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Lanthanides & Actinides
Solution Chemistry of Lanthanides

Solubility

  • Ln3+ are not especially soluble in water
  • no simple relationship of solubility to cation radius
    • depends on small difference between large solvation and lattice energies
    • depends on entropy effects
  • oxalates/double sulfates/double nitrateslighter Ln more soluble
  • basic nitratesheavier Ln more soluble

    basis of classical separation procedures

Hydrated Lanthanide Ions

  • Primary hydration numbers in (aq) are 8, 9

    from Luminescence & NMR measurements

  • Primary hydration number Ø with Lanthanide Contraction
  • Secondary hydration number with Lanthanide Contraction
    • increased polarization of 1ry hydration sphere by a smaller cation enhances hydrogen bonding to water in the secondary hydration sphere
  • Aqua ions hydrolyze ~ increasingly so from La to Lu as they become smaller
    • Ln(H2O)n3+ + H2O Æ Ln(H2O)n-12+ + H3O+
  • Salts with common anions frequently contain Ln(H2O)93+

    with tri-capped trigonal prismatic (ttp) geometry

Coordination Compounds
  • Strongly complexing, chelating ligands necessary to yield isolable products from (aq)
  • Some O-donor chelating ligands that form complexes include:

    NO3-

    • binds in its chelate mode
    • notable for high Coordination Numbers
      • Ce(NO3)52- 10-coordinate bicapped dodecahedron

        Ce(NO3)63- 12-coordinate icosahedron

    Oxalate

    Citrate

    Tartrate

    b-diketonates R(CO)(CH-)(CO)R

  • Classic bidentate complexes formed as Ln(L-L)3L' (L' = H2O, py, etc...) or Ln(L-L)4
    • dehydrate Ln(L-L)3(H2O) in vacuo -> Ln(L-L)3 now coordinatively unsaturated!
    • Ln(L-L)3 with bulky R
      • thermally stable
      • volatile & sublimable
      • soluble in non-polar solvents
      • Widely used as NMR shift reagents

        polar molecules may coordinate {Ln(L-L)3}, their NMR resonances are perturbed by the paramagnetism of Ln

e.g. Eu(facam)3
  • "Anti-knock" activity as petroleum additives
  • Macrocyclic Ligands

    Crown Ethers

    • differing cavity/M sizes allow range of C.N & stoichiometry

      e.g. with 18-crown-6

      La-Gd [Ln(18-C-6)(NO3)2]3[Ln(NO3)6]10-coordinate

      La-Nd [Ln(18-C-6)(NO3)3]12-coordinate

      Tb-Lu [Ln(NO3)3(H2O)3](18-C-6)18-C-6 is not coordinated ~ cavity too large

  • Complexes with
    • uncharged monodentate ligands
    • ligands with donor atoms other than O
MUST be prepared in the absence of H2O
  • e.g. Ln(en)43+ (8-coordinate) from polar organic solvents (e.g. acetone)
  • (18-crown-6) complexes dissociate instantly in water!
  • Coordination by halides is weak

    but LnX63- (unusually octahedral) are isolable from non-aqueous solution

Solution Chemistry of Other Lanthanide Oxidation States
Ln(IV)

Cerium is the only Ln4+ with significant aqueous or coordination chemistry

E° (Ce4+(aq)/Ce3+(aq)) = 1.72 V (others est. 2.9 V)
  • prepared by the action of a strong oxidizing agent, e.g. S2O82-, on Ce3+(aq)
  • widely used as an oxidant itself:- e.g. quantitative analysis / organic chemistry
  • E° (Ce4+/Ce3+) is markedly dependent on complexation and hydrolysis
    • strong oxidizing agent in perchloric acid solution
    • in other acids coordination occurs

      e.g. Ce(NO3)62- is generally used for oxidations as its NH4+ salt

    • on pH:
      1. hydrolysis to Ce(OH)3+ occurs
      2. then polymerization
      3. ultimately precipitation of yellow gelatinous CeO2.xH2O
  • 4+ charge stabilizes halogeno-complexes e.g. CeF84-

    {aside: CeF62- is 8-coord through fluoride-bridging, but CeCl62- is octahedral}

Ln(II)

Significant solution chemistry of Ln2+ is essentially confined to SmII, EuII, YbII

Preparation:

  • electrolytic reduction of Ln3+(aq)
  • Eu2+ (the most stable LnII) is prepared by reduction of Ln3+(aq) with Zn/Hg

Properties

  • Ln2+ Aquo-ion colours
    • Sm2+ blood-red
    • Eu2+ colourless
    • Yb2+ yellow
  • Ln2+(aq) are readily oxidized by air
    • BUT Eu2+(aq) is easily handled
  • Sm2+(aq) & Yb2+(aq) reduce water
  • Eu2+(aq) is relatively stable in the dark
  • Carbonate and sulfate salts have been isolated
  • Sm2+ and Yb2+ salts are susceptible to oxidation by their water of crystallization
  • Eu & Yb dissolve in l-ammonia to give intense blue, highly reducing solutions
    • contain [Ln(NH3)x]2+ and solvated electrons?
    • Solutions decompose on standing, precipitating the amide Ln(NH2)2
  • Properties of Ln2+ are closely-related to those of the alkaline earths

    In particular Eu2+ is often likened to Ba2+

    1. Similar Salt Solubilities (like Ba, sulfates are insoluble, hydroxides are soluble)
    2. Behaviour in l-NH3 is very similar
    3. Similar Coordination Chemistry (Not extensive / Hard ligands)
    4. But Very different redox chemistry!
--Info & DownloadsBibliography  [textbook & online resources]

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