Metal−metal and metal−ligand bonding at a QTAIM catastrophe: a combined experimental and theoretical charge density study on the alkylidyne cluster Fe3(μ-H)(μ-COMe)(CO)10

Farrugia, L.J. and Senn, H.M. (2010) Metal−metal and metal−ligand bonding at a QTAIM catastrophe: a combined experimental and theoretical charge density study on the alkylidyne cluster Fe3(μ-H)(μ-COMe)(CO)10. Journal of Physical Chemistry A, 114(51), pp. 13418-13433. (doi: 10.1021/jp1098624)

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The charge density in the tri-iron methoxymethylidyne cluster Fe<sub>3</sub>(μ-H)(μ-COMe)(CO)<sub>10</sub>(1) has been studied experimentally at 100 K and by DFT calculations on the isolated molecule using the Quantum Theory of Atoms in Molecules (QTAIM). The COMe ligand acts as a nearly symmetric bridge toward two of the Fe atoms (Fe−C = 1.8554(4), 1.8608(4) Å) but with a much longer interaction to the third Fe atom, Fe−C = 2.6762(4) Å. Complex 1 provides a classic example where topological QTAIM catastrophes render an exact structure description ambiguous. While all experimental and theoretical studies agree in finding no direct metal−metal interaction for the doubly bridged Fe−Fe vector, the chemical bonding between the Fe(CO)<sub>4</sub> unit and the Fe<sub>2</sub>(μ-H)(μ-COMe)(CO)<sub>6</sub> moiety in terms of conventional QTAIM descriptors is much less clear. Bond paths implying direct Fe−Fe interactions and a weak interaction between the COMe ligand and the Fe(CO)<sub>4</sub> center are observed, depending on the experimental or theoretical density model examined. Theoretical studies using the Electron Localizability Indicator (ELI-D) suggest the metal−metal bonding is more significant, while the delocalization indices imply that both Fe−Fe bonding and Fe···C<sub>alkylidyne</sub> bonding are equally important. The source functions at various interfragment reference points are similar and highly delocalized. The potential-energy surface (PES) for the migration of the alkylidyne group from a μ<sub>2</sub> to a semi-μ<sub>3</sub> coordination mode has been explored by DFT calculations on 1 and the model complexes M<sub>3</sub>(μ-H)(μ-CH)(CO)<sub>10</sub> (M = Fe, 2; Ru, 3; and Os, 4). These calculations confirm a semi-μ<sub>3</sub> bridging mode for the alkylidyne ligand as the minimum-energy geometry for compounds 2−4 and demonstrate that, for 1, both Fe−Fe and Fe···C<sub>alkylidyne</sub> interactions are important in the cluster bonding. The PES between μ<sub>2</sub> and semi-μ<sub>3</sub> alkylidyne coordination for 1 is extremely soft, and the interconversion between several topological isomers is predicted to occur with almost no energy cost. Analysis of the density ρ(r) and the Laplacian of the density <sup>∇2</sup>ρ(r<sub>b</sub>) in the methoxymethylidyne ligand is consistent with a partial π-bond character of the C−O bond, associated with an sp<sup>2</sup> hybridization for these atoms.

Item Type:Articles
Glasgow Author(s) Enlighten ID:Farrugia, Dr Louis and Senn, Dr Hans
Authors: Farrugia, L.J., and Senn, H.M.
College/School:College of Science and Engineering > School of Chemistry
Journal Name:Journal of Physical Chemistry A
Publisher:American Chemical Society
ISSN (Online):1520-5215

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