Geometric control of cage architecture; observation of ligand-selective behaviour in the structures of [{As-2(NCy)(4)}(2)M-4] (M = Na, Cu; Cy = C6H11)

The structure of [{As-2(NCy)(4)}(2)Cu-4] reveals a dramatic change in the m etal coordination mode compared to that found in the Sb analogue [{Sb-2(NCy )(4)}(2)Cu-4], resulting in the distortion of the Cu-4 core from a square-p lanar to a butterfly shape and providing the first illustration of ligand-s elective cage modification in such heterobimetallic species; in contrast th e square-planar Na-4 arrangement found in [{Sb-2(NCy)(4)}(2)M-4] (M = Na, C u, Ag) is retained in [{As-2(NCY)(4)}(2)Na-4].