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. 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. In January 2021, the aromaticity has been observed in a large metal cluster anion consisting of 12 bismuth atoms stabilised by a center thorium cation. This compound was shown to be surprisingly stable, unlike many previous known aromatic metal clusters. Organothorium compounds Most of the work on organothorium compounds has focused on the cyclopentadienyl complexes and cyclooctatetraenyls. Like many of the early and middle actinides (up to americium, and also expected for curium), thorium forms a cyclooctatetraenide complex: the yellow , thorocene. It is isotypic with the better-known analogous uranium compound uranocene. It can be prepared by reacting with thorium tetrachloride in tetrahydrofuran (THF) at the temperature of dry ice, or by reacting thorium tetrafluoride with . It is unstable in air and decomposes in water or at 190 °C. Half sandwich compounds are also known, such as , which has a piano-stool structure and is made by reacting thorocene with thorium tetrachloride in tetrahydrofuran. The simplest of the cyclopentadienyls are and : many derivatives are known. The former (which has two forms, one purple and one green) is a rare example of thorium in the formal +3 oxidation state; a formal +2 oxidation state occurs in a derivative. The chloride derivative is prepared by heating thorium tetrachloride with limiting used (other univalent metal cyclopentadienyls can also be used). The alkyl and aryl derivatives are prepared from the chloride derivative and have been used to study the nature of the Th–C sigma bond. Other organothorium compounds are not well-studied. Tetrabenzylthorium, , and tetraallylthorium, , are known, but their structures have not been determined. They decompose slowly at room temperature. Thorium forms the monocapped trigonal prismatic anion , heptamethylthorate(IV), which forms the salt (tmeda = ). Although one methyl group is only attached to the thorium atom (Th–C distance 257.1 pm) and the other six connect the lithium and thorium atoms (Th–C distances 265.5–276.5 pm), they behave equivalently in solution. Tetramethylthorium,