Halides

Fluorides

UF₆

• 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

Chlorides

UCl₄

• is the usual starting material for the synthesis of other U^IV 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⁺UCl₆^2-

UCl₃

• 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

UCl₆

• 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 UX_x + NaX in melts or solvents (e.g. SOCl₂), but in water only for some fluorides
• Occurrence:
  • U^III: UCl₅^2-, U₂Cl₇^2- and UCl₄^- (a useful U^III reagent)
  • U^IV: UF₇^3- and UF₈^4- are common, UF₆^2- and UCl₆^2- are also known

    also pseudohalide complexes, e.g. [U(NCS)₈]^4-

  • U^V: U^V is usually unstable in (aq),

    but UF₅ in 48% HF Æ M⁺UF₆^- (M⁺ = Rb⁺, Cs⁺, H₃O⁺) salts

    also UCl₆^- and UCl₆^3-

  • U^VI: UF₇^- and UF₈^2- are known, the latter is more thermally-stable

Hydrides

Principal Uranium Hydride is UH₃

~ important as a source material for U^III and U^IV chemistry

Oxides

• Many binary phases UO_x 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 (†UO_2.25)

• UO₂ (black-brown) has the Fluorite structure
• stoichiometric material is best obtained from:-
• Interstitial Oxide Ions may be incorporated into the structure Æ UO_2+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 UO_2.25 (U₄O₉) (black) the interstitials are ordered forming a distinct phase in the phase diagram

U₃O₈ (†UO_2.67) & 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 U^5.33
  • Evidence suggests Class II/III mixed valence
  • i.e. each Uranium has a time-averaged configuration [Rn]5f^0.67
  • 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

Uranates

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:

  

U³⁺

• powerful reducing agent, reduces H₂O to H₂ (solutions in 1M HCl stable for days)
• obtained by reduction of UO₂²⁺ electrolytically or with Zn/Hg
• UF₃•H₂O & U₂(SO₄)₃•5H₂O can be obtained from appropriate solutions

U⁴⁺

• 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)

UO₂⁺

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

UO₂²⁺

• 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

Bibliography