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Lanthanides & Actinides
Uranium Chemistry




  • The most important fluoride
  • preparation:
    UO2 + 4HF Æ UF4 + 2H2O

    3UF4 + 2ClF3 (from Cl2 + 3F2 Æ 2ClF3) Æ 3UF6 + Cl2

  • properties:
    • mpt. 64°C, vapour pressure = 115 mmHg at 25°C
    • made on a large scale to separate uranium isotopes
      • gas diffusion or centrifugation separates 235UF6 from 238UF6
      • uranium richer in 235U is termed enriched, richer in 238U depleted
      • U.K. capacity (BNFL) = 6 kilotonne per year
    • powerful fluorinating agent, e.g. + CS2 Æ SF4

Other Fluorides

  • UF6 + Me3SiCl Æ Me3SiF + 1/2Cl2 + UF5 (melts to an electrically-conducting liquid)
  • UF6 + 2Me3SiCl Æ 2Me3SiF + Cl2 + UF4 ¨ 500-600°C ¨ UO2 + CFCl2CFCl2
  • Mixed-Valence fluorides such as U2F9 also form
  • Reduction of UF4 with 1/2H2 yields UF3
    • LaF3 structure
    • like LnF3 is insoluble in water



  • is the usual starting material for the synthesis of other UIV compounds
  • preparation: liquid-phase chlorination of UO3 by refluxing hexachloropropene
  • properties:
    • soluble in polar organic solvents & in water
    • forms various adducts (2 - 7 molecules) with O and N donors

      e.g. UCl4(CH3CN)4 ~ an ideal dodecahedron

      but UCl4(DMSO)3 is actually [UCl2(DMSO)6]2+UCl62-


  • Usually encountered as UCl3(thf)x (a rather intractable material)
  • Unsolvated binary gives its name to the UCl3 structure!
  • Actinide trihalides form a group with strong similarities (excepting redox behaviour) to the lanthanides


  • From chlorination of U3O8 + C
  • Highly oxidizing
  • Moisture-sensitive : UCl6 + 2H2O Æ UO2Cl2( Uranyl Chloride) + 4HCl
  • In CH2Cl2 solution UCl6 decomposes to U2Cl10 (Mo2Cl10 structure)

Halogeno Complexes

  • All Halides can form halogeno complexes, but F- and Cl- are best-known
  • Preparation: from UXx + NaX in melts or solvents (e.g. SOCl2), but in water only for some fluorides
  • Occurrence:
    • UIII: UCl52-, U2Cl72- and UCl4- (a useful UIII reagent)
    • UIV: UF73- and UF84- are common, UF62- and UCl62- are also known
      also pseudohalide complexes, e.g. [U(NCS)8]4-
    • UV: UV is usually unstable in (aq),
      but UF5 in 48% HF Æ M+UF6- (M+ = Rb+, Cs+, H3O+) salts
      also UCl6- and UCl63-
    • UVI: UF7- and UF82- are known, the latter is more thermally-stable


Principal Uranium Hydride is UH3

~ important as a source material for UIII and UIV chemistry



  • Many binary phases UOx have been reported
    • many are not genuine phases
    • genuine phases show range of O-content
  • The most important genuine phases are UO2, U4O9, U3O8, UO3


UO2 & U4O9 (UO2.25)

  • UO2 (black-brown) has the Fluorite structure
  • stoichiometric material is best obtained from:-
  • Interstitial Oxide Ions may be incorporated into the structure Æ UO2+x

    e.g. Octahedral hole at (1/2,1/2,1/2) is an obvious interstitial location

    but Neutron Diffraction studies indicate:
    1. oxide vacancies in the normal fluorite lattice
    2. two types of interstitial sites, O' and O'' displaced from the nominal (1/2,1/2,1/2) position
    3. 2O', 2O'' and 2 vacancies cluster together to form a defect 2 : 2 : 2 Willis Cluster
  • At UO2.25 (U4O9) (black) the interstitials are ordered forming a distinct phase in the phase diagram

U3O8 (UO2.67) & UO3

U3O8 (dark green)

  • conveniently made by heating uranyl nitrate or ethanoate in air

    > 650°C Higher uranium oxides decompose to U3O8

    > 800°C U3O8 loses oxygen

  • Structure:
    • Mixed oxide - average oxidation state U5.33
    • Evidence suggests Class II/III mixed valence
    • i.e. each Uranium has a time-averaged configuration [Rn]5f0.67
    • An orthorhombic, pillared-layer structure
    • All U atoms have essentially identical environments
    • Contains pentagonal bipyramidal UO7 units

UO3 (orange yellow)

  • produced by a variety of methods:-
  • Structure:
    • > 6 modifications have been characterised
    • Most contain O=U=O 'uranyl' groups linked by 4x equatorial bridging O
      Þ distorted octahedral environments


Fusion of uranium oxides with alkali or alkaline earth carbonates Æ orange/yellow/brown mixed-oxides, Uranates


Aqueous Chemistry

  • Complex aqueous chemistry due to:-
    • extensive possibilities for complexation
    • hydrolytic reactions, often leading to polymeric ion species
  • Reduction Potentials appropriate for 1M HClO4 indicate:


  • powerful reducing agent, reduces H2O to H2 (solutions in 1M HCl stable for days)
  • obtained by reduction of UO22+ electrolytically or with Zn/Hg
  • UF3H2O & U2(SO4)35H2O can be obtained from appropriate solutions


  • only slightly hydrolysed in 1M acid solution U4+ + H2O U(OH)3+ + H+
    but, it can give rise to polymeric species in less acid solutions
  • regarded as a 'stable' oxidation state of uranium in (aq)


  • extremely unstable to disproportionation
  • evidence for its existence in (aq) from stopped-flow techniques
  • more stable in DMSO (half-life ~ 30 mins)


  • the Uranyl ion
  • very stable
  • forms many complexes
  • a dominant feature of uranium chemistry
  • reduced to U4+ by e.g. Zinc, Cr2+
    • re-oxidation by H218O2 Æ U18O22+
    • re-oxidation by 18O2 Æ U(18O16O)2+
  • linear, symmetrical (O=U=O)2+ structure

    Why is UO22+ trans linear, whereas WO22+ is cis, bent?

    • WO22+ (6d0)is cis, bent because it allows p-donation from the 2 O to 2 independent d-orbitals, with a single d-orbital shared
    • ThO2 (6d05f0) is bent (122deg.) for similar reasons i.e. no f-orbital participation
    • UO22+ (6d05f0) is trans, linear because of the participation of its 5f orbitals

      U(5f) are of considerably lower energy than Th(5f) - see section 1

  • Details of the MO diagram for AnO22+ are controversial,
    but f-orbitals have ungerade symmetry, d-orbitals are gerade
    Þ no d-f mixing in centrosymmetric AnO22+ unit
  • UO22+ readily adds 4-6 donors in its equatorial plane Æ distinctive complexes
    e.g. cyclic hexadentates Æ strong complexes

    e.g. Uranyl nitrate hydrates all contain the UO2(NO3)2(H2O)2 unit

    extraction of uranyl nitrate from aqueous nitric acid into non-polar solvents is a classic separation method

  • UO22+ salts show characteristic (yellow) fluorescence

--Info & DownloadsBibliography  [textbook & online resources]

Source: Dr. S.J. Heyes; University of Oxford
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