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the ground state electron configurations, which generally leads to a variety of
computationally difficult to model, closely spaced, open-shell states.
1.3.2 Electronic Strucure of Actinyls
The electronic structure of the actinyl unit AnO2 2+ is closely tied to the orbitals of
the actinide atoms. If we consider the fully ionic picture, the actinyl cation is treated as
An 6+ and the two oxygen anions as O 2– . In this picture the oxygen atoms adopt a closed-
shell configuration 2s 2 2p 6 of an inert gas while uranium adopts closed-shell 5f 0 state
(Figure 1–2).
As for neptunium and plutonium they adopt open-shell 5f 1 and 5f 2 states (1�u or
1�u), with one and two unpaired electron configurations, respectively.
There has been much disagreement over the ordering of the highest filled orbitals
of UO2 2+ —examples of alternate orderings include [23, 25, 26]—even so, most
calculations predict 2�u as the highest occupied molecular orbital (HOMO), as shown in
Figure 1–2. Also, since U 6+ is an f 0 ion, the highest filled MOs of UO2 2+ are thought to be
mostly O(2p 6 ) in character. Because of the cylindrical symmetry of the linear UO2 2+ unit,
the allowed U(5f) (ungerade) and U(6d) (gerade) interactions are mutually exclusive.
Thus only the 5f (�u, �u) MOs have the correct symmetry to interact with the 3�u, 3�u
MOs and in turn only 6d (�g, �g) MOs can interact with the 2�g, 2�g MOs. Therefore,
even though the 12 valence electrons all originate from the two oxygen atoms, both the
6d and 5f actinyl orbitals are thought to have significant participation in forming what is
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