ACTINIDES

Survey of Actinide Oxidation States

The known oxidation states of the actinides are indicated and surveyed:

[importance: > > ]

7

6

5

4

3

2

Ac

Th

Pa

U

Np

Pu

Am

Cm

Bk

Cf

Es

Fm

Md

No

Lr

+2

• Unusual oxidation state
• Common only for the heaviest elements
• No²⁺ & Md²⁺ are more stable than Eu²⁺
• Actinide An²⁺ ions have similar properties to Lanthanide Ln²⁺ and to Ba²⁺ ions
• rationalization of stabilities: Open University Course Book p. 54-56

+3

• The most common oxidation state
• The most stable oxidation state for all trans-Americium elements (except No?)
• Of marginal stability for early actinides Th, Pa, U (But: Group oxidation state for Ac)
• General properties resemble Ln³⁺ and are size-dependent
  • stability constants of complex formation are similar for same size An³⁺ & Ln³⁺
  • isomorphism is common
  • later An³⁺ & Ln³⁺ must be separated by ion-exchange/solvent extraction
• Binary Halides, MX₃ easily prepared, & easily hydrolysed to MOX
• Binary Oxides, M₂O₃ known for Ac, Th and trans-Am elements

+4

• Principal oxidation state for Th

  Th⁴⁺ chemistry shows resemblance to Zr⁴⁺ / Hf⁴⁺ - is it a transition metal?

• Very important, stable state for Pa, U, Pu
• Am, Cm, Bk & Cf are increasingly easily reduced - only stable in certain complexes

  e.g. Bk⁴⁺ is more oxidizing than Ce⁴⁺

• MO₂ known from Th to Cf (fluorite structure)
• MF₄ are isostructural with lanthanide tetrafluorides
• MCl₄ only known for Th, Pa, U & Np
• Hydrolysis / Complexation / Disproportionation are all important in (aq)

+5

• Principal state for Pa

  Pa⁵⁺ chemistry resembles that of Nb⁵⁺ / Ta⁵⁺ - is it a transition metal?

• For U, Np, Pu and Am the AnO₂⁺ ion is known (i.e. quite unlike Nb/Ta)
• Comparatively few other An^V species are known

  e.g. fluorides, PaF₅, NbF₅, UF₅; fluoro-anions, AnF₆^-, AnF₇^2-, AnF₈^3-

  e.g. oxochlorides, PaOCl₃, UOCl₃; uranates, NaUO₃

+6

• AnO₂²⁺ ions are important for U, Np, Pu, Am

  UO₂²⁺ is the most stable

• Few other compounds e.g. AnF₆ (An = U, Np, Pu), UCl₆, UOF₄ etc..., U(OR)₆

+7

• Only the marginally stable oxo-anions of Np and Pu, e.g. AnO₅^3-