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)

Abstract

The charge density in the tri-iron methoxymethylidyne cluster Fe₃(μ-H)(μ-COMe)(CO)₁₀(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)₄ unit and the Fe₂(μ-H)(μ-COMe)(CO)₆ 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)₄ 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_alkylidyne 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 μ₂ to a semi-μ₃ coordination mode has been explored by DFT calculations on 1 and the model complexes M₃(μ-H)(μ-CH)(CO)₁₀ (M = Fe, 2; Ru, 3; and Os, 4). These calculations confirm a semi-μ₃ 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_alkylidyne interactions are important in the cluster bonding. The PES between μ₂ and semi-μ₃ 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 ^∇2ρ(r_b) in the methoxymethylidyne ligand is consistent with a partial π-bond character of the C−O bond, associated with an sp² hybridization for these atoms.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• 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: 1089-5639
• ISSN (Online): 1520-5215