Covalency in AnCp_4 (An = Th-Cm): A comparison of molecular orbital, natural population and atoms-in-molecules analyses

摘要

The geometric and electronic structures of the title compounds are calculated with scalar relativistic, gradient-corrected density functional theory. The most stable geometry of ThCp_4 (Cp = η~5- C_5H_5) and UCp_4 is found to be pseudo-tetrahedral (S_4), in agreement with experiment, and all the other AnCp_4 compounds have been studied in this point group. The metal-Cp centroid distances shorten by 0.06 ? from ThCp_4 to NpCp_4, in accord with the actinide contraction, but lengthen again from PuCp_4 to CmCp_4. Examination of the valence molecular orbital structures reveals that the highest-lying Cp π_(2,3)-based orbitals split into three groups of pseudo-e, t_2 and t_1 symmetry. Above these levels come the predominantly metal-based 5f orbitals, which stabilise across the actinide series, such that in CmCp_4, the 5f manifold is at more negative energy than the Cp π_(2,3)-based levels. The stability of the Cm 5f orbitals leads to an intramolecular ligand→metal charge transfer, generating a Cm(iii) f~7 centre and increased Cm-Cp centroid distance. Mulliken population analysis shows metal d orbital participation in the e and t_2 Cp π_(2,3)-based orbitals, which gradually decreases across the actinide series. By contrast, metal 5f character is found in the t_1 levels, and this contribution increases four-fold from ThCp_4 to AmCp_4. Examination of the t_1 orbitals suggests that this f orbital involvement arises from a coincidental energy match of metal and ligand orbitals, and does not reflect genuinely increased covalency (in the sense of appreciable overlap between metal and ligand levels). Atoms-in-molecules analysis of the electron densities of the title compounds (together with a series of reference compounds: C _2H_6, C_2H_4, Cp~-, M(CO)_6 (M = Cr, Mo, W), AnF_3CO (An = U, Am), FeCp _2, LaCp_3, LaCl_3 and AnCl_4 (An = Th, Cm)) indicates that the An-Cp bonding is very ionic, increasingly so as the actinide becomes heavier. Caution is urged when using early actinide/lanthanide comparisons as models for minor actinides/middle lanthanides.