LANTHANIDES

Solution Chemistry

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 1^ry 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)₅^2- 10-coordinate bicapped dodecahedron

      Ce(NO₃)₆^3- 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

• 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 LnX₆^3- (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. ³ 2.9 V)

• prepared by the action of a strong oxidizing agent, e.g. S₂O₈^2-, 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)₆^2- 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. CeF₈^4-

  {aside: CeF₆^2- is 8-coord through fluoride-bridging, but CeCl₆^2- is octahedral}

Ln(II)

Significant solution chemistry of Ln²⁺ is essentially confined to Sm^II, Eu^II, Yb^II

Preparation:

• electrolytic reduction of Ln³⁺(aq)
• Eu²⁺ (the most stable Ln^II) 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!