The role of the ligand field was also evaluated using the Condon-Slater
parameters for interelectronic repulsion at CAS(7,7)SCF level though the
so-called ab initio ligand field theory (AILFT). As was pointed out
recently by Neese et al. , NEVPT2 results are not relevant to the
bonding analysis and frequently introduce large
errors.6 As shown in Table 3 for both molecules, the
ratio Fk/Fkfree-ion(k =2, 4, 6) < 1 with variations over 3%, which implies a
nephelauxetic effect responsible for the reduction of the
electron-electron repulsion due to the strong interaction of the 5f
orbitals with the carbonate ligands. LFDFT yields similar parameters for
the free-ion (Table 3), however they differ significantly when the
molecular structure is considered, i.e. Fk and ζ are
reduced compared to AILFT ; with reductions ca. 44% with respect
to the free-ion. This shows a more effective penetration of the ligand’s
electron cloud into that of the metal compared to
AILFT6. The difference could reside in the localized
description of f-electrons under HF and post-HF methods compared to the
well-known overdelocalization of DFT; though, this could be argued
against when using hybrid functionals as in this case. Carnall and
coworkers have obtained these parameters from BkF4experimental spectra.10,11 These parameters are larger
than those obtained by LFDFT, but in agreement with the nephelauxetic
series. This could prove the effect of covalency on the reduction of the
electrostatic repulsion in actinides.
The analysis of the optimized molecular orbitals shows an important
bonding interaction between 5f and ligands orbitals. The active behavior
of the ligand orbitals was corroborated through analysis of the
occupation numbers after a careful inclusion of the symmetry adapted
orbitals.
Table 3. One-electron Slater-Condon Fk (k =
2, 4, 6) and effective spin-orbit coupling parameters