameters and unit cell dimensions for the tetragonal actinide tetrachlorides(Th, Pa, U, and Np) and tetrabromides (Th and Pa)|pages=686–691|author=Brown D.|author2=Hall T.L.|author3=Moseley P.T|doi=10.1039/DT9730000686|issue=6}}</ref>
| I4<sub>1</sub>/amd
| 141
| tI20
| 837.7
| 837.7
| 748.1
| 4
| 4.72
|-
| PaBr<sub>4</sub>
| brown
| tetragonal<ref name="pabr4">{{cite journal|display-authors=4|last1=Tahri|first1=Y.|last2=Chermette|first2=H.|last3=El Khatib|first3=N.|last4=Krupa|first4=J.|last5=Simoni|first5=E.|title=Electronic structures of thorium and protactinium halide clusters of [ThX8]4− type|journal=[[Journal of the Less Common Metals]]|volume=158|pages=105–116|date=1990|doi=10.1016/0022-5088(90)90436-N}}</ref><ref name="pabr5b" />
| I4<sub>1</sub>/amd
| 141
| tI20
| 882.4
| 882.4
| 795.7
|
|
|-
| [[Protactinium(V) chloride|PaCl<sub>5</sub>]]
| yellow
| [[Monoclinic crystal system|monoclinic]]<ref name="pacl5">{{cite journal|doi=10.1107/S0365110X67000155|last1=Dodge|first1=R. P.|last2=Smith|first2=G. S.|last3=Johnson|first3=Q.|last4=Elson|first4=R. E.|title=The crystal structure of protactinium pentachloride|journal=[[Acta Crystallographica]]|date=1967|volume=22|issue=1 |pages=85–89|bibcode=1967AcCry..22...85D }}</ref>
| C2/c
| 15
| mS24
| 797
| 1135
| 836
| 4
| 3.74
|-
| PaBr<sub>5</sub>
| red
| monoclinic<ref name="pabr5b" /><ref name="pabr5">{{cite journal|last1=Brown|first1=D.|last2=Petcher|first2=T. J.|last3=Smith|first3=A. J.|title=The crystal structure of β-protactinium pentabromide|journal=[[Acta Crystallographica B]]|volume=25|pages=178|date=1969|doi=10.1107/S0567740869007357|issue=2|bibcode=1969AcCrB..25..178B }}</ref>
| P2<sub>1</sub>/c
| 14
| mP24
| 838.5
| 1120.5
| 1214.6
| 4
| 4.98
|-
| PaOBr<sub>3</sub>
|
| monoclinic<ref name="pabr5b" />
| C2
|
|
| 1691.1
| 387.1
| 933.4
|
|
|-
| Pa(PO<sub>3</sub>)<sub>4</sub>
|
| orthorhombic<ref name="papo3">{{cite journal|doi=10.1016/j.jssc.2004.08.009|last1=Brandel|first1=V.|date=2004|pages=4743|volume=177|journal=[[Journal of Solid State Chemistry]]|last2=Dacheux|first2=N. |title=Chemistry of tetravalent actinide phosphates—Part I|issue=12|bibcode = 2004JSSCh.177.4743B }}</ref>
|
|
|
| 696.9
| 895.9
| 1500.9
|
|
|-
| Pa<sub>2</sub>P<sub>2</sub>O<sub>7</sub>
|
| cubic<ref name="papo3" />
| Pa3
|
|
| 865
| 865
| 865
|
|
|-
| Pa(C<sub>8</sub>H<sub>8</sub>)<sub>2</sub>
| golden-yellow
| monoclinic<ref name="cene">{{cite journal|doi=10.1021/ic50136a011|last1=Starks|date=1974|first1=David F.|pages=1307|volume=13|last2=Parsons|journal=[[Inorganic Chemistry (journal)|Inorganic Chemistry]]|first2=Thomas C.|last3=Streitwieser|first3=Andrew|last4=Edelstein|first4=Norman|title=Bis(π-cyclooctatetraene) protactinium|issue=6|author-link3=Andrew Streitwieser}}</ref>
|
|
|
| 709
| 875
| 1062
|
|
|}

Here, ''a'', ''b'', and ''c'' are lattice constants in picometers, No is the space group number, and ''Z'' is the number of [[formula unit]]s per [[unit cell]]; ''fcc'' stands for the [[Cubic crystal system|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 the white pentoxide [[Protactinium(V) oxide|Pa<sub>2</sub>O<sub>5</sub>]], which can be produced by igniting protactinium(V) hydroxide in [[air]] at a temperature of 500&nbsp;°C.<ref name="g1268">[[#Greenwood|Greenwood]], p. 1268</ref> Its crystal structure is cubic, and the chemical composition is often non-stoichiometric, described as PaO<sub>2.25</sub>. Another phase of this oxide with orthorhombic symmetry has also been reported.<ref name="pao2" /><ref name="pacl4b" /> The black dioxide [[Protactinium(IV) oxide|PaO<sub>2</sub>]] is obtained from the pentoxide by reducing it at 1550&nbsp;°C with hydrogen. It is not readily soluble in either dilute or concentrated [[nitric acid|nitric]], [[hydrochloric acid|hydrochloric]], or [[sulfuric acid]], but easily dissolves in [[hydrofluoric acid]].<ref name="pao2" /> The dioxide can be converted back to pentoxide by heating in oxygen-containing atmosphere to 1100&nbsp;°C.<ref name="pacl4b">{{cite journal|last1=Elson|first1=R.|last2=Fried|first2=Sherman|last3=Sellers|first3=Philip|last4=Zachariasen|first4=W. H.|title=The tetravalent and pentavalent states of protactinium|journal=[[Journal of the American Chemical Society]]|volume=72|pages=5791|date=1950|doi=10.1021/ja01168a547|issue=12|bibcode=1950JAChS..72.5791E }}</ref> The monoxide PaO has only been observed as a thin coating on protactinium metal, but not in an isolated bulk form.<ref name="pao2" />

Protactinium forms mixed binary oxides with various metals. With alkali metals ''A'', the crystals have a chemical formula APaO<sub>3</sub> and [[perovskite structure]]; A<sub>3</sub>PaO<sub>4</sub> and distorted rock-salt structure; or A<sub>7</sub>PaO<sub>6</sub>, where oxygen atoms form a hexagonal close-packed lattice. In all of these materials, the protactinium ions are octahedrally coordinated.<ref name="g1269">[[#Greenwood|Greenwood]], p. 1269</ref><ref>{{cite journal|doi=10.1107/S056774087100284X|last1=Iyer|first1=P. N.|date=1971|pages=731|volume=27|journal=[[Acta Crystallographica B]]|last2=Smith|first2=A. J.|title=Double oxides containing niobium, tantalum or protactinium. IV. Further systems involving alkali metals|issue=4|bibcode=1971AcCrB..27..731I }}</ref> The pentoxide Pa<sub>2</sub>O<sub>5</sub> combines with rare-earth metal oxides R<sub>2</sub>O<sub>3</sub> to form various nonstoichiometric mixed-oxides, also of perovskite structure.<ref>{{cite journal|last1=Iyer|first1=P. N.|last2=Smith|first2=A. J.|title=Double oxides containing niobium, tantalum, or protactinium. III. Systems involving the rare earths|journal=[[Acta Crystallographica]]|volume=23|pages=740|date=1967|doi=10.1107/S0365110X67003639|issue=5|bibcode=1967AcCry..23..740I }}</ref>

Protactinium oxides are [[Basic oxide|basic]]; they easily convert to hydroxides and can form various salts, such as [[sulfate]]s, [[phosphate]]s, [[nitrate]]s, etc. The nitrate is usually white but can be brown due to [[radiolysis|radiolytic]] decomposition. Heating the nitrate in air at 400&nbsp;°C converts it to the white protactinium pentoxide.<ref name="target" /> The polytrioxophosphate Pa(PO<sub>3</sub>)<sub>4</sub> can be produced by reacting the difluoride sulfate PaF<sub>2</sub>SO<sub>4</sub> with [[phosphoric acid]] (H<sub>3</sub>PO<sub>4</sub>) under an inert atmosphere. Heating the product to about 900&nbsp;°C eliminates the reaction by-products, which include [[hydrofluoric acid]], [[sulfur trioxide]], and phosphoric anhydride. Heating it to higher temperatures in an inert atmosphere decomposes Pa(PO<sub>3</sub>)<sub>4</sub> into the diphosphate PaP<sub>2</sub>O<sub>7</sub>, which is analogous to diphosphates of other actinides. In the diphosphate, the PO<sub>3</sub> groups form pyramids of C<sub>2v</sub> symmetry. Heating PaP<sub>2</sub>O<sub>7</sub> in air to 1400&nbsp;°C decomposes it into the pentoxides of phosphorus and protactinium.<ref name="papo3" />

===Halides===
Protactinium(V) fluoride forms white crystals where protactinium ions are arranged in pentagonal bipyramids and [[Coordination number|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, but 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.<ref name="g1270">[[#Greenwood|Greenwood]], p. 1270</ref>

[[File:PaCl5.svg|thumb|right|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 halide atoms lie in one graphite-like plane and form planar pentagons around the protactinium ions. The 7-coordination of protactinium originates from the five halide atoms and two bonds to protactinium atoms belonging to the nearby chains. These compounds easily hydrolyze in water.<ref name="g1271" /> The pentachloride melts at 300&nbsp;°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&nbsp;°C, and protactinium(IV) fluoride is obtained from the oxide and a mixture of hydrogen and [[hydrogen fluoride]] at 600&nbsp;°C; a large excess of hydrogen is required to remove atmospheric oxygen leaks into the reaction.<ref name="pao2" />

Protactinium(V) chloride is prepared by reacting protactinium oxide with [[carbon tetrachloride]] at temperatures of 200–300&nbsp;°C.<ref name="pao2" /> The by-products (such as PaOCl<sub>3</sub>) are removed by fractional sublimation.<ref name="pacl5" /> Reduction of protactinium(V) chloride with hydrogen at about 800&nbsp;°C yields protactinium(IV) chloride – a yellow-green solid that sublimes in vacuum at 400&nbsp;°C. It can also be obtained directly from protactinium dioxide by treating it with carbon tetrachloride at 400&nbsp;°C.<ref name="pao2" />

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. They can alternatively be produced by reacting protactinium pentachloride with hydrogen bromide or thionyl bromide.<ref name="pao2" /> Protactinium(V) bromide has two similar monoclinic forms: one is obtained by sublimation at 400–410&nbsp;°C, and another by sublimation at a slightly lower temperature of 390–400&nbsp;°C.<ref name="pabr5b">{{cite journal|doi=10.1038/217737a0|last1=Brown|first1=D.|last2=Petcher|first2=T. J.|last3=Smith|first3=A. J.|title=Crystal Structures of some Protactinium Bromides|date=1968|pages=737|volume=217|journal=[[Nature (journal)|Nature]]|issue=5130|bibcode = 1968Natur.217..737B |s2cid=4264482}}</ref><ref name="pabr5" />

Protactinium iodides can be produced by reacting protactinium metal with elemental iodine at 600&nbsp;°C, and by reacting Pa<sub>2</sub>O<sub>5</sub> with AlO<sub>3</sub> at 600&nbsp;°C.<ref name="pao2" /> Protactinium(III) iodide can be obtained by heating protactinium(V) iodide in vacuum.<ref name="g1271" /> As with oxides, protactinium forms mixed halides with alkali metals. The most remarkable among these is Na<sub>3</sub>PaF<sub>8</sub>, where the protactinium ion is symmetrically surrounded by 8 F<sup>−</sup> ions, forming a nearly perfect cube.<ref name="g1275" />

More complex protactinium fluorides are also known, such as Pa<sub>2</sub>F<sub>9</sub><ref name="g1271">[[#Greenwood|Greenwood]], p. 1271</ref> and ternary fluorides of the types MPaF<sub>6</sub> (M = Li, Na, K, Rb, Cs or NH<sub>4</sub>), M<sub>2</sub>PaF<sub>7</sub> (M = K, Rb, Cs or NH<sub>4</sub>), and M<sub>3</sub>PaF<sub>8</sub> (M = Li, Na, Rb, Cs), all of which are white crystalline solids. The MPaF<sub>6</sub> formula can be represented as a combination of MF and PaF<sub>5</sub>. These compounds can be obtained by evaporating a hydrofluoric acid solution containing both complexes. For the small alkali cations like Na, the crystal structure is tetragonal, whereas it becomes orthorhombic for larger cations K<sup>+</sup>, Rb<sup>+</sup>, Cs<sup>+</sup> or NH<sub>4</sub><sup>+</sup>. A similar variation was observed for the M<sub>2</sub>PaF<sub>7</sub> fluorides: namely, the crystal symmetry was dependent on the cation and differed for Cs<sub>2</sub>PaF<sub>7</sub> and M<sub>2</sub>PaF<sub>7</sub> (M = K, Rb or NH<sub>4</sub>).<ref name="trif">{{cite journal|last1=Asprey|first1=L. B.|last2=Kruse|first2=F. H.|last3=Rosenzweig|first3=A.|last4=Penneman|first4=R. A.|title=Synthesis and X-Ray Properties of Alkali Fluoride-Protactinium Pentafluoride Complexes|journal=[[Inorganic Chemistry (journal)|Inorganic Chemistry]]|volume=5|pages=659|date=1966|doi=10.1021/ic50038a034|issue=4}}</ref>

===Other inorganic compounds===
Oxyhalides and oxysulfides of protactinium are known. PaOBr<sub>3</sub> 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.<ref name="pabr5b" /> 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&nbsp;°C.<ref name="pao2" />

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&nbsp;°C, and the nitride is a product of ammonia and protactinium tetrachloride or pentachloride. This bright yellow solid is thermally stable to 800&nbsp;°C in vacuum. Protactinium carbide (PaC) is formed by the reduction of protactinium tetrafluoride with barium in a carbon crucible at a temperature of about 1400&nbsp;°C.<ref name="pao2" /> Protactinium forms [[borohydride]]s, which include Pa(BH<sub>4</sub>)<sub>4</sub>. It has an unusual polymeric structure with helical chains, where the protactinium atom has coordination number of 12 and is surrounded by six BH<sub>4</sub><sup>−</sup> ions.<ref name="g1277">[[#Greenwood|Greenwood]], p. 1277</ref>

===Organometallic compounds===
[[File:Uranocene-3D-balls.png|thumb|upright|The proposed structure of the protactinocene (Pa(C<sub>8</sub>H<sub>8</sub>)<sub>2</sub>) molecule]]
Protactinium(IV) forms a tetrahedral complex tetrakis(cyclopentadienyl)protactinium(IV) (or Pa(C<sub>5</sub>H<sub>5</sub>)<sub>4</sub>) with four [[Cyclopentadienyl complex|cyclopentadienyl]] rings, which can be synthesized by reacting protactinium(IV) chloride with molten Be(C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>. One ring can be substituted with a halide atom.<ref name="g1278">[[#Greenwood|Greenwood]], pp. 1278–1279</ref> Another organometallic complex is the golden-yellow bis(π-cyclooctatetraene) protactinium, or [[protactinocene]] (Pa(C<sub>8</sub>H<sub>8</sub>)<sub>2</sub>), 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 [[cyclooctatetraene|cyclooctatetraenide]] (K<sub>2</sub>C<sub>8</sub>H<sub>8</sub>) in [[tetrahydrofuran]].<ref name="cene" />