Abstract
Abstract Exceptionally electron-rich, nearly trigonal-planar tricyanidometalate anions Fe(CN)37− and Ru(CN)37− were stabilized in LiSr3Fe(CN)3 and AE3.5M(CN)3 (AE=Sr, Ba; M=Fe, Ru). They are the first examples of group 8 elements with the oxidation state of −IV. Microcrystalline powders were obtained by a solid-state route, single crystals from alkali metal flux. While LiSr3Fe(CN)3 crystallizes in P63/m, the polar space group P63 with three-fold cell volume for AE3.5M(CN)3 is confirmed by second harmonic generation. X-ray diffraction, IR and Raman spectroscopy reveal longer C−N distances (124–128 pm) and much lower stretching frequencies (1484–1634 cm−1) than in classical cyanidometalates. Weak C−N bonds in combination with strong M−C π-bonding is a scheme also known for carbonylmetalates. Instead of the formal notation Fe−IV(CN−)37−, quantum chemical calculations reveal non-innocent intermediate-valent CN1.67− ligands and a closed-shell d10 configuration for Fe, that is, Fe2−.
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