[Co 2 @(Ge 17 Ni)] 4- : the first edge-sharing double-cage endohedral germanide.

We report here a new double-cage endohedral Ge cluster, [Co 2 @(Ge 17 Ni)] 4- , fused through two [Co@(Ge 9 Ni)] moieties with a shared Ni-Ge edge. This ternary Co-Ge-Ni species not only represents the first double-cage example of an 18-vertex Zintl cluster, but also fills in the missing link of the edge fusion model in the double-cage systems.