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Spectroscopic X-ray and Mössbauer Characterization of M6 and M5 Iron(Molybdenum)-Carbonyl Carbide Clusters: High Carbide-Iron Covalency Enhances Local Iron Site Electron Density Despite Cluster Oxidation

“…Two-electron oxidations of clusters containing [Fe 4 C] 2– , [Fe 5 C] 2– , and [Fe 6 C] 2– cores have shown decreases in quadrupole doublet isomer shift values by 0.063, 0.03, and 0.04 mm/s, respectively. 15,28,50 These directional shifts are in line with the canonical understanding of oxidations as observed in Mössbauer spectroscopy despite the complexes having significant CO coordination environments. Infrared spectra are only reported for the latter complexes ([Fe 6 C] 2– → [Fe 6 C] 0 and [Fe 5 C] 2– → [Fe 5 C] 0 ), but indicate respective ∼55 cm –1 and ∼65 cm –1 blue-shifts in terminal CO stretching frequencies upon oxidation of the clusters.…”

“…The observed isomer shift for 2 , however, resides at a significantly higher value than that previously reported values of δ = 0.109 and 0.055 mm/s. 15,49,50 This deviation is partially attributed to either the broadened, unresolved spectra or Mössbauer data collection at room temperature. 15,49,50 Nevertheless, such an extreme (+0.38 mm/s) shift corresponding to a seemingly negligible formal oxidation state change ( 1 : Fe 0.33 → 2 : Fe 0.4 ) is curious and merits further discussion.…”

“…15,49,50 This deviation is partially attributed to either the broadened, unresolved spectra or Mössbauer data collection at room temperature. 15,49,50 Nevertheless, such an extreme (+0.38 mm/s) shift corresponding to a seemingly negligible formal oxidation state change ( 1 : Fe 0.33 → 2 : Fe 0.4 ) is curious and merits further discussion. Because of the dramatic change in the zero-field Mössbauer data for 2 compared to 1 and 3 , we probed for possible temperature-dependent morphological changes in 2 via a set of variable-temperature powder XRD experiments.…”

“…Of particular interest are those incorporating modified cage-like core structures, in which 4–6 metal atoms reside at the vertices of an octahedron surrounding a central light atom. Most notable are the 5 and 6 atom clusters of iron, in square pyramidal and octahedral geometries, respectively, which allow for almost modular substitution with Rh and Co atoms. ,− Recently, Rose and co-workers reported the crystal structure of a substituted six Fe octahedron, in which one axial iron is replaced with molybdenum. − Additionally, Rauchfuss et al recently published the first report of a synthetic multinuclear iron-carbonyl cluster containing both a totally inorganic carbide and sulfide ligand …”

Identification of a Single Light Atom within a Multinuclear Metal Cluster Using Valence-to-Core X-ray Emission Spectroscopy

“…Despite the differences in coordination, the iron environments are similar, as shown by Mössbauer spectroscopy. 37 …”