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 5f^n-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