These orbitals were selected based on their bonding or antibonding
character with respect to the 5f shell (Fig. 4). As shown, the
occupation numbers show a dependence with the size of the active space.
For both molecules, the minimal space given is f = 7, but when ligand
orbitals are added to the 5f shell, occupation change to 7.32 and 7.59
for 1 and 2 respectively with CAS(13,10). This change
implies the sharing of electrons between the ligand and the metal,
allowing one to characterize the ligand-metal interaction partially
covalent. It is important to note that the occupation numbers presented
in Fig. 4 were obtained from an SCF calculation which, as known, has a
localization error and therefore the prediction of the covalent
character is underestimated. This means that other tools are necessary
to obtain a better understanding of the nature of the bond and whether
the electron density from the ligand is being localized into the metal
or shared as an open-shell interaction.