Na 2 [B 18 Se 16 ]: The First 3D Polymeric Selenoborato‐ closo ‐dodecaborate

Systematic studies on selenoborates containing a B 12 cluster entity and alkali metal cations led to the new crystalline phase Na 2 [B 18 Se 16 ] which consists of a icosahedral B 12 cluster completely saturated with trigonal‐planar BSe 3 units and sodium counter‐ions. The cluster entities are connected to each other via seleno‐ and perselenobridges. Thus a three‐dimensional network results with [(B 18 Se 12 Se 6/2 ) n ] 2− ‐ and [(B 18 Se 15 Se 6/2 ) n ] 2− anions in a ratio of 2:1. The new chalcogenoborate was prepared in a solid state reaction from sodium selenide, amorphous boron and selenium in evacuated carbon coated silica tubes at a temperature of 850 °C. Na 2 [B 18 Se 16 ] crystallizes in the trigonal space group ${\rm P}{\bar 3}$ (no. 147) with a = 17.021(1) Å, c = 8.903(1) Å and Z = 3.