Jach Franziska, Block Theresa, Prots Yurii, Schmidt Marcus, Bobnar Matej, Pöttgen Rainer, Ruck Michael, Höhn Peter
Max-Planck-Institute for Chemical Physics of Solids, Nöthnitzer Straße 40, 01187 Dresden, Germany.
Faculty of Chemistry and Food Chemistry, Technische Universität Dresden, 01062 Dresden, Germany.
Dalton Trans. 2022 May 24;51(20):7811-7816. doi: 10.1039/d2dt00833e.
While a negative oxidation state occurs rarely for metals in general, this is commonly known for metal carbonyl anions, carbonyl metalates. Although CO and CN are isoelectronic, cyanidometalates usually do not exhibit metal centers with negative oxidation states. However, we report on the electron-rich tetrahedral tetracyanidoferrate(-II) anion [Fe(CN)], which was stabilized in (SrN)[Fe(CN)] (space group 3, = 702.12(2) pm, = 4155.5(2) pm). Microcrystalline powders were synthesized by a solid-state route, single crystals were obtained from Na metal flux. In comparison to classical cyanidometalates, C-N distances are longer and stretching frequencies are lower as indicated by X-ray diffraction, IR and Raman spectroscopy. Weak C-N, strong Fe-C bonds as well as the anion geometry resemble the isoelectronic tetrahedral carbonyl ferrate [Fe(CO)]. Fe Mössbauer spectroscopic measurements reveal a negative isomer shift in agreement with substantially delocalized d electrons due to strong π back-bonding. These results point to a very similar bonding situation of both 18 tetracyanido and tetracarbonyl ferrates including non-innocent redox-active ligands and a d closed shell configuration on iron. Hereby, new tetracyanidoferrate(-II) provides a missing link for a more in-depth understanding of the chemical bonding trends of highly-reduced cyanidometalates in the quest for even higher reduced transition metals in this exceptional class of compounds.
