Jump to main content
Jump to site search

Issue 29, 2010
Previous Article Next Article

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

Author affiliations

Abstract

The geometric and electronic structures of the title compounds are calculated with scalar relativistic, gradient-corrected density functional theory. The most stable geometry of ThCp4 (Cp = η5-C5H5) and UCp4 is found to be pseudo-tetrahedral (S4), in agreement with experiment, and all the other AnCp4 compounds have been studied in this point group. The metal–Cp centroid distances shorten by 0.06 Å from ThCp4 to NpCp4, in accord with the actinide contraction, but lengthen again from PuCp4 to CmCp4. Examination of the valence molecular orbital structures reveals that the highest-lying Cp π2,3-based orbitals split into three groups of pseudo-e, t2 and t1 symmetry. Above these levels come the predominantly metal-based 5f orbitals, which stabilise across the actinide series, such that in CmCp4, the 5f manifold is at more negative energy than the Cp π2,3-based levels. The stability of the Cm 5f orbitals leads to an intramolecular ligand→metal charge transfer, generating a Cm(III) f7 centre and increased CmCp centroid distance. Mulliken population analysis shows metal d orbital participation in the e and t2 Cp π2,3-based orbitals, which gradually decreases across the actinide series. By contrast, metal 5f character is found in the t1 levels, and this contribution increases four-fold from ThCp4 to AmCp4. Examination of the t1 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: C2H6, C2H4, Cp, M(CO)6 (M = Cr, Mo, W), AnF3CO (An = U, Am), FeCp2, LaCp3, LaCl3 and AnCl4 (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.

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

Back to tab navigation
Please wait while Download options loads

Publication details

The article was received on 12 Jan 2010, accepted on 05 Mar 2010 and first published on 25 Mar 2010


Article type: Paper
DOI: 10.1039/C000704H
Citation: Dalton Trans., 2010,39, 6719-6725
  •   Request permissions

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

    M. J. Tassell and N. Kaltsoyannis, Dalton Trans., 2010, 39, 6719
    DOI: 10.1039/C000704H

Search articles by author