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Lanthanides & Actinides
General Observations on Actinides

General Observations (comparisons with Lanthanides)

  • Electronic Configurations of Actinides are not always easy to confirm
    • atomic spectra of heavy elements are very difficult to interpret in terms of configuration
  • Competition between 5fn7s2 and 5fn-16d7s2 configurations is of interest
    • for early actinides promotion 5f Æ 6d occurs to provide more bonding electrons
      much easier than corresponding 4f Æ 5d promotion in lanthanides
    • second half of actinide series resemble lanthanides more closely
  • 5f orbitals have greater extension wrt 7s and 7p than do 4f relative to 6s and 6p orbitals

    e.g. ESR evidence for covalent bonding contribution in UF3, but not in NdF3

  • 5f / 6d / 7s / 7p orbitals are of comparable energies over a range of atomic numbers
    • especially U - Am

      Þ tendency towards variable valency

      Þ greater tendency towards (covalent) complex formation than for lanthanides, including complexation with p-bonding ligands

      Þ electronic structure of an element in a given oxidation state may vary between compounds and in solution

      Þ often impossible to say which orbitals are being utilized in bonding

  • Ionic Radii of ions show a clear "Actinide Contraction"
    • Actinide 3+ or 4+ ions with similar radii to their Lanthanide counterparts show similarities in properties that depend upon ionic radius
  • Electronic Spectra
    • narrow bands (compared to transition metal spectra)
    • relatively uninfluenced by ligand field effects
    • intensities are ca. 10x those of lanthanide bands
    • complex to interpret

Optical Absorption Spectrum of U4+(aq)

  • Magnetic Properties
    • hard to interpret
    • spin-orbit coupling is large
      & Russell-Saunders (L.S) Coupling scheme doesn't work
    • ligand field effects are expected where 5f orbitals are involved in bonding

--Info & DownloadsBibliography  [textbook & online resources]

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