Q8(b). High oxidation states of early actinides; stable +2 of Am and No

High oxidation states in early actinides

1. In Th, Pa, U, Np and Pu the 5f, 6d and 7s orbitals are close in energy. Uranium is 5f³6d¹7s²: all six electrons are available, giving U(VI) (UO₂²+, UF₆); Np and Pu reach still higher states. Oxidation numbers above +3 are easily accessible and often preferred for these elements.
2. The 5f orbitals are less compact than the 4f: they extend into the bonding region and are less shielded, so 5f electrons take part in bonding (their complexes' spectra depend on the ligands). The 4f orbitals of the lanthanides are buried under 5s²5p⁶, behave as core orbitals, and the lanthanides keep to +3.
3. Across the series the nuclear charge rises and the 5f orbitals contract and fall in energy; from americium on, +3 becomes the main state and the later actinides behave like the lanthanides.

Stable +2 for americium and nobelium

1. Americium (Z=95, 5f⁷7s²): Am²⁺ is 5f⁷, a half-filled shell with maximum exchange energy, just as Eu²⁺ (4f⁷) is the most stable +2 lanthanide. Am(II) occurs in solid halides; Am(III) still dominates in water.
2. Nobelium (Z=102, 5f¹⁴7s²): No²⁺ is 5f¹⁴, a completely filled shell. Removing a third electron would break the filled shell, so +2 is the most stable state of No in aqueous solution, the only actinide for which this is so (the analogue of Yb²⁺, 4f¹⁴).
3. For the other actinides the 5f shell is neither half-filled nor filled after losing two electrons, so +2 has no special stability.

Application. the wide range of uranium and plutonium oxidation states is used in nuclear fuel reprocessing (PUREX): U(VI) and Pu(IV) are extracted together, then Pu is reduced to Pu(III) and separated.