Editing Thorium (section)

===Coordination compounds===
In an acidic aqueous solution, thorium occurs as the tetrapositive [[aqua ion]] {{chem2|[Th(H2O)9](4+)}}, which has [[tricapped trigonal prismatic molecular geometry]]:{{sfn|Wickleder|Fourest|Dorhout|2006|pp=117–134}}<ref>{{cite journal |last=Persson |first=I. |date=2010 |title=Hydrated metal ions in aqueous solution: How regular are their structures? |journal=Pure and Applied Chemistry |volume=82 |issue=10 |pages=1901–1917 |doi=10.1351/PAC-CON-09-10-22|doi-access=free }}</ref> at pH&nbsp;&lt;&nbsp;3, the solutions of thorium salts are dominated by this cation.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=117–134}} The {{chem2|Th(4+)}} ion is the largest of the tetrapositive actinide ions, and depending on the coordination number can have a radius between 0.95&nbsp;and&nbsp;1.14&nbsp;Å.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=117–134}} It is quite acidic due to its high charge, slightly stronger than [[sulfurous acid]]: thus it tends to undergo hydrolysis and polymerisation (though to a lesser extent than {{chem2|[[iron|Fe]](3+)}}), predominantly to {{chem2|[Th2(OH)2](6+)}} in solutions with pH 3 or below, but in more alkaline solution polymerisation continues until the gelatinous hydroxide {{chem2|Th(OH)4}} forms and precipitates out (though equilibrium may take weeks to be reached, because the polymerisation usually slows down before the precipitation).{{sfn|Greenwood|Earnshaw|1997|pp=1275–1277}} As a [[HSAB theory|hard Lewis acid]], {{chem2|Th(4+)}} favours hard ligands with oxygen atoms as donors: complexes with sulfur atoms as donors are less stable and are more prone to hydrolysis.<ref name="CottonSA2006">{{cite book |last=Cotton |first=S. |year=2006 |title=Lanthanide and Actinide Chemistry|publisher=[[John Wiley & Sons]]}}</ref>

High coordination numbers are the rule for thorium due to its large size. Thorium nitrate pentahydrate was the first known example of coordination number 11, the oxalate tetrahydrate has coordination number 10, and the borohydride (first prepared in the [[Manhattan Project]]) has coordination number 14.{{sfn|Greenwood|Earnshaw|1997|pp=1275–1277}} These thorium salts are known for their high solubility in water and polar organic solvents.<ref name="Yu. D. Tretyakov" />

Many other inorganic thorium compounds with polyatomic anions are known, such as the [[perchlorate]]s, [[sulfate]]s, [[sulfite]]s, nitrates, carbonates, [[phosphate]]s, [[vanadate]]s, [[molybdate]]s, and [[chromates]], and their hydrated forms.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=101–115}} They are important in thorium purification and the disposal of nuclear waste, but most of them have not yet been fully characterised, especially regarding their structural properties.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=101–115}} For example, thorium nitrate is produced by reacting thorium hydroxide with nitric acid: it is soluble in water and alcohols and is an important intermediate in the purification of thorium and its compounds.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=101–115}} Thorium complexes with organic ligands, such as [[oxalate]], [[citrate]], and [[EDTA]], are much more stable. In natural thorium-containing waters, organic thorium complexes usually occur in concentrations orders of magnitude higher than the inorganic complexes, even when the concentrations of inorganic ligands are much greater than those of organic ligands.{{sfn|Wickleder|Fourest|Dorhout|2006|pp=117–134}}

[[File:Thorium half sandwich.svg|thumb|upright|alt=Piano-stool molecule structure of (η8-C8H8)ThCl2(THF)2|Piano-stool molecule structure of ({{chem2|η^{8}\-C8H8)ThCl2(THF)2}}]]

In January 2021, the aromaticity has been observed in a large [[metal cluster]] anion consisting of 12 [[Bismuth|bismuth atoms]] stabilised by a center thorium cation.<ref>{{Cite magazine |last=Krämer |first=Katrina |date=2021-01-04 |title=Heavy-metal cluster sets size record for metal aromaticity |url=https://www.chemistryworld.com/news/heavy-metal-cluster-sets-size-record-for-metal-aromaticity/4012946.article |access-date=2 July 2022 |magazine=Chemistry World |language=en |archive-date=4 January 2021 |archive-url=https://web.archive.org/web/20210104151533/https://www.chemistryworld.com/news/heavy-metal-cluster-sets-size-record-for-metal-aromaticity/4012946.article |url-status=live }}</ref> This compound was shown to be surprisingly stable, unlike many previous known [[Metal aromaticity|aromatic metal clusters]].