Uranium tetrahalides

The formation of the halides of Ca and Mg are favored over the formation of the halides of uranium at all temperatures up to 1,500°C therefore, the reaction of a uranium tetrahalide with Ca° or Mg° will proceed exothermically to produce uranium metal. However, UCI4, UBr4, and UI4 are deliquescent and air-reactive practicality dictates that the production of uranium metal involves the reaction of UF4 with either Mg° or Ca° ... 

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. 

Answer 10.1 Early on in the series, as far as uranium, the maximum oxidation state corresponds to the total number of outer shell electrons. Uranium forms a hexachloride, in addition to the MEe also formed by Np and Pu. After uranium, neptunium forms the full range of tetrahalides, but, from plutonium onwards, the (-1-3) state dominates the chemistry of the binary halides, which strongly resemble those of the lanthanides. This may reflect decreased availability of 5f (and 6d ) electrons for bonding. As usual, F supports the highest oxidation states. 

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 crystalline tetrahalides of uranium are all stable and are the normal result of either direct union of the elements or action of the halogen acid on the metal. Neptunium tetrafluoride, a pale green sohd, is also known ... 

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. 

Halides of uranium. Our present knowledge of the structures of the majority of the 5f halides is summarized (with references) under Trihalides, Tetrahalides, etc. in Chapter 9. The halides of U include ... 

Ezhov, Yu. S., Akishin, P. A., Rambidi, N. G., Electron-diffraction study of uranium and thorinm tetrahalides in the gas phase. II. Uranium and thorinm tetrachlorides and tetrabromides, Zh. Strukt. Khim., 10, (1969), 571, inRnssian. Cited on page 202. 

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