This transition is also observed in the homoleptic structure (394.0 nm) where, due to the symmetry of the molecule, a lower change in the dipole moments is produced with respect to the carbonate-hydroxide complex. The substitution of one carbonate ligand by two hydroxyl ligands affects directly the transition dipole moment, resulting in an increase of the transition probability from 0.231 to 0.263 a.u. for the carbonate and carbonate-hydroxide complexes, respectively. This could explain the shoulder observed in the spectrum of the heteroleptic complex (Fig.2).

 Multiconfigurational calculations

The Bk(IV) free ion, considering a scalar relativistic limit and Hund’s rule, has an 8S ground state (L=0, S = 7/2) followed by a first excited multiplet 6P ~ 26500 cm-1 higher in energy. The spin-orbit coupling (SOC) removes the state degeneracy resulting in a series of multiplets characterized by the total angular momentum quantum number (J), with the ground term being (in a Russell-Saunders notation) 8S7/2. If the ion is now placed in a ligand environment, the effect of the ligand field produces the split of the 2J+1 degenerate state and the mixing of states with different mJ values.  This picture, based on a single ion structure, can be used in molecular systems where mixing between metal and ligand is negligible.