In polyoxometalate chemistry a large variety of compounds, dusters and solid-state structures can be formed by the linking together of well-defined metal-oxygen building blocks(1,2). These species exhibit unusual topological and electronic properties and find applications ranging from medicine(3) to industrial processes(4). The recently reported ring-shaped mixed-valence polyoxomolybdates of the type {Mo-154} (refs 5, 6) and {Mo-176} (refs 7, 8) represent a new class of giant clusters with nanometre-sized cavities and interesting properties for host-guest chemistry. Here we describe the formation of related clusters of the type {Mo-248} formed by addition of further units to the inner surface of the {Mo-176} ＇wheel＇. The additional units arrange themselves into two {Mo-36} ＇hub-caps＇ on the initial wheel-clusters that are not stable in isolation. These findings reveal a new pathway to the development of complex coordination clusters.