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Lanthanides & Actinides
Predominance of 3+ Oxidation State
Chemistry is principally of Ln3+
Why the prevalence of oxidation state III (Ln3+)?

Examine Thermodynamic Parameters:

I1/2/3/4 DatmH DhydH(Ln3+) DLH(LnX3)

these values are available in a Table(import DHatm from larger table for web!)


For any given Lanthanide
  • As successive electrons are removed from neutral Ln the stabilizing effect on the orbitals is related to their principal quantum number, 4f > 5d > 6s.
  • For Ln2+ {except for La & Gd} the configuration is [Xe]4fn
  • For Ln3+ the configuration is always [Xe]4fn

    the 4f binding energy is so great that remaining 4f electrons are regarded as "core-like" (i.e. incapable of modification by chemical means) (except Ce)

  • Note that as a rule of thumb: I4 ~ 2 I3 ~ 4 I2 ~ 8 I1

    Þ I4 > (I1 + I2 + I3 )

    Therefore in almost all cases Ln3+ provides the best energetics

Observing trends across the Lanthanide Series
  • The general trend is for increasing ionization energies with increasing Z (i.e. with increase in Zeff)
  • Marked Half-Shell Effects - magnitude as n in In
  • Also Quarter/Three-Quarter Shell Effects

    (compare with transition metals - these are not seen clearly with dn configurations)

    Explanation?: interelectronic repulsion is related not just to electron pairing but also to angular momentum of the electrons

    • e.g. in Pr2+ (4f3) Æ Pr3+ (4f2) ionization removes repulsion between e- of like rotation, whereas Pm2+ (4f4) Æ Pm3+ (4f3) removes the stronger repulsion between e- of unlike rotation (Þ latter Ionization Energy is correspondingly lower - hence the local minimum in the I3 graph at Pm)

    The three-quarter effect is the bigger: interelectronic repulsion is bigger in smaller Lnn+


DatmH follows the inverse trend to I3 {and therefore also to (I1 + I2 + I3 ) }

Þ metallic bonding is correlated with ease of ionization to Ln3+ state
this trend is modified slightly due to the different structures of the Ln metals

Some Thermodynamic Observations (Ionic Model style)

The trends in the formation of LnIII

Formation of Compounds {DfH(LnX3(s))} or Ln3+(aq) {E°(Ln3+(aq)/Ln(s))}

depend on the balance between:

Energy Supplied to effect Ln(s) Æ Ln(g) Æ Ln3+(g) + 3e- [DatmH + I1 + I2 + I3]


Energy gained from Ln3+(g) + 3X-(g) Æ LnX3(s) [DLH(LnX3(s))] or Ln3+(g) Æ Ln3+(aq) [DhydH(Ln3+)]

The energies determining trends in E°(Ln3+(aq)/Ln(s)) are graphed below:

Production of Ln3+(g) shows

  • a smooth trend based on size effects (trend based on Zeff)
  • shell structure effects superimposed with clear maxima at half-shell (f7) and full-shell (f14)

    also smaller quarter and three-quarter shell effects

Hydration Energy of Ln3+ (also Lattice Energies of LnX3(s)) shows

  • only a smooth ionic-size-based trend (the trend based on Zeff) and no shell structure effects

Balance of trends in Ionization + Atomization Energies with Hydration (Lattice) Energy

  • removes size effects
  • leaves only the Shell effects - see values of DfH(Ln3+(aq))


Overall: The most important energy correlations are with I3

"exceptions to +3 rule" can also be rationalized

Occurrence of +4 oxidation state

predicted from [DatmH + I1 + I2 + I3 + I4] which follows trends in I4

  • Ce, Pr Æ Ce4+ [4f0], Pr4+ [4f1] ~ early in series 4f orbitals still comparatively high in energy
  • Tb Æ Tb4+ [4f7 valence shell] ~ half shell effect

Occurrence of +2 oxidation state

predicted from [DatmH + I1 + I2] which follows trends in DatmH, which is reverse of trend in I3

  • Eu, Sm, Yb Æ Eu2+ [4f7], Sm2+ [4f6], Yb2+ [4f14]

    ~ clear influences of electronic shell structure & from DatmH

--Info & DownloadsBibliography  [textbook & online resources]

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