Structural comparison of the HA cluster compounds used in this study. The HA clusters are shown as stereo ball-and-stick representations. (a) Hexasodium α-metatungstate (HMT). (b) Trisodium phosphotungstate (TPT). In these tungstate clusters, large blue spheres represent tungsten, medium-sized orange spheres represent phosphorus and small red spheres represent oxygen. (c) Hexatantalum tetradecabromide (TaB): large blue spheres represent tantalum, medium-sized brown spheres represent bromine.