Pr(16)Mo(21)O(56).

The structure of hexa-deca-praseodymium henicosa-molybden-um hexa-penta-contaoxide, Pr(16)Mo(21)O(56), is isotypic with other rare earth representatives of formula type RE(16)Mo(21)O(56) (RE = La, Ce, Nd). It is characterized by Mo(10)O(18) (i)O(8) (a) units (where i = inner and a = apical O atoms) containing biocta-hedral Mo(10) clusters and octa-hedral MoO(6) units that share some of their O atoms to form the Mo-O framework. The two independent Mo(10) cluster units are centred at Wyckoff positions 2b and 2c and have point-group symmetry [Formula: see text]. The Mo atom of the MoO(6) unit is likewise situated at an inversion centre (2d). The eight crystallographically different Pr(3+) cations occupy irregular voids in the framework with coordination numbers to the O atoms varying between 8 and 11.