LiBa₄[Mo₂N₇] und LiBa₄[W₂N₇]: Neue Nitridomolybdate/wolframate mit dimeren Tetraeder-Anionen

Black crystals of the isotypic phases LiBa4[Mo2N7] and LiBa4[W2N7] (monoclinic, P2/n; a = 1552.2(3)/1555.2(4), b = 642.2(1)/644.4(1), c = 1039.9(2)/1042.4(2) pm; beta = 99.23(2)/99.30(3)degrees; Z = 4) were grown from the melt (Li, Ba3N2, Mo/W; molar ratio 20:5:1; T-max = 1323 K; reaction with N-2, 1 atm, 18 h) by cooling (20 K/h). The crystal structures contain two independent dimeric units [Mo2N7](9-)/[W2N7](9-) (staggered conformation; Mo/W-N-Mo/W: 173.6/173.7 degrees and 180 degrees/180 degrees; Mo/W-N-(term): 180(2)-185(1)/181(3)-185(1) pm; Mo/W-N-(bridge): 195(1)/194(1) pm and 193(1)/194(1) pm)