Thorium compounds

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Thorium forms a wide family of compounds because it’s one of the earliest actinides and its electrons readily participate in bonding. This gives thorium chemistry more covalent character and a broader range of compounds than many other elements in the same row. The chemistry of thorium is best understood in its four‑plus oxidation state, Th4+, which is the most stable and common form in chemicals.

Key ideas about thorium chemistry
- Th4+ is the main player: In solutions it exists as a hydrated aqua ion, [Th(H2O)9]4+, and it tends to hydrolyze and form polymeric hydroxides as pH changes. The most common thorium salt, Th(NO3)4·5H2O, shows thorium’s high coordination and water-rich chemistry.
- Reactions and safety in practice: Thorium is highly electropositive. It slowly reacts with air when in bulk, and finely divided thorium can be pyrophoric. It dissolves in concentrated nitric acid only with fluoride or similar ions present, otherwise it passivates.
- Large, highly coordinated ions: Th4+ is big and can have many ligands around it. This leads to very high coordination numbers in thorium compounds and to the formation of a variety of complex salts and coordination compounds.

Common thorium compounds and families
- Oxides and hydroxides: The most important oxide is thorium dioxide, ThO2 (also called thoria). Thorium forms thorium hydroxide, Th(OH)4, which can be prepared from thorium salts and oxides.
- Halides: Thorium forms several halides, including ThF4, ThCl4, ThBr4, and ThI4. These can be purified and sometimes exist in different solid forms or phases; some can form adducts with solvents like THF or pyridine.
- Nitrides and hydrides: Thorium nitrides ThN, Th3N4, and Th2N3 are known. Thorium also forms hydrides, notably ThH2 and Th4H15, the latter of which is superconducting at very low temperatures.
- Chalcogenides: Thorium forms sulfides (ThS, Th2S3, ThS2, Th2S5, etc.), selenides (ThSe3 among others), and tellurides (ThTe in some cases). There are also oxysulfides like ThOS and oxo-tellurides such as ThOTe.
- Borides and carbides: Binary thorium borides (ThB4, ThB6, ThB12) and carbides (ThC2, Th2C3, ThC) are known. These materials are very refractory and of interest for high‑temperature applications.
- Nitrides and phosphides with pnictogens: Thorium forms several thorium pnictides, such as Th3P4, Th2P11, ThP7, Th3As4, ThAs2, Th3Sb4, and related compounds with bismuth and other pnictogens.
- Organothorium and organometallic chemistry: Thorocene, Th(C8H8)2, is the thorium analogue of uranocene and represents a notable organothorium compound. Other thorium organometallics include various cyclopentadienyl and related complexes (for example, Th(C5H5)3 and ThIV(C5H5)4 and their derivatives). Some rare oxidation states are observed in organometallics, such as ThIII and ThII species in special compounds.
- Polyatomic anions and complex salts: Thorium forms various highly coordinated anionic complexes, such as complex nitrates and borohydride salts like Th(BH4)4, which are notable for their stability and high coordination numbers.

Why thorium chemistry is distinctive
- Early actinide behavior: Up to thorium (and uranium), the 5f and 6d electrons are relatively close in energy to other valence orbitals, which makes thorium chemistry resemble early transition metals in some ways. This gives thorium a wide range of possible bonds and coordination environments.
- Rich coordination chemistry: Because Th4+ is large and highly charged, thorium forms many high‑coordination compounds and a variety of binary and mixed compounds with oxides, halides, nitrides, sulfides, borides, and more.
- Practical relevance: Thorium compounds are important in areas such as purification, processing, and potential nuclear fuel cycles, reflecting their chemical versatility and stability.

In short, thorium’s +4 chemistry drives a diverse and expansive set of compounds, from simple oxides to complex organometallics, with unusually high coordination and varied bonding that set thorium apart in the world of inorganic chemistry.