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Concept
Uranium tetrachloride
Summary
Uranium tetrachloride is an inorganic compound, a salt of uranium and chlorine, with the formula UCl4. It is a hygroscopic olive-green solid. It was used in the electromagnetic isotope separation (EMIS) process of uranium enrichment. It is one of the main starting materials for organouranium chemistry.
Uranium tetrachloride is synthesised generally by the reaction of uranium trioxide (UO3) and hexachloropropene. Solvent UCl4 adducts can be formed by a simpler reaction of UI4 with hydrogen chloride in organic solvents.
According to X-ray crystallography the uranium centers are eight-coordinate, being surrounded by eight chlorine atoms, four at 264 pm and the other four at 287pm.
Dissolution in protic solvents is more complicated. When UCl4 is added to water the uranium aqua ion is formed.
UCl4 + xH2O → [U(H2O)x]4+ + 4Cl−
The aqua ion [U(H2O)x]4+, (x is 8 or 9) is strongly hydrolyzed.
[U(H2O)x]4+ [U(H2O)x−1(OH)]3+ + H+
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 (pH < 0). 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 [UCl]3+(aq) vary from −0.5 to +3 because of difficulty in dealing with simultaneous hydrolysis.
With alcohols, partial solvolysis may occur.
UCl4 + xROH UCl4−x(OR)x + xHCl
Uranium tetrachloride dissolves in non-protic solvents such as tetrahydrofuran, acetonitrile, dimethyl formamide etc. that can act as Lewis bases. Solvates of formula UCl4Lx 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.
UCl4 + H2O + S UCl3(OH) + SH+ +Cl−
The solvent molecules may be replaced by other ligand in a reaction such as
UCl4 + 2Cl− → [UCl6]2−.
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.
Official source
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