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Covalency in AnCp(4) (An = Th-Cm): a comparison of molecular orbital, natural population and atoms-in-molecules analyses

Tassell, MJ; Kaltsoyannis, N; (2010) Covalency in AnCp(4) (An = Th-Cm): a comparison of molecular orbital, natural population and atoms-in-molecules analyses. DALTON T , 39 (29) 6719 - 6725. 10.1039/c000704h.

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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 = eta(5)-C5H5) and UCp4 is found to be pseudo-tetrahedral (S4), 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 angstrom 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 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 CmCp4, 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 ThCp4 to AmCp4. 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 re. ect 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), AnF(3)CO (An = U, Am), FeCp2, LaCp3, LaCl3 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.

Type:Article
Title:Covalency in AnCp(4) (An = Th-Cm): a comparison of molecular orbital, natural population and atoms-in-molecules analyses
DOI:10.1039/c000704h
Keywords:VALENCE PHOTOELECTRON-SPECTRUM, AB-INITIO PSEUDOPOTENTIALS, ANO BASIS-SETS, WAVE FUNCTIONS, ACTINIDE COMPLEXES, TRANSITION-ELEMENTS, ELECTRON-DENSITY, BOND, LCAO, PU
UCL classification:UCL > School of BEAMS > Faculty of Maths and Physical Sciences > Chemistry

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