Comparative Experimental and Theoretical Study of the Fe L_2,3-Edges X‑ray Absorption Spectroscopy in Three Highly Popular, Low-Spin Organoiron Complexes: [Fe(CO)₅], [(η⁵‑C₅H₅)Fe(CO)(μ-CO)]₂, and [(η⁵‑C₅H₅)₂Fe]

The occupied and unoccupied electronic structures of three highly popular, closed shell organoiron complexes ([Fe­(CO)5], [(η5-C5H5)­Fe­(CO)­(μ-CO)]2, and [(η5-C5H5)2Fe]) have been theoretically investigated by taking advantage of density functional theory (DFT) calculations coupled to the isolobal analogy (Elian et al. Inorg. Chem. 1976, 15, 1148). The adopted approach allowed us to look into the relative role played by the ligand → Fe donation and the Fe → ligand back-donation in title molecules, as well as to investigate how CO- (terminal or bridging) and [(η5-C5H5)]−-based π* orbitals compete when these two ligands are simultaneously present as in [(η5-C5H5)­Fe­(CO)­(μ-CO)]2. Insights into the nature and the strength of the bonding between Fe and the C donor atoms have been gained by exploiting the Nalewajski–Mrozek bond multiplicity index (Nalewajski et al. Int. J. Quantum Chem. 1994, 51, 187), which have been found especially sensitive even to tiny bond distance variations. The bonding picture emerging from ground state DFT results proved fruitful to guide the assignment of original, high-resolution, gas-phase L2,3-edges X-ray absorption spectra of the title molecules, which have been modeled by the two-component relativistic time-dependent DFT including spin orbit coupling and correlation effects and taking advantage of the full use of symmetry. Assignments alternative to those reported in the literature for both [Fe­(CO)5] and [(η5-C5H5)2Fe] are herein proposed. Despite the high popularity of the investigated molecules, the complementary use of symmetry, orbital, and spectroscopy allowed us to further look into the metal–ligand symmetry-restricted-covalency and the differential-orbital covalency, which characterize them