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

The charge density in the tri-iron methoxymethylidyne cluster Fe-3(mu-H)(mu-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) angstrom) but with a much longer interaction to the third Fe atom, Fe-C = 2.6762(4) angstrom. 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 Fe-2(mu-H)(mu-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 center dot center dot center dot 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 mu(2) to a semi-mu(3) coordination mode has been explored by DFT calculations on 1 and the model complexes M-3(mu-H)(mu-CH)(CO)(10) (M = Fe, 2; Ru, 3; and Os, 4). These calculations confirm a semi-mu(3) 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 center dot center dot center dot C-alkylidyne interactions are important in the cluster bonding. The PES between mu(2) and semi-mu(3) 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 rho(r) and the Laplacian of the density del(2)rho(r(b)) in the methoxymethylidyne ligand is consistent with a partial pi-bond character of the C-O bond, associated with an sp(2) hybridization for these atoms.