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.