Composition space diagrams can be used to study the stability and variety of mixed transition metal oxide fluorides prepared under hydro(solvato)thermal conditions. Prior to this work the neutral (CuF(NC5H5)(4))(2)NbOF5 cluster had not been observed but it was expected because of the nucleophilic character of the oxygen and trans fluorine in the [NbOF5](2-) species, [H3O](2)[(CuF(NC5H5)(4))(2)NbOF5][F](2), a compound containing the 2:1 (Cu: Nb) cluster, was synthesized from the concentration variables, [CuO . 0.5 Nb2O5], [(HF), pyridine], and [H2O], by adding a fourth, [pyridine], and extending the composition space from a two-dimensional triangle to a three-dimensional prism. Crystal Data: [H3O](2)[(CuF(NC5H5)(4))(2)NbOF5][F](2), tetragonal, space group I4(1)/acd (no, 142), with a = 24.691(1) Angstrom, c = 14.556(1) Angstrom, Z = 8.