Composition space diagrams have been used to study phase stability of mixed transition metal oxide fluorides synthesized by hydrothermal reaction of the metal oxides in (HF)(x).pyridine/H2O/pyridine solution (150 degrees C, autogenous pressure). The combination of early (Ti, Zr, Hf, Nb, Ta, Mo, W) and late (Cu, Cd, Zn) transition metals and the effects-of varying the mole ratios of the metals, (HF)(x).pyridine, and water are examined in this study. Single-crystal products containing [Cu(py)(4)](2+), [Cd(py)(4)](2+), or [Zn(py)(4)](2+) cations and [TiF6](2-), [ZrF6](2-), [HfF6](2-), [NbOF5](2-), [TaOF5](2-), [MoO2F4](2-), or [WO2F4](2-) anions are recovered. Relative stability of the crystalline products is:governed by the negative charge distribution on the anions, as well as the concentrations of each reactant in solution. Two new structures are reported: Cu(NC5H5)(4)TaOF5 and Cu(NC5H5)(4)TiF6. 3H(2)O. Crystal data: for Cu(NC5H5)(4)TaOF5, monoclinic, space group C2/c (No. 15), with a 10.541(3) Angstrom, b = 13.547(6) Angstrom, c 16.04(1) Angstrom, beta = 97.73(5)degrees, and Z = 4; for Cu(NC5H5)(4)TiF6. 3H(2)O, monoclinic, space group C2/c (No. 15), with a = 12,767(4) Angstrom, b = 12.048(2) Angstrom, c = 17.787(6) Angstrom, beta = 109.9(1)degrees, and Z = 4.