Exploring Interatomic Electron Transfer and Metal–Ligand Binding Mechanism in Trimethylenemethane Iron Tricarbonyl: Insights from Potentials per Electron and Corresponding Force Density Fields

This study employs an analysis of the per-electron potentials and the superposition of the electrostatic and kinetic force fields, F_es(r) and F_k(r), and the gradients of the potential energy and one-electron densities to investigate the binding mechanism in trimethylenemethane iron tricarbonyl complex (TMM)Fe(CO)₃. Our approach permits the delineation of the “ligand-binding” force field generated by the metal nucleus but partially operating within the ligand atoms. A mechanical rationale for metal–ligand interactions is thus presented: In the corresponding area, the attractive force F_es(r) provides the backdrop against which the homotropic static force $F$(r) and the heterotropic kinetic force F_k(r) exert attractive and repulsive influences, respectively, toward the metal nucleus on a portion of the electrons belonging to the ligand atoms. This area thus represents electron sharing, which emerges as a quantum chemical response against the metal-to-ligand electron transfer. It has been demonstrated that the response is facilitated by the decreased potential energy density in the vicinity of the interatomic surface. Our findings indicate that the polar coordination bonds in (TMM)Fe(CO)₃ exhibit notable quantum chemical responses. However, the previously described nonbonded contact also features an unexpectedly pronounced response, despite the absence of a bond path. It can be proposed that the unforeseen response is a consequence of the formation of the 18-electron, closed valence shell, rather than an indication of the establishment of an organometallic chemical bond.