Covalency in AnCp4 (An = Th-Cm): a comparison of molecular orbital, natural population and atoms-in-molecules analyses.

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 = eta(5)-C(5)H(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 A 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 pi(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 pi(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 pi(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(2)H(6), C(2)H(4), Cp(-), M(CO)(6) (M = Cr, Mo, W), AnF(3)CO (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.