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C2FeN7Sr6, orthorhombic, P21212 (no. 18), a = 9.938(2) Å, b = 14.717(3) Å, c = 3.856(1) Å, V = 564.0 Å, Z = 2, Rgt(F) = 0.037, wRref(F) = 0.057, T = 295 K. Source of material Black, needle shaped single crystals of (Sr6N)[FeN2][CN2]2 were obtained by reaction of pellets of a mixture of Sr and Sr2N (prepared from strontium metal (Alfa, 99.98 %) and nitrogen (Praxair, 99.999 %) at 973 K) together with Fe powder (Alfa, 99.99 %) and graphite powder (Chempur, 99.9 %) in the molar ratio Sr : Sr2N : Fe : C = 1 : 5 : 1 : 2. The reaction was carried out after addition of NaN3 (Roth, 99.9 %) as the nitrogen source in sealed iron ampoules at elevated pressure (40 80 bar) and a temperature of 1373 K for 48 h, followed by slow cooling (10 K / h) to ambient temperature. Starting materials and reaction products are sensitive to moisture and air. Discussion Nitridoferrates of lithium and alkaline-earth metals contain a variety of nitridoferrate anions with different oxidation states of Fe (I III). Up to now, nitridoferrates(I) have been observed containing 1D chains # [FeN 2 2 2 / . ] in the solid solution series Li Li Fe N 2 1 [( ) ] .x x [1] , CaLi Li Fe N 2 1 2 [( ) ] .x x [2] , SrLi Li Fe N 2 1 2 [( ) ] .x x [2], and Ca {Li Li Fe N 2 1 2 [( ) ]} .x x [3], respectively. Trigonal planar complexes [Fe N3] have been reported in the crystal structures of AE3[FeN3] (AE: Sr [4], Ba [5]) and (Ca3N)2[FeN3] [6]. Linear dumbbells [Fe N2] are observed in Li4[FeN2] [7], planar complexes [Fe2 N4] are present in Ca2[FeN2] [8]. Both types of complex anions together are found in the crystal structure of Sr2[FeN2] [8]. Nitridoferrates containing carbodiimide groups have not been reported up to now. (Sr6N)[FeN2][CN2]2 is isotypic to (Sr6N)[CoN2][CN2]2 [9]. The peculiar structural features of this nitridoferrate are nearlinear carbodiimide [N=C=N]2− ions (∠(N−C−N) = 172.7(7)°) besides novel linear nitridoferrate(I) complexes [FeN2] (∠(N− Fe−N) = 179.0(2)°). The nitride species N3− are octahedrally coordinated by strontium. The coordination spheres of the nitrido ligands of [FeN2] are completed by two Sr5 tetragonal pyramids. The interatomic distance d(Fe—N) = 1.871(5) Å is in good agreement with respective values in related nitridoferrates (Li4[(Fe N2)] [7]: d(Fe—N) = 1.858(8) Å; Sr2[Fe N2] [8]: d(Fe—N) = 1.85(1) Å; SrLi Li Fe N 2 1 2 [( ) ] .x x [2]: d(Fe—N) = 1.896 Å extrapolated to x = 1). The carbodiimide unit [N=C=N]2− contains symmetric double bonds and two nearly equivalent nitrogen species (d(C—N1) = 1.23(1) Å and d(C—N2) = 1.24(1) Å). The coordination spheres of both N positions are also completed by (distorted) Sr5 tetragonal pyramids. Charge balance in the crystal structure of (Sr6N)[Fe I N2][CN2]2 is accomplished by Sr ions. The average Sr—N distance d = 2.779 Å is in good agreement with respective values in related Sr compounds (e.g.: (Sr6N)[CoN2][CN2]2 [9], d = 2.77 Å; Sr3[FeN3] [4], d = 2.74 Å). Z. Kristallogr. NCS 224 (2009) 5-6 / DOI 10.1524/ncrs.2009.0004 5 © by Oldenbourg Wissenschaftsverlag, München Crystal: black needle, size 0.1 × 0.1 × 0.2 mm Wavelength: Mo K0 radiation (0.71073 Å) %: 293.81 cm−1 Diffractometer, scan mode: RIGAKU AFC7 Mercury CCD, )/2 2"max: 60.9° N(hkl)measured, N(hkl)unique: 4635, 1704 Criterion for Iobs, N(hkl)gt: Iobs > 2 !(Iobs), 1545 N(param)refined: 76 Programs: SHELXS-97 [10], SHELXL-97 [11], DIAMOND [12] Table 1. Data collection and handling.

Crystal structure of hexastrontium [dinitridoferrate(I)] bis(carbodiimide) mononitride, (Sr6N)[FeN2][CN2]2