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delocalized and localized 5f electrons. This is reflected in the complex chemical and
physical properties of U, Np, and especially Pu. As well, with the 5f orbitals overlapping
the 6p, 6d, and 7s orbitals, the closeness in energy of the many electronic configurations
can potentially lead to multiple self-consistent forms of similar structure and energy.
Plutonium, in particular, is sensitive to chemical variations, especially in the presence of
hydrogen or water. Its ability to have multiple oxidation states in solution at the same
time depends on several key factors: the most important of these are the tendency of Pu IV
and Pu V to disproportionate, and the relatively slow kinetics of reactions that involve the
making or breaking of bonds in plutonyl ions (PuO2 2+ and PuO2 + ). Other factors include
ion concentration, ionic strength, pH, temperature, and the presence or absence of
complexing ligands [2].
Electronic structure calculations have shown that one must consider more than
just the occupied 5f orbitals for metal–ligand bonding in the early-actinyls. The virtual 5f
and 6d orbitals, as well as the semicore 6s and 6p orbitals also play an important role
(Figure 1–1) [2, 18].
Unlike their 4f counterparts the 5f electrons of the early actinides are not
completely shielded by the 6s and 6p electrons, resulting in a significant radial extention
of the 5f orbitals that allows for overlap with ligand orbitals and a covalent bonding
contribution. However, the covalent contribution is relatively small and bonding is still
thought to be predominantly ionic in character. In addition, there is an increasing
tendency for the 6d and 7s electrons to experience less shielding from the nuclear charge
as an increase occurs along the actinide series for a given oxidation state. This reduced
shielding leads to a contraction of the atomic radius, ―the actinide contraction‖. For the
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