Non-innocent cyanido ligands: tetracyanidoferrate(–II) as carbonyl copycat

Research output: Contribution to journalResearch articleContributedpeer-review


  • Franziska Jach - , Chair of Inorganic Chemistry II (Author)
  • Theresa Block - , University of Münster (Author)
  • Yurii Prots - , Max Planck Institute for Chemical Physics of Solids (Author)
  • Marcus Schmidt - , Max Planck Institute for Chemical Physics of Solids (Author)
  • Matej Bobnar - , Max Planck Institute for Chemical Physics of Solids (Author)
  • Rainer Pöttgen - , University of Münster (Author)
  • Michael Ruck - , Chair of Inorganic Chemistry II (Author)
  • Peter Höhn - , Max Planck Institute for Chemical Physics of Solids (Author)


While a negative oxidation state occurs rarely for metals in general, this is commonly known for metal carbonyl anions, i.e. 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)4]6−, which was stabilized in (Sr3N)2[Fe(CN)4] (space group R3c, a = 702.12(2) pm, c = 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)4]2− 57Fe 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 18e tetracyanido and tetracarbonyl ferrates including non-innocent redox-active ligands and a d10 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.


Original languageEnglish
Pages (from-to)7811-7816
JournalDalton Transactions
Issue number20
Publication statusPublished - 6 Apr 2022

External IDs

Scopus 85129750404
WOS 000782947600001
ORCID /0000-0002-2391-6025/work/142250226


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