Tricyanidoferrates(−IV) and Ruthenates(−IV) with Non‐Innocent Cyanido Ligands

Exceptionally electron‐rich, nearly trigonal‐planar tricyanidometalate anions [Fe(CN)(3)](7−) and [Ru(CN)(3)](7−) were stabilized in LiSr(3)[Fe(CN)(3)] and AE(3.5)[M(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 LiSr(3)[Fe(CN)(3)] crystallizes in P6(3)/m, the polar space group P6(3) with three‐fold cell volume for AE(3.5)[M(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(−))(3)](7−), quantum chemical calculations reveal non‐innocent intermediate‐valent CN(1.67−) ligands and a closed‐shell d(10) configuration for Fe, that is, Fe(2−).