Na2[B18Se16]: The First 3D Polymeric Selenoborato‐closo‐dodecaborate

Systematic studies on selenoborates containing a B12cluster entity and alkali metal cations led to the new crystalline phase Na2[B18Se16] which consists of a icosahedral B12cluster completely saturated with trigonal‐planar BSe3units and sodium counter‐ions. The cluster entities are connected to each other via seleno‐ and perselenobridges. Thus a three‐dimensional network results with [(B18Se12Se6/2)n]2−‐ and [(B18Se15Se6/2)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. Na2[B18Se16] 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.