Four new Th(IV), U(IV), and Np(IV) hexanuclear clusters with 1,2-phenylenediphosphonate as the bridging ligand have been prepared by self-assembly at room temperature. The structures of Th6Tl3[C6H4(PO3)-(PO3H)](6)(NO3)(7)(H2O)(6)center dot(NO3)(2)center dot 4H(2)O (Th6-3), (NH4)(8.11)-Np12Rb3.89[C6H4(PO3)(PO3H)](12)(NO3)(24)center dot 15H(2)O (Np6-1), (NH4)(4)U12Cs8[C6H4(PO3)(PO3H)](12)(NO3)(24)center dot 18H(2)O (U6-1), and (NH4)(4)U12Cs2 [C6H4(PO3)(PO3H)](12)(NO3)(18)center dot 40H(2)O (U6-2) are described and compared with other clusters of containing An(IV) or Ce(IV). All of the clusters share the common formula M-6(H2O)(m)[C6H3(PO3)(PO3H)](6)(NO3)(n)((6-n)) (M = Ce, Th, U, Np, Pu). The metal centers are normally nine-coordinate, with five oxygen atoms from the ligand and an additional four either occupied by NO3- or H2O. It was found that the Ce, U, and Pu clusters favor both C-3i and C-i point groups, while Th only yields in C-i, and Np only C-3i. In the C-3i clusters, there are two NO3- anions bonded to the metal centers. In the C-i clusters, the number of NO3- anions varies from 0 to 2. The change in the ionic radius of the actinide ions tunes the cavity size of the clusters. The thorium clusters were found to accept larger ions including Cs+ and Tl+, whereas with uranium and later elements, only NH4+ and/or Rb+ reside in the center of the clusters.