Thorium compounds
[1] While the later actinides from americium onwards are predominantly trivalent and behave more similarly to the corresponding lanthanides, as one would expect from periodic trends, the early actinides up to plutonium (thus including thorium and uranium) have relativistically destabilised and hence delocalised 5f and 6d electrons that participate in chemistry in a similar way to the early transition metals of group 3 through 8: thus, all their valence electrons can participate in chemical reactions, although this is not common for neptunium and plutonium.
However, in metallic thorium, the [Rn]5f16d17s2 configuration is a low-lying excited state and hence the 5f orbitals contribute, existing in a rather broad energy band.
[8][9] It dissolves in concentrated nitric acid containing a small amount of catalytic fluoride or fluorosilicate ions;[8][10] if these are not present, passivation can occur, similarly to uranium and plutonium.
[8] When thorium dissolves in hydrochloric acid, a black insoluble residue, probably ThO(OH, Cl)H is left behind,[8] similarly to protactinium and uranium.
[11] Finely divided thorium metal presents a fire hazard due to its pyrophoricity and must therefore be handled carefully.
[8] When heated in air, thorium turnings ignite and burn brilliantly with a white light to produce the dioxide.
[2] In 1997, reports of amber Th3+ (aq) being generated from thorium tetrachloride and ammonia were published: the ion was supposedly stable for about an hour before it was oxidised by water.
However, the reaction was shown the next year to be thermodynamically impossible and the more likely explanation for the signals was azido-chloro complexes of thorium(IV).
[16] In fact, the redox potentials of thorium, protactinium, and uranium are much more similar to those of the d-block transition metals than the lanthanides, reflecting their historic placement prior to the 1940s as the heaviest members of groups 4, 5, and 6 in the periodic table respectively.
[2] Due to the large size of the Th4+ cation, thorium salts have a weaker tendency to hydrolyse than that of many multiply charged ions such as Fe3+, but hydrolysis happens more readily at pH above 4, forming various polymers of unknown nature, culminating in the formation of the gelatinous hydroxide:[16] this behaviour is similar to that of protactinium, which also hydrolyses readily in water to form colloidal precipitates.
[1] The distinctive ability of thorium salts is their high solubility, not only in water, but also in polar organic solvents.
[22] When heated, it emits intense blue light, which becomes white when mixed with its lighter homologue cerium dioxide (CeO2, ceria): this is the basis for its previously common application in gas mantles.
[14] Incomplete reports of the lower bromides ThBr3, ThBr2, and ThBr are known (the last only known as a gas-phase molecular species): ThBr3 and ThBr2 are known to be very reactive and at high temperatures disproportionate.
[14] Many polynary halides with the alkali metals, barium, thallium, and ammonium are known for thorium fluorides, chlorides, and bromides.
[14] For example, when treated with potassium fluoride and hydrofluoric acid, Th4+ forms the complex anion ThF2−6, which precipitates as an insoluble salt, K2ThF6.
[10] The heavier chalcogens sulfur, selenium, and tellurium are known to form thorium chalcogenides, many of which have more complex structure than the oxides.
[29] All five chemically characterised pnictogens (nitrogen, phosphorus, arsenic, antimony, and bismuth) also form compounds with thorium.
[31] Finely divided thorium metal reacts very readily with hydrogen at standard conditions, but large pieces may need to be heated to 300–400 °C for a reaction to take place.
[32] Thorium borides, carbides, silicides, and nitrates are refractory materials, as are those of uranium and plutonium, and have thus received attention as possible nuclear fuels.
[33] Many other inorganic thorium compounds with polyatomic anions are known, such as the perchlorates, sulfates, sulfites, nitrates, carbonates, phosphates, vanadates, molybdates, chromates, and other oxometallates,[b] many of which are known in hydrated forms.
[25] These are important in thorium purification and the disposal of nuclear waste, but most have not yet been fully characterized, especially on their structural properties.
Another example of the high coordination characteristic of thorium is [Th(C5H5NO)6(NO3)2]2+, a 10-coordinated complex with distorted bicapped antiprismatic molecular geometry.
[18] The anionic [Th(NO3)6]2− is isotypic to its cerium, uranium, neptunium, and plutonium analogues and has a distorted icosahedral structure.
[34] Although these f-series cyclooctatetraenyls are not isotypic with the d-series cyclopentadienyls, including the more famous ferrocene, they have very similar structures, and were named to emphasise this resemblance.
[34] Half-sandwich compounds are also known, such as 2(η8-C8H8)ThCl2(THF)2, which has a piano-stool structure and is made by reacting thorocene with thorium tetrachloride in tetrahydrofuran.
The halide derivative Th(C5H5)3Cl can be made similarly by reducing the amount of K(C5H5) used (other univalent metal cyclopentadienyls can also be used), and the chlorine atom may be further replaced by other halogens or by alkoxy, alkyl, aryl, or BH4 groups.
Of these, the alkyl and aryl derivatives have been investigated more deeply due to the insight they give regarding the nature of the Th–C σ bond.
[35] Of special interest is the dimer [Th(η5-C5H5)2-μ-(η5,η1-C5H5)]2, where the two thorium atoms are bridged by two cyclopentadienyl rings, similarly to the structure of niobocene.
