alkali metals, barium, thallium, and ammonium are known for thorium fluorides, chlorides, and bromides. For example, when treated with potassium fluoride and hydrofluoric acid, forms the complex anion (hexafluorothorate(IV)), which precipitates as an insoluble salt, (potassium hexafluorothorate(IV)). Thorium borides, carbides, silicides, and nitrides are refractory materials, like those of uranium and plutonium, and have thus received attention as possible nuclear fuels. All four heavier pnictogens (phosphorus, arsenic, antimony, and bismuth) also form binary thorium compounds. Thorium germanides are also known. Thorium reacts with hydrogen to form the thorium hydrides and , the latter of which is superconducting below 7.5–8 K; at standard temperature and pressure, it conducts electricity like a metal. The hydrides are thermally unstable and readily decompose upon exposure to air or moisture. Coordination compounds In an acidic aqueous solution, thorium occurs as the tetrapositive aqua ion , which has tricapped trigonal prismatic molecular geometry: at pH < 3, the solutions of thorium salts are dominated by this cation. The ion is the largest of the tetrapositive actinide ions, and depending on the coordination number can have a radius between 0.95 and 1.14 Å. 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 ), predominantly to in solutions with pH 3 or below, but in more alkaline solution polymerisation continues until the gelatinous hydroxide forms and precipitates out (though equilibrium may take weeks to be reached, because the polymerisation usually slows down before the precipitation). As a hard Lewis acid, favours hard ligands with oxygen atoms as donors: complexes with sulfur atoms as donors are less stable and are more prone to hydrolysis. 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. These thorium salts are known for their high solubility in water and polar organic solvents. Many other inorganic thorium compounds with polyatomic anions are known, such as the perchlorates, sulfates, sulfites, nitrates, carbonates, phosphates, vanadates, molybdates, and chromates, and their hydrated forms. They are important in thorium purification and the disposal of nuclear waste, but most of them have not yet been fully characterized, especially regarding their structural properties. For example,