Thorium tetrahalides

Question 10.2 Study the information about the thorium tetrahalides in Table 10.4 and comment on the trends and patterns in it. 

The more important properties of the tetrahalides, from reference [11], are listed in Table 6.10. Many of these properties, especially for ThCU, ThBr4, and TI1I4, are known only semiquantitatively. Anhydrous Thp4 is made by passing an excess of HF vapor over ThOj or ThOFj at temperatures between 550 and 600 C. The anhydrous double fluoride KThFs is precipitated from aqueous solutions of thorium nitrate by addition of an excess of KF. It has been used for electrolytic production of thorium metal. Table 6.10 Thorium Tetrahalides ... 

For this analysis a set of consistent spectroscopic data and molecular constants for these species is presented based on recent results of IR spectroscopic measurements for UChlg), UF4(g) and ThF4(g). For the thorium tetrahalides, these estimates of molecular parameters have been somewhat overtaken by theoretical calculations using density functional theory. These confirm the tetrahedral stracture of the species, and on the whole, the current review has preferred these calculated values to the correlative estimates presented in this paper. 

Thorium reacts with all halogens forming tetrahalides. 

The compounds known are summarized in Table 10.1. The only compound of an early actinide in the -1-2 state is Thl2, a metallic conductor which is probably Th + (e )2 (D)2-Certain heavier actinides form MX2 (Am, Cf, Es), which usually have the structure of the corresponding EuX2 and are thus genuine M + compounds. All four trihalides exist for all the actinides as far as Es, except for thorium and protactinium. Tetrafluorides exist for Th-Cm and the other tetrahalides as far as NpX4 (and in the gas phase in the case of PuCE). Pentahalides are only known for Pa, U, and Np whilst there are a few MFe (M = U-Pu), uranium is the only actinide to form a hexachloride. The known actinide halides are generally stable compounds most are soluble in (and hydrolysed by) water. 

Actinide halides and oxyhalides are known to form numerous complexes with oxygen and nitrogen donor ligands and the preparation and properties of such compounds have recently been reviewed (12, 13). Relatively few protactinium halide complexes are known, but this situation reflects the lack of research rather than a tendency not to form complexes. However, there is sufficient information available for certain ligands to permit a comparison with the behavior of other actinide halides, and to illustrate the similarities and differences observed with the tetrahalides of thorium to plutonium inclusive and, to a lesser extent, with the protactinium and uranium pentahalides. 

The actinide tetrachloride-DMSO complexes are particularly interesting (18). There is a pronounced change in stability proceeding along the actinide series with 1 5 complexes being the most stable for thorium and protactinium and the 1 3 complexes for the remaining actinides. The 1 7 complex could not be obtained pure with thorium tetrachloride and under the preparative conditions required, namely, recrystallization from hot dimethyl sulfoxide, protactinium(IV) was oxidized. The solid 1 5 and 1 3 protactinium tetrachloride compounds are, in fact, unstable in dry nitrogen, behavior which contrasts markedly with the stability of the tetrahalide-phosphine oxide and DMA complexes. 

With the exception of thorium, the actinides form trihalides. For uranium and neptunium, reduction of the MX4 compounds with hydrogen is necessary, but for the elements from plutonium onwards the action of the carbon tetrahalide or aluminium halide on the dioxide is usually employed. The trifluorides are insoluble but the rest dissolve to give solutions containing ions. 

The tetrahalides are the thorium halides of greatest practical importance. The tetrafluoride ThF4 is the preferred starting material for large-scale production of thorium metal (Sec. 10.4). ThF4 has been proposed as fertile material in the fuel mixture of the molten-salt reactor. The tetraiodide has been used as feed material in the iodide process for making very pure thorium metal (Sec. 10.4). 

---