V - MSpace at the University of Manitoba

Under the ionic model MO diagram presented the 2�u MO would be considered
the most U-O bonding. The 2�u HOMO is much less stable, has less 2p character, and
has the least U-O bonding compared to the other three bonding orbitals. All three of these
features are thought to be due to the admixture of the U 6p(�u) semicore atomic orbital
[23].
One potential scenario for the make-up of the partial triple bond of the actinyl unit
is the following combination of three symmetry–allowed bond formations: 1.) O(2py) to
An(6dyz) with �g anti-bonding symmetry, 2.) O(2py) to An(5fyz2) with �u non-bonding
symmetry, and 3.) O(2pz) to An 5fz 3 with �u bonding symmetry [23] .
Orbitals in the equatorial plane, not utilized by the actinyl oxygens, typically are
viewed as acceptors to weak �–donor ligands. Such U ligand MOs as 6d(x 2 -y 2 ), 6d(xy),
5f(x 3 -3xy 2 ), and 5f(y 3 -3x 2 y), and to a smaller degree the 7s and 7p MOs are also thought
to play a role [23].
However, a DFT analysis of UO2(H2O)5 2+ [27] reveals that the UO2 2+ bonding
picture is of a much more covalent nature with atomic orbital occupations of 2s 1.94 2p 4.32
for oxygen and 5f 2.71 6d 1.26 7s 0.21 7p 0.16 for uranium. As such, modifications must be made
to the ionic model to account for this covalency. Also, once spin-orbit effects are properly
incorporated one can see why there has been, and continues to be, difficulty in finalizing
a more accurate MO diagram for uranyl and its actinyl counterparts.
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