Filled trivacant icosahedra as building fragments in 17-atom endohedral germanides [TM2@Ge17]n- (TM = Co, Ni).

The syntheses and the characterization of two 17-atom endohedral Ge clusters, [Co2@Ge17]6- (1a) and [Ni2@Ge17]4- (2a), are reported. The anions 1a and 2a, which close the gap between the known 16- and 18-atom Ge clusters, are investigated by single crystal X-ray diffraction and by quantum chemical calculations. The structures mark a new example on the pathway for cluster growth towards larger clusters with icosahedral symmetry. Furthermore, the [Co@Ge10]3- anion (3a) is obtained from liquid ammonia.