Electronic Structure of the Hieber Anion [Fe(CO)₃(NO)]⁻ Revisited by X-ray Emission and Absorption Spectroscopy

Abstract

While the Hieber anion [Fe­(CO)3(NO)]− has been reincarnated in the last years as an active catalyst in organic synthesis, there is still a debate about the oxidation state of the central Fe atom and the resulting charge of the NO ligand. To shed new light on this question and to understand the Fe–NO interaction in the Hieber anion, it is investigated in comparison to the formal 3d8 reference Fe­(CO)5 and the formal 3d10 reference [Fe­(CO)4]2– by the combination of valence-to-core X-ray emission spectroscopy (VtC-XES), X-ray absorption near-edge structure spectroscopy (XANES), and high-energy-resolution fluorescence-detected XANES. In order to extract information about the electronic structure, time-dependent density functional theory and ground-state density functional theory calculations are applied. This combination of experimental and computational methods reveals that the electron density at the Fe center of the Hieber resembles that of the isoelectronic [Fe­(CO)4]2–. These observations challenge recent descriptions of the Hieber anion and reopen the debate about the experimentally and computationally determined Fe oxidation state and charge on the NO ligand.

Exploring the Limits of Dative Boratrane Bonding: Iron as a Strong Lewis Base in Low-Valent Non-Heme Iron-Nitrosyl Complexes

“…In comparison, the only other known, stable ls-{FeNO} 10 complex is Hieber's anion, [Fe(CO) 3 (NO)] − . [64][65][66] In this case, the three strongly π-backbonding CO ligands take on the role of the boron Lewis acid, and allow for the stabilization of the highly reduced iron center in this unusual compound.…”

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

“…This work is summarized in refs − . Another iconic compound obtained in these studies is Hieber’s anion, [Fe­(CO) 3 (NO)] − , whose electronic structure is still an unsettled area of research. − This tetrahedral complex is diamagnetic and has Fe–NO and N–O bond lengths of 1.659 and 1.212 Å, respectively. The Fe–N–O angle is observed at 180°.…”

Probing Physical Oxidation State by Resonant X‐ray Emission Spectroscopy: Applications to Iron Model Complexes and Nitrogenase

“…[1][2][3][4][5][6][7][8][9][10][11] However,asother factors,including ligand identity, coordination number, and metal spin state,c ontribute to the rising edge position, [10,[12][13][14][15][16][17] caution must be exercised in using the metal K-edge energies as ag eneralized measure of oxidation state.This observation has led to significant debate in the literature as to how edges can be quantitatively interpreted. [18][19][20] Metal L-edge XAS (2p!3d) [21][22][23][24][25][26] can also provide covalencya nd metal oxidation state information, but experimental intensity and covalencyc an only be correlated through computational studies.T hese correlations may be further biased by the computational protocol or individual interpretation. [19,27] In this regard, very similar Cu L-edge data of formal Cu III complexes have been used to both support [27] and dismiss [19] aC u III physical oxidation state assignment.…”