Uranium tetrachloride is a hygroscopic, dark green solid, which sublimes in a high vacuum at ca. 500 °C. The crystal structure shows the uranium to be surrounded by eight chlorine atoms, four at 264 pm and the other four at 287pm. The molecule UCl4 is a Lewis acid and dissolves in solvents that can act as non-protic Lewis bases. Dissolution in protic solvents is more complicated. When UCl4 is added to water the uranium aqua ion is formed. The aqua ion 4+, is strongly hydrolyzed. The pKa for this reaction is ca. 1.6, so hydrolysis is absent only in solutions of acid strength 1 mol dm−3 or stronger. Further hydrolysis occurs at pH > 3. Weak chloro complexes of the aqua ion may be formed. Published estimates of the log K value for the formation of 3+ vary from −0.5 to +3 because of difficulty in dealing with simultaneous hydrolysis. With alcohols, partial solvolysis may occur. Uranium tetrachloride dissolves in non-protic solvents such as tetrahydrofuran, acetonitrile, dimethyl formamide etc. that can act as Lewis bases. Solvates of formula UCl4L are formed which may be isolated. The solvent must be completely free of dissolved water, or hydrolysis will occur, with the solvent, S, picking up the released proton. The solvent molecules may be replaced by other ligand in a reaction such as The solvent is not shown, just as when complexes of other metal ions are formed in aqueous solution. Solutions of UCl4 are susceptible to oxidation by air, resulting in the production of complexes of the uranyl ion.
Applications
Uranium tetrachloride is produced commercially by the reaction of carbon tetrachloride with pure uranium dioxide UO2 at 370 °C. It has been used as feed in the electromagnetic isotope separation process of uranium enrichment. Beginning in 1944, the Oak Ridge Y-12 Plant converted UO3 to UCl4 feed for the Ernest O. Lawrence's Alpha Calutrons. Its major benefit being the uranium tetrachloride used in the calutrons is not as corrosive as the uranium hexafluoride used in most other enrichment technologies This process was abandoned in the 1950s. In the 1980s, however, Iraq unexpectedly revived this option as part of its nuclear weapons program. In the enrichment process, uranium tetrachloride is ionized into a uranium plasma. The uranium ions are then accelerated and passed through a strong magnetic field. After traveling along half of a circle, the beam is split into a region nearer the outside wall, which is depleted, and a region nearer the inside wall, which is enriched in 235U. The large amounts of energy required in maintaining the strong magnetic fields as well as the low recovery rates of the uranium feed material and slower more inconvenient facility operation make this an unlikely choice for large scale enrichment plants. Work is being done in the use of molten uranium chloride–alkali chloride mixtures as reactor fuels in molten salt reactors. Uranium tetrachloride melts dissolved in a lithium chloride–potassium chloride eutectic have also been explored as a means to recover actinides from irradiated nuclear fuels through pyrochemical nuclear reprocessing.
