Comprehensive Re-examination of Intermetallic Charge Density for the Metal Carbonyl Dimers Mn2(CO)10 and [(η5‑C5H5)Fe(CO)2]2

In this study, we have employed high-resolution charge density (HRCD) techniques using X-ray diffraction data collected at 10 K to reanalyze the intermetallic electron density topologies of two representative metal carbonyl dimers, Mn2(CO)10 and [(η5-C5H5)Fe(CO)2]2 (Fp2)–both complexes widely used in organometallic reactions and textbook discussions of metal–metal interactions. A combination of traditional bond critical point indicators like Laplacian of electron density and metrics like the delocalization index offered insight into these metal–metal interactions and reconciled some irregularities in previous studies. Regarding the controversial question of Fe–Fe bonding in Fp2, our experimental HRCD data aligns with previous reports that the intermetallic region lacks a Fe–Fe bond critical point and, instead, features a ring critical point within the Fe2(μ-CO)2 core. While no formal Fe–Fe bond is present in Fp2 (according to the strict definition of the atoms-in-molecules theory), our theoretical analysis of the Fp2 isomer without bridging carbonyls indicates that Fe–Fe elongation coincides with the onset of a Fe–Fe bond path with characteristics akin to Mn2(CO)10. Thus, the Fe–Fe bonding in Fp2 is likely dynamic in solution, oscillating between multicenter bonding in the Fe2(μ-CO)2 core in the ground state and localized Fe–Fe bonding in the unbridged intermediate state.