Comparative Experimental and Theoretical Study of the C and O K-Edge X-ray Absorption Spectroscopy in Three Highly Popular, Low Spin Organoiron Complexes: [Fe(CO)(5)], [(eta(5)-C5H5)Fe(CO)(mu-CO)](2), and [(eta(5)-C5H5)(2)Fe]

The unoccupied electronic structures of three closed-shell, highly popular organoiron complexes ([Fe(CO)(5)], [(eta(5)-C5H5)Fe(CO)(mu-CO)](2), and [(eta(5)-C5H5)(2)Fe]; 0, I, and II, respectively) have been investigated both experimentally and theoretically by combining original gas-phase X-ray absorption spectroscopy (XAS) outcomes recorded at the C and O K-edge with results of scalar relativistic time-dependent density functional calculations carried out within the zeroth order regular approximation. Experimental evidence herein discussed complement the Fe L-2,L-3-edges XAS ones we recently recorded, modeled, and assigned for the same complexes (Carlotto et al. Inorg. Chem. 2019, 58, 5844). The first-principle simulation of the C and O K-edge features allowed us to univocally identify the electronic states associated to the ligand-to-metal charge transfer (LMCT) transitions both in I and in II. At variance to that, LMCT transitions with sizable oscillator strengths do not play any role in determining neither the C nor the O K-edge spectral pattern of 0. The higher pi-acceptor capability of the CO ligand, regardless of its terminal or bridging coordination, with respect to [(eta(5)-C5H5)](-) is herein ultimately confirmed.