LiSr2[ReN4] and LiBa2[ReN4] - isotypic nitridorhenates(VII)

The quaternary nitridorhenates(VII) LiAE(2)[ReN4] (AE = Sr, Ba) were synthe sized by reaction of the metals with molecular nitrogen at 850-900 degreesC . The plate-like, nearly colourless crystals were investigated by X-ray sin gle crystal methods and were identified as isotypic phases: LiSr2[ReN4] (Li Ba2[ReN4]); monoclinic, P2(1)/m; a = 614.64(8) pm (651.04(12) pm), b = 585. 97(6) pm (b = 598.86(9) pm), c = 689.70(17) pm (737.43(5) pm), beta = 106.3 75(4)degrees (108.535(2)degrees); Z = 2. Crystals of the strontium compound were systematically twinned along [001]. In the crystal structures of the quaternary compounds the alkaline earth- and nitride- ions are arranged in the motif of the InNi2-type structure. Strontium and barium are in a trigon al prismatic coordination by nitrogen (Sr-N: 261.0(7)-284.3(4) pm; Ba-N: 27 8.0(7)-303.0(6) pm). One half of the tetrahedral voids within the partial s tructure formed by stacking of trigonal prismatic rod layers is occupied by rhenium (formation of [(ReN4)-N-VII](5-)-tetrahedra; Re-N: 181.0(6)-184.5( 8) pm), lithium takes the positions of the remaining tetrahedral sites (Li- N: 2 x 198(1) pm, 224(2) pm and 228(2) pm for the strontium phase). In the barium compound the lithium positions show a larger shift from the tetrahed ral centres towards a tetrahedral plane (Li-N: 2 x 195(1) pm, 213(2) pm and 304(2) pm).