Issue 20, 2022

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


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.

Graphical abstract: Non-innocent cyanido ligands: tetracyanidoferrate(–ii) as carbonyl copycat

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Article information

Article type
17 Mar 2022
31 Mar 2022
First published
06 Apr 2022
This article is Open Access
Creative Commons BY license

Dalton Trans., 2022,51, 7811-7816

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

F. Jach, T. Block, Y. Prots, M. Schmidt, M. Bobnar, R. Pöttgen, M. Ruck and P. Höhn, Dalton Trans., 2022, 51, 7811 DOI: 10.1039/D2DT00833E

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