Editing Protactinium (section)

===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>