Uranium halide

Coordination polyhedra in the uranium halides [adapted from J.C. Taylor, Coord. Chem. Rev. 1976, 20, 203 reprinted with permission of Elsevier Science pnhhshers]. 

UFe and UCle are two remarkable substances, the former on account of its application in isotopic enrichment of uranium (Sections 9.4 and 10.4.5) the latter as the only actinide hexachloride. Colourless UFe can be made by a wide variety of routes, including  

The latter is the Fluorox process, which has the considerable advantage of not requiring the use of elemental fluorine. Dark green, hygroscopic, UCle may be synthesized by halogen exchange or disproportionation  

Both have octahedral molecular structures (U-F = 1.994 A in UFe) and are volatile under reduced pressure at temperatures below 100 °C. 

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Uranium can be prepared by reducing uranium halides with alkali or alkaline earth metals or by reducing uranium oxides by calcium, aluminum, or carbon at high temperatures. The metal can also be produced by electrolysis of KUF5 or UF4, dissolved in a molten mixture of CaCl2 and NaCl. High-purity uranium can be prepared by the thermal decomposition of uranium halides on a hot filament. 

Preparation of Uranium Metal. Uranium is a highly electropositive element, and extremely difficult to reduce. As such, elemental uranium caimot be prepared by reduction with hydrogen. Instead, uranium metal must be prepared using a number of rather forcing conditions. Uranium metal can be prepared by reduction of uranium oxides (UO2 [1344-59-8] or UO [1344-58-7] with strongly electropositive elements (Ca, Mg, Na), reduction of uranium halides (UCl [10025-93-1], UCl [10026-10-5] UF [10049-14-6] with electropositive metals (Li, Na, Mg, Ca, Ba), electro deposition from molten... 

Table 6. Physical Constants for Selected Uranium Halides...

Allyl Complexes. Allyl complexes of uranium are known and are usually stabilized by cyclopentadienyl ligands. AEyl complexes can be accessed via the interaction of a uranium halide and an allyl grignard reagent. This synthetic method was utilized to obtain a rare example of a "naked" homoleptic allyl complex, U(T -C2H )4 [12701 -96-17, which decomposes at 0°C. Other examples, which are more stable than the homoleptic allyl complex have been synthesized, ie, U(allyl)2(OR)2 (R = alkyl), U(allyl)2X (X = halide), and U(allyl)(bipy)2. 

Uranium halide complexes, 25 437-439 physical constants for, 25 4371 Uranium hexachloride, 25 439 Uranium hexafluoride, 11 845, 859, 25 438 Uranium hydrocarbyl complexes,... 

Predominantly cis-1,4-polybutadiene is produced by coordination polymerization with mixed catalysts.187,487,488 Three catalyst systems based on titanium, cobalt, or nickel are used in industrial practice. Iodine is an inevitable component in titanium-alkylaluminum sytems to get high cis content. Numerous different technologies are used 490,491 A unique process was developed by Snamprogetti employing a (Tr-allyl)uranium halide catalyst with a Lewis acid cocatalyst.492-494 This catalyst system produces poly butadiene with 1,4-ris content up to 99%. 

Uranium halides exist in the +3 oxidation state (UF3, UC13, UBR3, UI3), the +4 oxidation state (UF4, UC14, UBr4, and UI4), the +5 oxidation state (UF5, UC15), and the +6 oxidation state (UF6, UCU). 

UI3(THF)4 can easily be prepared in large scale, which makes easy the synthesis of U(III) complexes from it. According to Ref. 360, the use of solvated complexes of other uranium halides or uranium alkoxides involves some difficulties, in... 

These are listed in Table 20-11. While their preparations and properties show much similarity to those of the uranium halides, there is a steady decrease in stability of the higher oxidation states with increasing atomic number (which continues in the succeeding elements). In short, the actinide concept becomes more and more valid. 

These form a large and interesting series of substances, in oxidation states ranging form +3 to +6, illustrating principles of structure and property. The structures of the uranium halides are shown in Figure 10.4. 

In normal laboratory work, these are the most important uranium halides, especially UCI4. 

A single uranium halide adduct of a monodentate phosphine has been reported reaction of UCI4 with excess PMcs permits isolation of the trimethylphosphine adduct, UCl4(PMe3)3. ... 

Examples of ( 7 -C4N) coordination in pyrrole-derived macrocyles may be found in the reaction products of uranium halides with the tetraanion of the macrocycle [(-CH2-)5]4-calix[4]tetrapyr-role. " " As described in Equation (30), the reaction of Ul3(THF)4 with the potassium salt of the tetrapyrrolide in THF generates a dinuclear complex, [ [(-CH2-)5]4-calix[4]tetrapyrrole -UK(THF)3]2(/4-0) 2THF the 0x0 group is proposed to come from deoxygenation of a THF molecule ... 

Tsuda T, Boyd L, Hussey CL (2006) Uranium halide complexes in ionic liquids an electrochemical and structural study. J Chem Soc Dalton Trans 2006 2334-2341... 

Table 5.9 lists uranium halides together with some of their more significant properties. 

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